Audio Musings by Sean Olive

Harman Science of Sound Demonstrations at Rocky Mountain Audio Fest 2011

2011-10-06T13:16:00.000-07:00

October 14-16, I will be giving Science of Sound presentations for the Harman Luxury Audio Group (room #8020) at the Rocky Mountain Audio Fest (RMAF) in Denver, CO. My demonstration will be repeated every 1/2 hour on the hour and half-hour.

Drop by and find out more about the science behind Harman audio product development and testing including JBL and Revel loudspeakers. I will be demonstrating our latest release of the "How to Listen" software used for training and selecting listeners for product research and testing. Find out how discriminating and reliable you are as a critical listener.

Attendees will be given 30% discount coupons towards a copy of Floyd Toole's book "Sound Reproduction" (Focal Press), a book that describes much of the current scientific knowledge and perception of the sound quality of loudspeakers, listening rooms, and their acoustical interaction with each other. I will be raffling off a few copies to the best performing listeners.

I hope to see you there!

Topics Related to Perception and Measurement of Reproduced Sound

2011-04-21T11:43:00.000-07:00

On Tuesday, April 26th 2011, I will be giving a presentation at the meeting of the Los Angeles AES Chapter on several topics related to recent audio research at Harman International. The topics include:

I've briefly discussed these topics in Audio Musings over the past few months, and you can find summaries of them by clicking on the links above. I'll be giving an update on new findings, and briefly touch on topics not mentioned above. As a door prize, Harman will donate a free copy of Dr. Floyd Toole's book Sound Reproduction (shown on the right side bar) autographed by the author of the book.

AES members and nonmember guests are welcome to attend. The meeting will be held at the Sportmen's Lodge in Studio City. More details can be found at the Los Angeles AES website.

Version 2.04 of Harman How to Listen Now Available For Download!

2011-04-03T17:24:00.000-07:00

Version 2.04 of Harman How to Listen is now available for download here.

This update fixes the problem with the noise and hum attribute tests. We've also updated the user's manual to help navigate around some installation issues some users have reported.

Version 2.03 of Harman How to Listen Now Available For Download!

2011-03-25T15:19:00.000-07:00

You can download the latest update of Harman How to Listen (version 2.03) here. This update fixes a bug in the Windows version that prompted listeners to locate program material that was not packaged with the installer. There is no significant change to the Mac version. Enjoy!

Harman's "How to Listen" Listener Training Software Now Available as Beta

2011-03-15T00:16:00.000-07:00

Well, it's been some time coming, but the listener training software Harman How to Listen is finally available for free download here. This beta software is available in both Mac OSX and Windows versions.

We are pleased to offer the software packaged with four high quality music samples, courtesy of Bravura Records. The 24-bit music tracks are provided in both 96 kHz and 48 kHz formats in order to be compatible with older PC sound cards. We hope you try the software, and find that it improves you critical listening skills. This is a work in progress, and we expect to add more features and training tasks to this public version of the software over time. Enjoy!

How to Listen: A Course on How to Critically Evaluate the Quality of Recorded and Reproduced Sound

2010-12-09T11:58:00.001-08:00

Next month, I will be giving a one day course on How to Listen at the 2011 ALMA Winter Symposium in Las Vegas,held Jan. 4th and 5th, just prior to the CES show. The symposium will also feature other courses, workshops and paper sessions on loudspeaker and headphone design, testing and evaluation. You can register for my course and other events at the ALMA Symposium here. Below is a brief preface for my course How to Listen, which I encourage you to attend.

Figure 1: A listener training in the Harman International Reference Listening Room using the original version of the How to Listen training software.

Whether you are involved in the mixing of live and recorded sound, the design and calibration of sound systems, or just shopping for a new audio system, the question “Does it sound good?” is usually foremost on your mind. With sufficient prodding, most people can offer an opinion on the overall sound quality of a recording and its reproduction. Beyond that, listeners generally lack the necessary experience, training and vocabulary to describe which specific aspects of the sound they like and dislike. Sadly, the audio industry has no standardized terminology that allows musicians, audio engineers and audiophiles to communicate with each other about sound quality in a concise and meaningful way. Courses in critical listening are not commonly available in audio programs at universities, and are even less available to the general public. In summary, there is a real need for a comprehensive course that teaches audio enthusiasts how to critically evaluate sound quality.

A Scientific Approach Towards Training Listeners

To address this need, the author has developed a critical listening course called How to Listen. The course aims to teach students how to evaluate sound quality using percepts well established in the auditory perception field. These sound quality percepts are taught and demonstrated in a controlled way using real-time processing of recorded sounds. This has two benefits. First, the intensity of each attribute can be adjusted according to the aptitude and performance of the listener. Second, closely tying the physical properties of the stimulus to its perception and evaluation (a science known as psychoacoustics) there is theoretical basis behind the training approach. For example, the listener training data can be used to better understand how we perceive sound quality, which physical aspects of sound matter most of its perceived quality, and possibly identify the important underlying sound quality attributes that influence our preferences. Critical listening is treated as a science, rather than the black art it currently is.

How to Listen also includes classroom topics in the fundamentals of human auditory perception, sound quality research in variables that significantly influence the quality of recorded and reproduced sound (e.g. loudspeakers, rooms, recordings, microphones) and a brief tutorial in how to conduct sound quality listening tests that produce accurate, reliable and valid results.

But before we get too far ahead of ourselves, there must be good reasons for training listeners since it requires an investment in time and resources. There is also the question of external validity: Can the sound quality preferences of trained listeners be extrapolated to the preferences of untrained listeners, and does this hold true across different cultures? These questions will be answered in the following sections.

Why Train Listeners?

There are several compelling reasons for training listeners. First, trained listeners have been shown to produce more discriminating and reliable judgment of sound quality than untrained listeners [1]. Fewer listener can be used to achieve a similar level of statistical confidence, which can result in savings in time and money. For example, a panel of 15 trained listeners can provide sound quality ratings with reliable statistical confidence in less than 8 hours. To achieve a similar level of confidence using untrained listeners would require about 10 times more listeners, 10 times more days to complete the testing, and cost 10 times more money to pay the listeners and staff conducting the tests. If the study is conducted by an independent research firm using 200-300 untrained listeners, the cost can easily exceed $100k.

A second reason for training listeners is that they are able to report precisely what they like and dislike about the sound quality using well-defined, meaningful terms. This feedback can provide important guidance for reengineering the product for optimal sound quality.

Besides training listeners for product research, there are benefits in training audio marketing and sales people to become better critical listeners. Training makes them better equipped to communicate sound quality issues to audio engineers and customers. As audio companies expand sales and operations in China, India, and other developing countries, there is a growing need to develop a common cross-cultural understanding as to what constitutes good sound and unacceptable sound.

Does Training Bias Listeners?

An important question is whether the training process itself biases the sound quality preferences of listeners. If the trained listener preferences are different from those of the targeted demographic, there is a danger the product may not be well received in the marketplace. This raises the age old question, “Is preference in sound quality a matter of personal taste - much like food, wine and music - or is it universal?”

To study this question, the author compared the performances and loudspeaker preferences of trained listeners versus untrained listeners [1]. Over 300 untrained listeners were tested over a period of 18 months where they compared four different loudspeakers under controlled, double-blind listening conditions. Their preferences were then compared to the preferences of the trained Harman listening panel.

The results, plotted in Figure 2, show that the rank ordering of the loudspeaker preferences were the same for both the trained and untrained listeners. There were two main differences in how the two groups of listeners responded. First, the trained listeners tended to give lower loudspeaker ratings overall. Second, the trained listeners distinguished themselves from the untrained listeners by generally giving more discriminating and consistent loudspeaker preference ratings.

Figure 2: The mean loudspeaker preference ratings and 95% confidence intervals are shown for four loudspeakers evaluated in a controlled, double-blind listening test. The results of different groups of untrained listeners are compared to those of the 12 Harman listeners.

Relative Performances of Trained Versus Untrained Listeners

A common performance metric used to quantify the listener’s discrimination and consistency in rating sound quality is the F-statistic. This calculation is done by performing an analysis of variance (ANOVA) on the main variable being tested. In the above study [1], the performances of trained versus untrained listeners were compared by calculating the loudspeaker F-statistic for each individual listener.

Figure 3 shows the relative performance of different groups of untrained listeners based on their mean F-statistics compared to the F-statistics of the trained listeners. The relative performances of the untrained groups were: audio retailers (35%), audio reviewers (20%), audio marketing/sales staff (10%), and college students (4%). The poor performance of the students was explained by their tendency to give all four loudspeakers very similar and high ratings. A likely explanation for this was that they experienced a level of sound quality that was much higher than their everyday common experience: compressed MP3 music reproduced through headphones. The good news is that the students seemed to appreciate the higher fidelity sound based on the high ratings. In time, they will hopefully seek out better quality audio systems.

Figure 3: The relative performance of different groups of untrained listeners compared to the trained Harman listeners. Performance is based on the group’s average loudspeaker F-statistic which represents their ability to give discriminating and consistent preference ratings.

Are There Cross-Cultural Preferences in Sound Quality?

One of the oldest controversies in audio is the notion that different cultures or geographical regions of the world have different sound quality preferences [see reference 2]. For example, it is often claimed that Japanese listeners have different loudspeaker preferences than Americans due to differences in language, music, cultural practices and norms, and the acoustics of their homes. So far, very little formal research has done on this subject. In some preliminary studies, the author has found no significant differences in sound quality preferences for loudspeakers and automotive audio systems among Chinese, Japanese and American listeners.

How to Listen: A New Listener Training Software Application

Research has found most sound quality percepts fall under the attribute categories of timbre, spatial, dynamic or related nonlinear distortion. Within these four attributes there are additional sub-attributes that describe more specific sonic characteristics of the attribute. For example, Bright-Dull and Full-Thin are timbre sub-attributes related to the relative emphasis and de-emphasis of high and low frequencies, respectively. Sub-attributes for spatial quality deal with the location and width of the auditory image(s), and the perceived sense of spaciousness or envelopment. Distortion sub-attributes include the presence of noise, hum, audible clipping and distortions specific to the audio device(s) under test.

How to Listen focuses on teaching listeners to evaluate sound quality differences based on these four attributes and their sub-attributes (see Figure 4). While listening to music recordings, one or more attributes are manipulated in a controlled way so that listeners recognize and report the magnitude of these changes using the appropriate terms and scales. An analogy to this would the Wine Aroma Wheel where expert wine tasters are trained to identify the intensities of different aroma-flavors perceived in the wine.

Figure 4: A list of the 17 different training tasks that focus on one or more of the four sound quality attributes: spectral (timbral), spatial, distortion and dynamics.

To facilitate the training process, a proprietary computer-based software program called “How to Listen” was developed by Harman software engineers Sean Hess and Eric Hu. The software runs on both Mac and PC computers, and can play both stereo and multichannel music files. A real-time DSP engine built into the software application allows real-time manipulation of sound quality attributes in response to the listeners’ responses and performance.

There are currently five different types of training tasks that focus on one or more sound quality attributes (see Figure 4):

Band Identification
Spectral Plot
Spatial Mapping
Attribute Test
Preference Test

Band Identification (see Figure 5) teaches listeners to identify spectral distortions based on their frequency, level and Q-factor using combinations of peak/dip and highpass/lowpass filters. In each trial, the listener compares the unequalized version of the music track (FLAT) to a version that has been equalized (EQ) using one of the filters drawn on the screen. The listener must select the correct filter (Filter 1 or 2) they believe has been applied to the equalized version.

Figure 5: A screen capture of the listener training task “Band Identification” in Harman’s “How to Listen” training software. The listener compares the unequalized music “Flat” to an equalized version (EQ) and must select the EQ filter that is associated with its sound.

The difficulty of each training task automatically increases or decreases based on the listener’s performance. The listener is give immediate feedback on their responses, and they can audition all possible response choices when they enter an incorrect response.

The training task Spectral Plot requires the listener to compare different music programs that have been equalized a number of different ways. The listener must select the equalization curve that best matches its sound quality. This task teaches listeners to behave like human spectrum analyzers. Once fully trained, the listener can draw a graph of the audio system’s frequency response based on how it sounds.

The Spatial Mapping task requires the listener to graphically indicate on a two-dimensional map where a sound appears in the listening space. The Attribute training task requires the listener to correctly rank order two or more sounds on a given attribute scale based on the intensity of the attribute (e.g. bright-dull). For the Preference task, the listener must give preference ratings where the sound quality of the music has been modified for one or more sound quality attributes. The performance of the listener is calculated based on a statistical post-hoc test that determines the discrimination and reliably of the listeners’ preference ratings. Together, these different training tasks teach listeners to critically evaluate any type of sound quality variation they are likely to encounter when listening to recorded and reproduced sound.

Conclusions

The evaluation of sound quality remains an elusive art in the audio industry. Better awareness, understanding, and appreciation of sound quality may be possible if there existed a method to teach listeners how to evaluate the quality of reproduced sound and report what they hear using well-defined and meaningful terminology. How to Listen is a listener training course that aims to achieve those goals. Listeners are taught to identify and rate audible changes to different sound quality percepts related to the spectral, spatial, dynamic and distortion qualities of recorded music. Performance metrics based on the discrimination, accuracy and reliably of the listeners’ responses are factored into whether the listener meets the criterion of being a “trained” listener. The question of whether a listener is truly golden eared or not, is no longer a matter of conjecture and debate since How to Listen will ultimately reveal the true answer.

References

[1] Sean E. Olive, "Differences in Performance and Preference of Trained Versus Untrained Listeners in Loudspeaker Tests: A Case Study," J. AES, Vol. 51, issue 9, pp. 806-825, September 2003. Download for free here, courtesy of Harman International.

[2] Sean E. Olive, “Are There Cross-Cultural Preferences in the Quality of Reproduced Sound?” Audio Musings, July 2, 2010.here.

Harman Debunks Youthful Music Myths

2010-09-18T15:42:00.000-07:00

Robert Archer, a writer at CEPro magazine has written a nice article called "Harman Debunks Youthful Music Myths." The article is based on an interview he did with me a couple of weeks ago, and summarizes some recent Harman research on Generation Y's sound quality preferences for different digital music file formats (MP3 versus CD) and loudspeakers. The details of the preliminary research were first reported back in June in a blog posting, "Some New Evidence that Generation Y May Prefer Accurate Sound Reproduction."

The early results of that research suggest that today's youth prefer higher quality music formats and accurate loudspeakers when given the opportunity to A/B them under controlled, double-blind listening conditions. While it is refreshing news that good sound is not lost on today's youth, the challenge is to figure out how to market and sell it to them.

Unfortunately, good A/B audio demonstrations are becoming nearly extinct. Internet and Big Box store sales of audio equipment and music generally don't provide such listening opportunities. In the end, consumer education, meaningful audio specifications and measurements that are indicative of a product's true sound quality, and accurate, unbiased product reviews, will help consumers make more informed audio and music purchase decisions as they relate to sound quality. Until then, most consumers will never know for sure whether or not they've purchased something that is truly "good enough."

The Danger From Headphones

2010-07-13T23:16:00.000-07:00

Below is an English translation of a recent article "Gefahr aus dem Kopfhörer" (The Danger From Headphones) written by Matthias Hohensee over at Valley Talk. His article refers to my recent investigations into whether younger generations prefer lossy MP over higher quality music file formats. The preliminary results of that study were reported in the article I recently posted called, “Some New Evidence that Generation Y May Prefer Accurate Sound Reproduction”.

Matthias makes a good point about listener preference for MP3 becoming a moot issue with higher quality file formats becoming the standard, as bandwidth and music storage costs drop. I only briefly mentioned this in my slide presentation (see slide 7), but it deserves repeating. The days of low quality music downloads are numbered, I hope. Then, the main sound quality issue will become the recordings themselves, and the quality of the headphones and loudspeakers through which the recordings are heard. What are your thoughts on this matter?

The Danger from Headphones

by Matthias Hohensee

from Valley Talk 6.30.2010

Can the Germans be really proud of MP3 or has the digital stroke of genius desensitized the hearing of a complete generation?

When Angela Merkel recently visited the prestigious Stanford University in Silicon Valley and enumerated German technology services, she also mentioned the data compression method MP3. The technology that was largely developed by scientists at the Fraunhofer Institute has changed the music industry, even though it’s mainly U.S. companies that profit from the MP3 player market.

But can the Germans be really proud of MP3? Or has the digital stroke of genius desensitized the hearing of a complete generation? At least the observations of Jonathan Berger suggest this. Over the years the Stanford professor of music has been asking his students if they are satisfied with compressed music files, or if they prefer the full Hi-Fi sound.

He came to a surprising result: For years, the number of those who preferred the sound of ‘packed’ music to the uncompressed audio spectrum seems to grow steadily. Berger concludes that the taste of sound has changed.

Good sound is measurable

Sean Olive, on the other hand, considers Berger's insight as nonsense: "Good, accurately reproduced sound is not a question of taste, but scientifically measurable." And this is the way he should see it. After all, Olive is the head of acoustic research at Harman International. The U.S. manufacturer is considered to be THE address for sophisticated sound systems.

Alarmed by Berger's observations Olive recently invited Los Angeles high school students to the Harman studios for extensive tests. "Everyone could hear the difference between different compacted sound files - and preferred less compressed songs," says the scientist relieved.

Danger from headphones

Now, Olive is not really unbiased, after all Harman sells nearly three billion dollars worth of high-end audio technology per year. But in fact, technical progress makes Olive's worries already obsolete. In times of high-speed Internet, data compression does not play the same role as it did in the nineties when the music piracy supplier Napster made MP3 popular.

The songs that were exchanged back then were extremely compressed in order to distribute them via the still slow Internet connections – but also to spare the limited memory of computers and MP3 players. Today the vendors such as Apple and Amazon are selling songs which are formatted in such a way that only real audiophiles can hear the difference to music CDs.

And so the real dangers for the hearing of ‘generation iPod’ aren’t the highly compressed music files, but simply the volume adjustment of their headphones.

Acknowledgements: Thank you to the author Matthias Hohensee for permission to repost his article here, and to Alena Winterhoff for the English translation.

Why Live-versus-Recorded Listening Tests Don't Work

2010-07-09T18:22:00.000-07:00

Figure 1: Singer Frieda Hempel conducting a Tone Test at Edison Studios, NYC in 1918. Note that many of the listeners' ears are covered by the blind folds making it a double blind and double deaf listening test, since the experimenter Edison was deaf himself.

Recently I was asked how I could possibly prove or assert that listeners prefer accurate loudspeakers without having performed a live-versus-recorded listening test. This is a test where the listener compares a live musical performance to a recording of the performance reproduced through loudspeakers. The closer the sound quality of the reproduction is to that of the live performance, the more accurate the loudspeaker is deemed to be - at least in theory. In practice, these tests are usually ridden with so many uncontrolled listening test nuisance variables that the results are essentially meaningless. This article examines why live-versus-recorded listening tests are not suitable for serious scientific investigations of the perceived sound quality of recorded and reproduced sound.

Edison’s Tone Tests: “People will hear what you tell them to hear”
Thomas Edison was among the first audio engineers to embrace live-versus-recorded demonstrations. In 1910, he invented the Edison Diamond Disk Phonograph, which he claimed had “no tone” of its own. To prove it, a series of road shows involving 4,000 live-versus-recorded demonstrations of his phonograph were conducted in auditoriums across the United States At some point during the live music performance there would be a switch over to the recorded performance, and apparently audience members could not tell the difference between the live and recorded performances

After a 1916 live-versus-recorded demonstration in Carnegie Hall, the New York Evening Mail stated “the ear could not tell when it was listening to the phonograph alone, and when to actual voice and reproduction together. Only the eye could discover the truth by noting when the singer’s mouth was open or closed” [1].

By today’s standards, the fidelity of Edison’s disc phonograph was egregious in terms of its noise, distortion, limited dynamic range, bandwidth and frequency response (you can hear some of Edison’s recordings online here). It’s hard to imagine that listeners were fooled into thinking his Diamond Disk recording was indistinguishable from the live performance. In fact, we now know that Edison manipulated the tests to produce the results he wanted. First, he carefully chose the music and musicians to work within the technical limitations of his technology. Edison detested music with extreme dynamics, high tones, vibrato and complex textures because they were a challenge to his deafness and his Tone Tests. He selected and coached musicians to mimic the sound of their recordings to minimize the audible differences between live and recorded performances [1],[2].

Second, Edison was the consummate audio salesman and was known to say, “People will hear what you tell them to hear” [2]. The expectations and perceptions of his listeners were manipulated before the test to produce a more predicable outcome. Audience members were given a concert program before his Tone Tests that clearly told them exactly what they would hear, how amazing it will sound, and what an appropriate response would be:

“Those who hear this test will realize fully for the first time how literally true it is that Mr. Edison has made possible the re-creation of the artist’s voice. No more exacting test could be made to demonstrate that the New Edison actually does re-create the voice of the artist than to play it side by side with the artist who made the records. This is the final proof. Close your eyes. See if you can distinguish the voice of the New Edison from that of the artist. Did you ever believe it possible to re-create a voice? Note that the voice of the artist and the voice of the Edison are indistinguishable” [emphasis is mine] [ 3].

Figure 2: Another Edison Tone Test where extraneous biases related to sight and smell may have compromised the results based on the large number of listeners covering their noses. Perhaps a bad case of singer's halitosis made it possible to identify the live performance from the recorded one based on smell alone?

Other Live-Versus-Recorded Demonstrations

Following Edison’s live-versus-recorded demonstrations, other tests have been conducted by Harry Olson at RCA, and G.A. Briggs (Wharfedale) and Peter Walker at Quad in the 1950’s. [4]. A common problem with these demonstrations was double reverberation: the reverberation of the room was heard both in the recording, and again when it was reproduced through loudspeakers in the same room. This made it easier for listeners to tell the difference between the recorded and live performances.

Acoustic Research's Live-Versus-Recorded Demonstrations

During the 1960’s, Acoustic Research (AR), an American loudspeaker company, performed over 75 live-versus-recorded concerts in cities around the USA featuring The Fine Arts String Quartet, and the AR-3 loudspeaker [5],[6]. To solve the double reverberation problem, the recordings of the quartet were made in an anechoic chamber, or outdoors. Outdoor live-versus-recorded demonstrations had the added benefit that there were no room reflections in either the recording or the live performance. This made the demonstrations less sensitive to off-axis problems in the microphones and loudspeakers. It also relaxed the demands on the recording-reproduction to accurately capture and reproduce the complex spatial properties of a reverberant performing space.

The AR demonstrations apparently generated an enormous amount of free publicity in newspapers and audio magazines where it was reported that the reproduction of the recordings was virtually indistinguishable from the live performance. AR sales increased dramatically, to the point where in 1966 AR apparently owned 32% market share of loudspeakers sold in the United States.

A Live-Versus-Recorded Method For Testing Loudspeaker Accuracy

Edgar Villchur, head of Acoustic Research, to his credit, was a firm believer that loudspeakers should accurately reproduce the art (the recorded music) and not editorialize or enhance it. In a 1962 paper, he described a live-versus-recorded method for evaluating the accuracy of loudspeakers [7]. The method used a reference loudspeaker (the live performance) that was placed in the listening room with the loudspeaker-under-test. The goal of the loudspeaker-under-test was to accurately reproduce a previous recording of the reference loudspeaker playing white noise in an anechoic chamber. The original white noise signal was also fed to the reference loudspeaker during the listening test. The more similar the loudspeaker-under-test sounded to the reference speaker, the more accurate it was deemed to be, at least in theory.

Villchur acknowledged that the sensitivity and validity of the method depended on the quality of the reference loudspeaker, its directivity, and the choice of program material. White noise was more revealing of loudspeaker inaccuracies than music. His reference loudspeaker consisted of a single 2-inch midrange from an AR-3 loudspeaker selected because he found using multiple drivers caused acoustical inference that was audible in the anechoic chamber, but not so audible in a reverberant listening room; these differences would produce errors in the listening test. One wonders how a tiny 2-inch driver could have produced adequate high treble and low bass without distortion. These limitations would significantly limit the accuracy and usefulness of this listening test method.

Another problem with this method was that the anechoic loudspeaker recordings were made at a single point in space, and did not capture the directivity and off-axis characteristics of the reference loudspeaker. Unless the speaker-under-test had the same directivity and off-axis characteristics of the reference loudspeaker, it could never sound exactly the same in a reflective listening room. To compensate for these errors, Villchur used a trial-an-error process to find the best microphone position relative to the reference loudspeaker where the timbre of the anechoic recording best matched the timbre of the reference loudspeaker when placed in a room. Adjusting the recording to mimmic the sound of live performance was the reverse process of what Edison’s musicians did, but essentially it produced the same bias. (Edison would have been proud!)

Finally, it is not clear how Villchur controlled loudspeaker positional biases when comparing the reference loudspeaker to the loudspeaker-under-test. Loudspeaker positional biases have been shown to produce audible effects that are sometimes larger than the audible differences between different loudspeakers under test [9]. At Harman, these positional biases are eliminated via an automated speaker shuffler that places each loudspeaker in the same position of the room.

Summary of Problems with Live-versus-Recorded Tests

By today’s standards, the live-versus-recorded tests performed to date lack the necessary scientific controls and rigor to consider their results or conclusions accurate, repeatable and valid. Below are a few of the most significant psychological, physical, methodological or experimental listening variables that plague these types of tests. While it is possible to control some of these variables, others are either impossible, impractical or too expensive to control.

Sighted and Cross-Modality Biases

To date, most of the live-versus-sighted tests have been performed sighted, where non-auditory cues were available to allow the listener to identify whether they were hearing the live or reproduced sound source. These tests could have been easily made blind via an acoustically transparent curtain; however, scientific validity was apparently not the primary purpose of the test. The visual cues from the musicians (bowing, lip syncing) would also enhance the realism and presence of the reproduction, a well-known cognitive effect observed in research of binaural and virtual reality displays.

Listener Expectation, Authority Bias, Group Interaction Bias

In many of the public live-versus-recorded demonstrations, listeners expectations were manipulated by knowledge given to them by the organizers of the demonstrations. In some cases, listeners were told what the expected response should be before the test began (see Edison's concert programs above). In large groups settings, listeners' responses can be easily swayed by the opinions and reaction of other members in the group (a herd mentality), especially when an authority member is present. These biases are easily removed from live-versus-recorded tests by repeating the test for each individual listener. The live and recorded performances would have to be replicated for every listener, which makes the tests too difficult, expensive, time consuming, and impractical to use.

Qualifications of Listeners

None of the live-versus-recorded tests I've read about have reported the hearing and critical listening qualifications of the listeners who participated in them. These are important variables in the sensitivity and reliability of the test results, and can be easily quantified.

Live and Recorded Performances Must Be Identical

For live-versus-recorded tests to be valid, the live and recorded performance should be identical, having the same notes, intonation, tempo, dynamics, loudness, balance between instruments, and the same location and sense of space of the instruments. Otherwise, there are extraneous cues that allow listeners to readily identify the live and recorded performances. Midi-controlled instruments (e.g. player pianos) are but one example of how this problem could be resolved.

Positional Biases from Live and Reproduced Sound Sources

Unless the live and reproduced (e.g. loudspeakers) sound sources occupy the same physical locations, the listener can always identify the live versus recorded versions based on the localized positions of the sound sources.

Errors in the Recording

The usefulness of live-versus-recorded methods for perceptual measurements of sound quality in the playback chain is severely limited by errors in the recording. The recording errors are not easily separated from the errors in the playback chain (see circle-of-confusion). Microphones and microphone techniques both contain errors that limit the timbral, spatial and dynamic accuracy of the recordings through which we judge loudspeakers. Apparently the most effective live-versus-recorded demonstrations were conducted outdoors - effectively an anechoic environment - where the off-axis performances of the microphones and loudspeakers, and the complex spatial cues of a reflective room were largely removed as factors from the experiment. However, results from outdoor live-versus-recorded tests cannot be generalized to how the loudspeakers would perform in real rooms, where the off-axis sounds provide a significant contribution towards the listener's impression of the loudspeaker.

Lack of Proper Scientific Protocols, Listener Response Data, Statistical Analysis, Results

The most interesting characteristic of live-versus-recorded tests is that they never seem to provide listener response data, statistical analysis or published results. Eyewitness reports written in newspapers or magazines do not constitute scientific evidence.

Accuracy is Not Applicable to Most Recordings Made Today

Most recordings made today are not intended to sound like the live performance. Anyone who heard Taylor Swift's live performance with Stevie Nicks at the 2010 Grammy Awards understands why.(Note: you can relive the magical moment on Youtube. Warning: this may be offensive for the musically-inclined). About 90% of commercial recordings are studio creations consisting of a series of overdubs, processed with auto-tuning, equalization, dynamic compression, and reverb sampled from an alien nation. For these recordings, there is no equivalent live performance to which the recording/reproduction can be compared for accuracy. The only reference is what the artist heard over the loudspeakers in the recording control room. If the important performance aspects of the playback system through which the art (the music and recording) was created can be reproduced in the home, then the consumer will hear an accurate reproduction of the music, as the artist intended. It is possible to achieve this if we adopt a science in the service of art philosophy towards audio recording and reproduction.

Conclusions

In reviewing the history of live-versus-reproduced tests, most have been performed as elaborate sales and marketing demonstrations designed to fool listeners into believing that a product sounded much better and more accurate than it actually was. While live-versus-recorded tests have proven their merit as an effective marketing and sales tool, they have not yet proven themselves as a serious method for scientific experiments intended to advance our psychoacoustic understanding of music recording and reproduction.

The reason for this, I believe, is that live-versus-recorded tests do not adequately control important listening test nuisance variables, a prerequisite for accurate, reliable and scientifically valid results. It is not entirely coincidental, that (to my knowledge) none of the live-versus-recorded tests to date have produced a single scientific publication or new psychoacoustic knowledge.

Hopefully, you now understand why I don’t conduct live-versus-recorded loudspeaker listening tests.

References

[1] Harvith, J., and Harvith, S. Edison, Musicians and the Phonograph: A Century in Retrospect, Greenwood Press, N.Y (1987).

[2] Andre Milliard, “Edison’s Tone Tests and the Ideal of Perfect Sound Reproduction,” from Lost and Found Sounds’, NPR.
[3] Program for Edison Demonstration http://www.nipperhead.com/old/tonetest04.htm
[4] Wharfedale History: http://www.wharfedale.co.uk/About/History/tabid/66/Default.aspx

[5] Acoustic Research http://en.wikipedia.org/wiki/Acoustic_Research

[6] Edgar Villchur, http://edgarvillchur.com/

[7] Villchur, Edgar, “A Method of Testing Loudspeakers with Random Noise”, J. Audio Eng. Society, Vol. 10, Issue 4, pp, 306-309 (October 1962),

[8] Kissinger, John R.The Development of the Simulated Live-vs-Recorded Test into a Design Tool, presented at the 35th AES Convention, preprint 609, (October 1968

[9] Olive, Sean E.; Schuck, Peter L.; Sally, Sharon L.; Bonneville, Marc E. “The Effects of Loudspeaker Placement on Listeners' Preference Ratings”,JAES Volume 42 Issue 9 pp. 651-669; September 1994.

Harman Kardon's Quest to Standardize Sound

2010-07-07T13:59:00.000-07:00

Above: Trained listener Alex Miller is evaluating the sound quality of three loudspeakers in Harman's Multichannel Listening Lab. The automated speaker shuffler ensures that each speaker is heard in the exact same position. The acoustically transparent, visually opaque scrim means the tests are double-blind and not influenced by brand, price or other sighted biases. The computer randomly selects the presentation order of the speakers in each trial and listener controls the switching so that experimenter bias is removed from the test.

There is a nice story written by Stuart Miles over at Pocket-Lint called "Harman Kardon's Quest to Standardize Sound". Check it out and leave a comment whether or not you think standards that define the sound quality of an audio product is something that would benefit the consumer.

This story came from a recent Harman Kardon press event held at Northridge where they kicked off Harman Kardon's Science of Sound campaign that aims to promote the science and philosophy behind how we develop and test our products.

A group of 18 European journalists attended the press event and received presentations from Dr. Floyd Toole and myself about how good sound is not a matter of personal taste, but rather something that can be quantified through scientific-based listening tests, measurements, and psychoacoustics.

The journalists experienced how we train listeners and then participated in brief double-blind listening tests of the Harman Kardon MS100 and MAS100 in our Reference Listening Room and the Multichannel Listening Lab (shown in the picture above).

Are There Cross-Cultural Preferences in The Quality of Reproduced Sound?

2010-07-03T14:59:00.000-07:00

Do we need a new user menu where you dial in your nationality to match your taste in sound quality?

[click on image to see a larger version].

The field of audio is ripe with myths and unsubstantiated opinions. One of the most enduring opinions is that there are cross-cultural preferences in the sound quality of reproduced sound. Some of the more common cross-cultural assertions I hear repeated among audiophiles, audio reviewers and audio marketing executives include these:

Americans prefer more bass than Europeans and Japanese
Japanese prefer less bass and more midrange (and listen at lower volumes)
Germans prefer brighter sound
The British prefer “tighter” or more over-damped bass

To my knowledge, these statements are anecdotal, and have not been tested in any rigorous scientific way. Marketing has already given us misguided menus in media players and automotive head units that adjust the equalization based on music genre (e.g.jazz, classical, hip hop, rock, country music, Christian music, and heavy metal, etc). Do we really need another one based on where we were born? What could the “Canadian” sound have in common with a predisposition towards liking cold long winters, hockey, Molson beer, maple syrup, beaver tails, national health care, and the music of KD Lang and Celine Dion?

While it is easy to dismiss the importance of cross-cultural preferences, the subject is gaining serious attention from audio manufacturers expanding into new markets like China, India, Russia and South America. Now the same age-old questions are being asked: Are there cross-cultural preferences in the quality of reproduced sound or is good sound universal and transcend cultural differences?

Possible Reasons Why Cross-Cultural Preferences in Sound Quality May Exist

Very little research in cross-cultural sound quality preferences exists. Nonetheless, here are some proposed reasons why they may exist according to various sources.

Language, Dialect, Music

Certain spectral balances may compliment and enhance the timbre and intelligibility of different languages and dialects. Similarly the culture’s ethnic music and its instrumentation may be enhanced from certain loudspeakers or EQ. Wouldn’t this enhancement be added to the recording by the artist or the producer when it was mixed? If so, why do we need to duplicate in the playback chain? Is there such a thing as too much enhancement (think Dolly Parton)?

Influence of Regional Building Construction and Room Acoustics

One explanation for regional tastes for certain types of loudspeakers is related to the design and construction of the region's homes and apartments. This would affect the noise isolation and acoustical properties of the room, and its interaction with the loudspeaker. Massive, rigid plaster walls commonly found in older construction in Europe would provide more noise isolation and less absorption of bass than less massive and rigid walls used in typical American construction today. It is argued that a loudspeaker with less bass might sound better in the European room. It should be pointed out that if the different rooms and loudspeakers combine in ways that in the final analysis produce the same sound, this doesn't really constitute a difference in preferred sound quality. Different means are being used to achieve the same end goal. Fortunately, there are technological solutions for dealing with loudspeaker-room interactions at low frequencies so that decent bass performance can be achieved regardless of the room’s size, dimensions and stiffness of its walls.

Influence of Social Norms and Practices

Cultural practices and norms may influence how much bass people like, and how loud they listen to their music. For example, Japanese apartment dwellers may prefer to listen to reproduced sound at lower volumes to avoid disturbing their neighbors, which is a serious social infraction. On the other hand, American urban apartment dwellers may be more tolerant of bass and higher playback levels due to better noise isolation from the wall construction. Tolerance to your neighbor's subwoofers and loud music comes more easily if you know they own a handgun. The right to listen to loud music and bass in America is sort of protected under the second amendment (i.e. the right to bear arms). :)

Possible Reasons Why Cross-Cultural Preferences May Not Exist or Matter

The following arguments do not directly prove that cross-cultural sound quality preferences do not exist. They do provide evidence that the cultural entertainment, broadcast, recording and audio industries have largely decided to ignore cross-cultural preferences. Either they don't believe they exist, or if they do, catering to them doesn't make sense from a business or philosophical viewpoint.

Audio Manufacturers: One Product, One Sound

Most audio companies sell the same model of product in every country, only changing the language of the packaging/owners manual and the power supply voltage to meet the local requirements. Measurements of loudspeakers from different countries of origin tend to aim towards the same performance target. There is nothing in the objective measurements or the listening test results that indicate a unique sound, voicing or preference that can be attributed to the country of origin whether the loudspeaker is British, German, Canadian, American, French, Italian, Danish or Japanese [1]-[3]. Accurate sound seems to be the common universal attribute that matters most. These studies did not formally or systematically study the culture or race of the listener as a factor in loudspeaker preference, so the definitive study remains to be done.

Recording/Film Industries: One Product,One Sound

To my knowledge, record companies do not release different mixes of their recordings to satisfy different cultural tastes in sound quality. Fans of Lady Ga Ga apparently equally like (or dislike) her sound on the recordings whether they are in America, Europe or Asia. Similarly, there is no option in the iTunes store where you indicate your nationality or culture before downloading your music.

Universal Loudspeaker / Audio Standards in Broadcast

If you look at international audio standards for broadcasting (AES, IEC, ITU, EBU), and read the loudspeaker papers written by researchers within the BBC (British), CBC (Canadian) and NHK (Japanese), you will find a common set of performance criteria: flat on-axis response, extended bandwidth in bass and treble, smooth off-axis response and low distortion. At the broadcast level, the playback chain in different countries is not being influenced by cross-cultural preferences in the targeted audience where the content will be heard.

Concert Halls and Live Music Performance

Acoustical design of concert halls have generally followed well established standards and practices based on research using international listening panels. Qualities such as spatial envelopment, reverberation, clarity and richness of timbre are universally accepted as desirable qualities. The classical and romantic composers specifically wrote their music for these particular acoustics, and to radically alter the acoustics would not well serve the art.

The Global Economy

In the new global economy, the political, cultural, socioeconomic and technological barriers have been largely removed. As communication between different cultures improves, this will likely influence their attitudes, tastes and perception towards culture, music and sound reproduction. If there are cross-cultural differences in sound quality preferences, it seems likely that in the future these differences will converge, and taste in sound quality will become more homogeneous (hopefully, in a positive way).

Audio is science in the service of art

This philosophy assumes that music, its performance and recording are part of the art, and the goal of sound reproduction is to accurately reproduce the art. To serve the art, there is no room for cultural preferences or individual tastes in the design of the audio equipment used for reproduction of the art. It is presumed that any cultural sound quality preferences will be encoded in when the music when it is performed and recorded, and doesn’t need to be added again in the playback chain.

Here is a parallel analogy in painting: When a Monet art exhibit travels to different countries, the art is not altered, transformed or "improved" to suit the local tastes of the country. Art lovers want to see the original Monet, not a new and improved version with edge enhancements, higher contrast and 3D effects. The same is true of the sound of Vienna Philharmonic when they do a world tour. When they tour Japan, they don’t leave half the bass section at home because the Japanese do not supposedly like bass. So why would we want to tamper with the original sound of the Vienna Philharmonic when playing recordings of them through our audio system?

Research in Cross-Cultural Preference in Sound Quality of Recorded and Reproduced Sound

In the realm of perception there is an essential pan-human unity, and that most differences among cultures is only a “fine tuning” [4].

To date, very little cross-cultural research has been done in the perception of sound quality. One of the challenges in cross-cultural research is ensuring that the listener instructions, sound quality descriptors and semantic definitions of the scales have the same meaning across cultures. Fortunately, there are methods for removing language from the perceptual task. Multidimensional scaling allows listeners to judge different pairs of sounds based on their similarity. Then the perceptual attributes of the sounds (e.g. timbre or spatial related) can be identified through multivariate statistical methods like principal component analysis. In a study of different guitar timbres, Martens et al. found that native speakers of English, Japanese, Bengali, and Sinhala perceived the same underlying dimensions, but used different adjectives/semantics to describe the attribute [5].

In another study that compared Japanese and English speaking listeners’ perception of music recordings made with four different 5-channel microphone techniques, the authors found a common understanding of three critical dimensions in which the quality of the recordings differed [6].

Recently, we have begun testing cross-cultural sound quality preferences of music reproduced through different loudspeakers, equalizations, and automotive audio systems using American, Japanese and Chinese speaking listeners. While this work is still ongoing, the preliminary results do not show any evidence of cross-cultural preferences among the different groups. Accurate sound reproduction seems to be the common link across the preferences of the different cultures.

Conclusions

Very little research has been done in cross-cultural preferences in the sound quality of reproduced sound. What we know is that differe Preliminary investigations by the author in preferred spectral balance of music reproduced through loudspeakers have not revealed any significant differences in cross-cultural preferences to date. If cross-cultural preferences exist, the music and audio industries have largely ignored catering to them, instead distributing products that are optimized for a single universal audience.

Finally, an important question is whether audio companies should even be catering to these cross-cultural preferences if research eventually finds that they indeed exist? If the audio industry takes an “audio science in the service of art” philosophy where the goal is to faithfully and accurately reproduce the art as the artist intended, the question of cross-cultural preferences becomes moot. If certain cultures don’t like the sound of the art, then that becomes an issue between the artist and the recording producer/record executive - not the audio manufacturer.

For more discussion on this topic, please head over to WhatsBestForum.

References

[1] Floyd E. Toole, "Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 1" J. AES Vol. 23, issue 4, pp. 227-235, April 1986. (download for free courtesy of Harman International).

[2]Floyd E. Toole, "Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 2," J. AES, Vol. 34, Issue 5, pp. 323-248, May 1986. (download for free courtesy of Harman International).

[3] Sean E. Olive, "Differences in Performance and Preference of Trained Versus Untrained Listeners in Loudspeaker Tests: A Case Study," J. AES, Vol. 51, issue 9, pp. 806-825, September 2003. (download for free courtesy of Harman International).

[4] John W. Berry, Ype H. Poortinga, Janak Pandey, Handbook of Cross-Cultural Psychology, Volume 1 Theory and Method, 2nd edition, Aug. 21, 1996.

[5] Martens, William L.; Giragama, Charith N. W.; Herath, Susantha; Wanasinghe, Dishna R.; Sabbir, Alam M.” Relating Multilingual Semantic Scales to a Common Timbre Space - Part II,” presented at the 115th Audio Engineering Convention, preprint 5895 (October 2003).

[6] William L. Martens, Sungyoung Kim, Atushi Marui,”Comparison of Japanese and English Language descriptions of piano performances using popular multichannel microphone arrays,” J Acoust Soc Am. 2008 May ;123 (5):3690

Science in the Service of Art

2010-06-28T08:39:00.000-07:00

Last week, I've was given my own front page forum over at WhatsbestForum called "Science in the Service of Art", where I can write about any topic I wish. My first posting is called "Audio Science in the Service of Art".

I will probably post the same articles I write over there, on this blog as well. But for now, I recommend you go over there, read my article, and then leave your comments about what we need to do in order to improve the quality and consistency of recorded and reproduced music.

Harman is committed to a scientific approach towards the design and testing of audio products in the consumer, professional, and automotive audio spaces. Last week, Harman Kardon began a PR campaign called the "Science of Sound " where "Science in the Service of Art" is a major theme. You can read about this on the Harman Kardon web sites (click on the "about" link at the top of the page). Enjoy!

Some New Evidence That Generation Y May Prefer Accurate Sound Reproduction

2010-06-18T17:24:00.000-07:00

Sound quality in mainstream music recording and reproduction is all but dead, at least according to the media reports published over the past year [1]-[6]. On the music production side, music quantity (as in volume and decibels) matters more than quality and dynamic range. Record executives and producers are forcing artists to squash the dynamics and life from their music in order to be the loudest record on the charts [5],[6]. Listening to one of these albums can induce an instant migraine, making you wonder if the record companies aren’t secretly owned by the makers of Excedrin (see slides 2-4 in this article’s accompanying PDF slide presentation or this YouTube Video ).

On the music reproduction side, convenience, portability and low cost are the purchase driving factors in this Mobile-Ipod Age of entertainment; sound quality need only be “good enough” [3]. The problem is that no one seems to be able to define what “good enough” sound quality means for Generation Y. Given that they represent the largest and youngest demographic in terms of music and audio equipment consumption, it's important to understand the attitudes and tastes of these twenty-somethings before it's too late. Getting these Millennials hooked on good sound now, means they're more likely to upgrade the audio systems in their future homes and automobiles acquired as they grow older and wealthier.

A common belief being spread by the media is that Generation Y is indifferent to sound quality, or worse, they prefer the tinny, sizzling sound of low-bit rate MP3 over higher quality lossless music formats (slide 4). This is based on an informal study conducted by a Stanford music professor, Jonathan Berger, who over a 7 year period found his students increasingly preferred music coded in lower quality lossy MP3 formats over higher quality lossless music formats [1]-[5]. “I think our human ears are fickle” says Berger. “What’s considered good or bad sound changes over time. Abnormality can become a feature” [1].

While Berger’s unpublished study raises more scientific questions than it provides answers, nonetheless it has been widely reported by the media, and has captured the attention of consumer and automotive audio marketing executives, who ultimately decide what level of sound quality is “good enough” for Generation Y (slides 4-7). There's an increased risk that sound quality may become the sacrificial lamb for products targeted at Millennials (they can’t tell the difference, after all) with the savings diverted to more salient “purchase drivers” such as industrial design, more features, advertising, and celebrity endorsements.

If someone doesn’t soon stand up for Generation Y and show some evidence that they care about sound quality, its death may become a self-fulfilling prophesy.

Some New Experiments on Generation Y Sound Quality Preferences For Music Reproduction

To this end, I recently conducted two listening experiments on a group of high school students (the younger half of Generation Y) to determine if their sound quality preferences in music reproduction were: a) consistent with those of older trained listeners used for product evaluation at Harman International, or alternatively b) indifferent or skewed towards preferring less accurate sound (slide 8).

Two research questions were asked in separate tests:

Do the students prefer the sound quality of lossy MP3 (128 kbps) music reproduction over the original lossless CD version?
Do the students prefer music reproduced through a more accurate loudspeaker given four different options that vary in accuracy and sound quality?

The students, who ranged in ages from 15 to 18 years, were visiting Harman on a class field trip (slide 9). A description of the listening tests and the results are summarized in the following sections.

Do High School Students Prefer Lossy MP3 Music Over Lossless CD-Quality Formats?

In the first double-blind listening experiment (slide 11), the students were presented two versions of the same program selection encoded in:

MP3 (Lame 3.97, version 2.3; constant bit rate @ 128 kbps). Note that this a 2 year old MP3 encoder that may be more representative of what Berger used in his study.
CD - The original lossless CD-quality version (16-bit, 44.1 kHz).

After hearing the same music several times in both MP3 and CD formats, the listeners indicated on a scoresheet which one they preferred: A or B. They were also asked to indicate the magnitude of preference (slight/moderate/strong), and provide comments describing the differences in sound quality they heard.

A total of 12 trials were completed in which preference choices were recorded using four different short program loops in three separate trials (slide 10). Three music programs and a recording of applause at a live concert were chosen based on their ability to provide audible differences between the lossy and lossless formats. The applause provided listeners a familiar acoustic signal that the author felt most listeners could easily judge based on its apparent naturalness.

The order of programs and MP3/CD formats was randomized by the listening test software to eliminate any order-related effects. Switching between A and B was performed by the test administrator via a custom Harman listening test software application. The listening test was conducted in the Harman International Reference Room, which provided a quiet, and controlled acoustic environment typical of a domestic listening room. Listening was done through a high quality, stereo playback system (JBL LSR 6336 with four JBL HB5000 subwoofers) calibrated at the listening locations. A comfortable playback level (on average 78 dB (B)) was used throughout the tests.

Two groups of nine listeners each participated in two separate listening sessions, which lasted about 30 minutes each.

Listening Test Results: Students Prefer Music in Lossless CD Versus MP3 Formats

When all 12 trials were tabulated across all listeners, the high school students preferred the lossless CD format over the MP3 version in 67% of the trials (slide 16). The CD format was preferred in 145 of 216 trials (p<0.001).

As expected, there were differences among individual students in their ability to formulate consistent preference choices (slide 17). Nearly 40% of the listeners gave a sufficient number of preference choices (9 of 12) to establish a statistically significant preference for CD (p <= 0.054). Only one of the 18 listeners preferred MP3 over CD (7 versus 5 trials), although the preference was not statistically significant ( p = 0.19). Other listeners were either guessing, and/or were inconsistent in their choices. With additional training and trials, the performance of these listeners would likely improve.

On average, the magnitude of preference for CD over MP3 was also stronger based on the frequency of responses assigned to the categories of preference: slight, moderate and strong preference (slide 18). When CD format was preferred, listeners assigned a proportionally higher number of moderate-to-strong responses compared to when MP3 was the preferred choice.

The preference for CD over MP3 formats was relatively independent of the program selection (slide 19). CD format was preferred for all four programs, with only a slight drop (68.5 % to 63%) for program JW.

Finally, the comments given by the more consistent listeners (slide 20) reveal the nature of audible differences between MP3 and CD. The CD version was often described as sounding more dynamic and brighter, with more impact on percussive sounds. MP3 versions of the programs were described as sounding duller, dynamically compressed with swirling-pitch modulation artifacts on vocal and strings.

Do High School Students Prefer Neutral/Accurate Loudspeakers?

Given that the high school students preferred the higher quality music format (CD over MP3), would their taste for accurate sound reproduction hold true when evaluating different loudspeakers? To test this question, the students participated in a double-blind loudspeaker test where they rated four different loudspeakers on an 11-point preference scale. The preference scale had semantic differentials at every second interval defined as: 1 (really dislike), 3 (dislike), 5 (neutral), 7 (like) and 9 (really like). The relative distances in ratings between pairs of loudspeakers indicated the magnitude of preference: ≥ 2 points represent a strong preference, 1 point a moderate preference and ≤ 0.5 point a slight preference.

The four loudspeakers were floor-standing the models (slide 22): Infinity Primus 362 ($500 a pair), Polk Rti10 ($800), Klipsch RF35 ($600), and Martin Logan Vista ($3800). Each loudspeaker was installed on the automated speaker shuffler in Harman International’s Multichannel Listening Lab, which positions each loudspeaker in same the location when the loudspeaker is active. In this way, the loudspeaker positional biases are removed from the test. Each loudspeaker was level-matched to within 0.1 dB at the primary listening location.

Listeners completed a series of four trials where they could compare each of the four loudspeakers reproducing a number of times before rating each loudspeaker on an 11-point preference scale. Two different music programs were used with two observations. At the beginning of each trial, the computer randomly assigned four letters (A,B,C,D) to the loudspeakers. This meant that the loudspeaker ratings in consecutive trials were more or less independent (slide 23).

Results: High School Students Prefer More Accurate, Neutral Loudspeakers

When averaged across all listeners and programs, there was moderate-strong preference for the Infinity Primus 362 loudspeaker over the other three choices (slide 25). In the results shown in the accompanying slide, as an industry courtesy, the brands of the competitors’ loudspeakers are simply identified as Loudspeakers B,C and D.

As a group, the listeners were not able to formulate preferences among the three lower rated loudspeakers B,C, and D, which were all imperfect in different ways. For an untrained listener, sorting out these different types of imperfections and assigning consistent ratings can be a difficult task without practice and training [5].

The individual listener preferences (slide 26) reveal that 13 of the 18 listeners (72%) preferred the Infinity loudspeaker based on their ratings averaged across all programs and trials.

When comparing the student's rank ordering of the loudspeakers to those of the trained Harman listeners (slide 27), we see good agreement between the two groups. The one exception is Loudspeaker C, which the trained listeners strongly disliked. The general agreement between trained and untrained listener loudspeaker preferences illustrated in this test is consistent with previous studies where a different set of listeners and loudspeakers were used [5],[6]. As found in the previous study, the trained listeners, on average, rated each loudspeaker about 1.5 preference rating lower than the untrained listeners, and the trained listeners were more discriminating and consistent in their ratings[5],[7].

The comprehensive set of anechoic measurements for each loudspeaker is compared to its preference rating (slide 28). There are clear visual correlations between the set of technical measurements and listeners’ loudspeaker preference ratings. The most preferred loudspeaker (Infinity Primus 362) had the flattest measured on-axis and listening window curves (top two curves), and the smoothest first reflection, sound power and first reflection/sound power directivity index curves (the third, fourth, fifth and sixth curves from the top). The other loudspeaker models tended to deviate from this ideal linear behavior, which resulted in lower preference ratings. Again, this relationship between loudspeaker preference and a linear frequency response is consistent with similar studies conducted by the author and Toole [9],[10].

Finally, sound quality doesn't necessarily cost more money to obtain as illustrated in these experiments. The most accurate and preferred loudspeaker - the Infinity Primus 362 - was also the least expensive loudspeaker in the group at $500 a pair. It doesn't cost any more money to make a loudspeaker sound good, as it costs to make it sound bad. In fact, the least accurate loudspeaker (Loudspeaker C) cost almost 8x more money ($3,800) than the most accurate and preferred model. Sound quality can be achieved by paying close attention to the variables that scientific research says matter, and then applying good engineering design to optimize those variables at every product price point.

Conclusions

A group of 18 high school students participated in two double-blind listening tests that measured their sound quality preferences for music reproduced in lossy (MP3 @ 128 kbps) and lossless (CD quality) formats, as well as music reproduced through loudspeakers that varied in accuracy. In both tests, the high school students preferred the most accurate option, preferring CD over MP3, and the most accurate loudspeaker over the less accurate options.

While this study is still in its early phase, these preliminary results suggest that these teenagers can reliably discriminate among different degradations in sound quality in music reproduction. When given the opportunity to hear and compare different qualities of sound reproduction, the high school students preferred the higher quality, more accurate reproduction over the lower quality choices.

The audio industry should not discount the potential opportunities to provide a higher quality audio experience to members of Generation Y. The popular belief that they don’t care about or appreciate sound quality needs to be critically reexamined. This data suggests there are opportunities to sell good sounding audio products to Generation Y as long as the products hit the right features and price points,. The audio industry should also provide these consumers the necessary education and information (i.e. meaningful performance specifications) to identify the good sounding products from the duds. Science can already do this (review slide 28), it’s simply a matter of making the information more widely available.

References

[1] Joseph Plambeck, “In Mobile Age, Sound Quality Steps Back,” New York Times, May 9, 2010.

[2] Andrew Edgecliffe-Johnson, “Could a Pair of Headphones Save the Music Business?” Financial Times, June 12 2010.

[3] Robert Capps, “The Good Enough Revolution: When Cheap and Simple Is Just Fine” Wired Magazine, August 24, 2009.

[4] Dale Dougherty, “The Sizzling Sound of Music,” O’Reilly Radar, March 1 2009.

[5] Nora Young, Full Interview: Jonathan Berger on mp3s and “Sizzle”, CBC Radio , March 24, 2009.

[6] The Loudness Wars: Why Music Sounds Worse, from All Things Considered, NPR Music, December 31, 2009.

[5] Sean E. Olive, "Differences in Performance and Preference of Trained Versus Untrained Listeners in Loudspeaker Tests: A Case Study," J. AES, Vol. 51, issue 9, pp. 806-825, September 2003. (download for free courtesy of Harman International).

[6] Sean Olive, “Part 1 - Do Untrained Listeners Prefer the Same Loudspeakers as Untrained Listeners?” Audio Musings, December 26, 2008.

[7] Sean Olive, Part 2 - Differences in Performance of Trained Versus Untrained Listeners, Audio Musings, December 27, 2008.

[8] Sean Olive, “Part 3 - Relationship between Loudspeaker Measurements and Listener Preferences”, Audio Musings, December 28, 2008.

[9] Floyd E. Toole, "Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 1" J. AES Vol. 23, issue 4, pp. 227-235, April 1986. (download for free courtesy of Harman International).

[10] Floyd E. Toole, "Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 2," J. AES, Vol. 34, Issue 5, pp. 323-248, May 1986. (download for free courtesy of Harman International).

Evaluating the Sound Quality of Ipod Music Stations: Part 3 Measurements

2010-05-01T13:34:00.000-07:00

In Part 3 of this article, the acoustical measurements of three popular Ipod Music Stations (Harman Kardon MS100, Bose SoundDock 10 and Bowers & Wilkins Zeppelin) are examined to see if they corroborate listeners’ sound quality ratings of the products based on controlled double-blind listening tests. Part 2 summarized the results of those listening tests, and Part 1 described the listening test methodology used for this research.

Throughout this article, I will refer to some slides of a presentation that can be downloaded as a PDF or viewed as a YouTube video.

Mono or Stereo Acoustical Measurements?

There is a substantial body of scientific research on the subjective and objective testing of conventional stereo loudspeakers [1]-[5]. Unfortunately, the same is not true for Ipod Music Stations: this raises several research questions about how they should be evaluated and measured.

The first important question is whether the acoustical measurements should be done in mono or stereo. Due to the proximity of the left and right channel transducer arrays in Music Stations, there is the potential for constructive and destructive interference when both channels are active that will vary according to frequency and the relative inter-channel levels and phases of the music signals. To study this phenomena, the left and right channels were measured and analyzed as both single and combined channels. Generally, we found very little difference in the frequency responses (magnitude and phase) of the left and right channels. Combining the two channels only led to the expected 6 dB increase in sound pressure level (SPL).

Anechoic Measurements of the Music Stations

Each Music Station was measured at distance of 2 meters in the large anechoic chamber at Harman International. The chamber is anechoic down to 60 Hz and this is extended to 20 Hz through a calibration procedure. Each Music Station was subjected to the same battery of measurements used for designing and testing Revel, Infinity and JBL home loudspeakers. A total of 70 frequency response measurements were taken at 10 degree increments in both horizontal and vertical orbits (slide 4). These measurements were then spatially averaged and weighted to characterize the direct, early and late reflected sounds in a typical listening room, in addition to the calculated directivity indices (slides 5-8).

The family of measurement curves (slide 9) reveal significant differences among the three Music Stations in terms of their smoothness and low frequency extension below 70 Hz.

Music Station A has the smoothest frequency response across the family of curves, which corroborates listeners’ comments about its neutral sound and absence of colorations (see slide 11 of Part 2). There is also physical evidence in the measurements that explain listener comments about Music Station A sounding a bit bright and thin, due to a combination of the upward spectral tilt in its listening window curve, and its higher low frequency cutoff.

Music Station B has even more peaks and dips in the curves that contribute to the higher frequency of listener comments regarding audible coloration. Particularly problematic is the large broad resonance at 500 Hz that is visible in both the direct and reflected sounds produced by the product. However, there is nothing in the measurements to explain listeners’ complaints about its boomy bass.

Music Station C clearly has the least tidy set of measurement curves with a significant hole centered at 2 kHz in the on-axis curve. There are visible resonances in the measurements that elicited frequent listener comments about “midrange unevenness” and “coloration.” Finally, the sound power response and directivity indices reveal that this Music Station becomes increasingly directional at higher frequencies compared to its competitors. This could contribute to coloration and dullness at off-axis listening positions and at further listening distances.

Relationship between Anechoic Measurements and Listener Preference

The anechoic measurements of the Music Stations are shown again in Slide 10 along with the listener preference ratings. From this, we see that the overall smoothness of the family of curves appeared to be important underlying factor that influenced listeners’ Music Station preference ratings.

Correlations Between Anechoic Measurements and Perceived Spectral Balance: The Direct Sound Influences the Perceived Spectral Balance Above 300 Hz

There has been a 30+ year debate in the audio community regarding which set of acoustical measurements best predict the loudspeaker’s perceived sound quality in a typical listening room. There are several different camps that include the direct sound response advocates, the sound power response advocates, the in-room measurement advocates, and others, like myself, who argue that you need a combination of all of the above measurements to accurately predict how the loudspeakers will sound in a room.

One way to tackle this debate is to study the correlation between different loudspeaker measurements and listeners’ perceived spectral balance of the loudspeakers in a room. Slide 11 shows the perceived spectral balance ratings of the Music Stations versus the family of anechoic curves that include the listening window (direct sound), first reflections and sound power response.

For Music Station A, there is good agreement between the perceived spectral balance and the listening window curve, which represents the direct sound over a ± 30 degree horizontal angle. For Music Station B, there is generally poor agreement: listeners complained about boomy bass, yet there is nothing in these measurements to suggest why. There is clearly some information missing in the anechoic measurements and/or perhaps the subjective ratings are faulty. We will come back to this topic later.

For Music Station C, there is good agreement between the perceived spectral balance and the listening window curve (direct sound), with indications that the resonances centered at 1.5 and 3.5 kHz were heard and registered by the listeners.

In summary, it seems that for at least two of the Music Stations, the perceived spectral balance can be approximated by looking at the listening window curves that represent the direct sound. However, there is information missing in the anechoic measurements that don’t explain perceptual effects below 300 Hz.

In-Room Measurements of the Music Stations

Below about 300 Hz, the room acoustics and the Music Station/listener positions can have a significant influence on the perceived quality of reproduced sound. Yet, these physical effects are not captured in the anechoic measurements described in the previous section. To further examine these effects, steady-state frequency response measurements of the Music Stations were taken at the primary listening seat at 6 different microphone positions, and then spatially averaged to remove highly localized acoustical interference effects (slide 12). The 1/6-octave smoothed curves for each Music Station are shown in slide 13. Below 200 Hz, there is evidence of room resonances (high Q peaks and dips) and boundary effects that were absent in the previous anechoic measurements (slide 9). Music Station A had less apparent boundary gain than the other two products, probably because the boundary effect was accounted for in its design.

Correlation Between In-Room Measurements and Perceived Spectral Balance: The Influence of Room and Boundary Effects Below 300 Hz

The in-room measurements are plotted in slide 13 along with listeners’ perceived spectral balance ratings. Here, the in-room measurements have been super-smoothed (1-octave) to better correspond to the frequency resolution of the subjective ratings.

Below 300 Hz, there is better agreement between the in-room measurements and listeners’ spectral ratings than observed using the anechoic measurements (slide 11). However, above 300 Hz, there is generally better agreement between the anechoic measurement and spectral ratings, particularly using the listening window curve that represents the direct sound. This confirms the important role that the direct sound plays in our perception of reproduced sound. Below 300 Hz, the room’s standing waves and boundary effects play a dominant role in the quality and quantity of bass we hear. Previous studies [5] have shown bass quality accounts for 30% of listener preference, and cannot be ignored.

Dynamic Compression Measurements

Our scientific understanding of the perception and measurement of nonlinear distortions in loudspeakers is still quite poor. There are currently no standard loudspeaker measurements that adequately capture the perceptual significance of dynamic compression and the associated distortions it produces. This is an area of audio that is in need of more research.

Listeners reported that Music Station A had fewer audible nonlinear distortions than the other two Music Stations. However, it was not clear if the distortions were real or due to a cognitive bias known as the “Halo effect.” Examining the objective distortion measurements will hopefully clarify what is real and not real.

The dynamic linearity of the Music Stations was tested by measuring their anechoic frequency response at different playback SPL’s from 76 to 100 dB SPL (@ 1 meter distance) in 6 dB increments. A relatively short length 4 s log sweep was used as a test signal to minimize the thermal effects on the transducers. Consequently, the measured dynamic compressions shown below were largely related to the behavior of the electronic limiters in the Music Stations, designed to prevent the amplifier clipping, which could otherwise potentially damage the transducers.

Slide 16 shows the dynamic compression for each Music Station. The frequency response measured at 82, 88, 94 and 100 dB SPL’s have been normalized to the 76 dB measurement. Any dynamic compression effects would be exhibited as a deviation from 0 dB. In examining these graphs, Music Station A produced 6 dB more output (100 dB @ 1 meter) than the other Music Stations without significant compression effects.

On the surface, the relationship between these measurements and listeners’ distortion ratings seems to be straightforward: the Music Stations with the higher amounts of compression received lower distortion ratings (slide 17). However, the SPL’s at which the compression effects occurred (> 94 dB) were higher than those used in the listening test.

Harmonic Distortion Measurements

Harmonic distortion (second and third harmonic only) measurements were made in the anechoic chamber at a SPL of 95 dB. The distortion levels of the harmonics are plotted along with the fundamental for each of the Music Stations in slide 18. Note that the levels of the harmonics have been raised 20 dB for the sake of clarity.

All of the Music Stations exhibited relatively high distortion at low frequencies below 100 Hz, with generally less harmonic distortion at higher frequencies. Music Station B differentiated itself by having higher levels of second and third harmonic distortion between 100 Hz to 1 kHz. Music Station C had the lowest distortion even though it received the lowest preference and distortion ratings from the listeners.

In conclusion, the harmonic distortion measurements of the Music Stations are not particularly good at predicting listeners’ distortion ratings, or overall preference in sound quality. This confirms many previous loudspeaker studies that have reported that harmonic distortion measurements are poor predictors of listeners’ overall impression of the loudspeaker. This can be explained by the fact that the distortions are often below the threshold of audibility, and the measurements themselves do not account for the masking properties of human hearing.

Conclusions

This article has shown evidence that a combination of comprehensive anechoic and in-room measurements can help explain listeners’ preferences and spectral balance ratings of the Music Stations evaluated in controlled listening tests.

Above 300 Hz, the anechoic derived listening window curve correlated well with listeners’ spectral balance ratings, whereas the in-room measurements better explained the Music Station’s acoustical interactions with the room below 300 Hz. In these particular tests, the overall smoothness of the on and off-axis frequency response curves provided the best overall indicator of listeners’ preferences and their comments.

Dynamic compression measurements revealed significant differences among the Music Stations in terms of their linear SPL output capability. The most preferred Music Station could play 6 dB louder (100 dB SPL @ 1 meter) than the other units without exhibiting significant dynamic compression. It is unlikely that this was a factor in the listening tests since the units were evaluated at a comfortable average level of 78 dB (B-weighted, slow). Finally, distortion measurements revealed some differences among the products but had no clear correlation with listeners’ sound quality ratings. This highlights the need for further research into the perception and measurement of nonlinear distortion in loudspeakers so that loudspeaker engineers can optimize their designs using psychoacoustic criteria.

References

[2] Floyd E. Toole, "Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 2," J. AES, Vol. 34, Issue 5, pp. 323-248, May 1986. (download for free courtesy of Harman International).

[3] W. Klippel, "Multidimensional Relationship between Subjective Listening Impression and Objective Loudspeaker Parameters", Acustica 70, Heft 1, S. 45 - 54, (1990).

[4] Sean E. Olive, “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part I - Listening Test Results,” presented at the 116th AES Convention, preprint 6113 (May 2004).

[5] Sean E. Olive, “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part 2 - Development of the Model,” presented at the 117th AES Convention, preprint 6190 (October 2004).

Evaluating the Sound Quality of Ipod Music Stations: Part 2 Listening Tests

2010-04-10T14:39:00.000-07:00

Part 1 of this article described a listening test method used at Harman International for evaluating the sound quality of Ipod Music Docking Stations. In part 2, I present the results of a recent competitive benchmarking listening test where three popular Music Stations of comparable price were evaluated by a panel of trained listeners. Were listeners able to reliably formulate a preference among the different Ipod Music Stations using this test method? And what were the underlying sound quality attributes that explain these preferences? Read on to find out.

Throughout this article, I will refer to slides in an accompanying PDF presentation, or you can watch a YouTube video of the presentation.

The Products Tested
A listening test was performed on three Ipod Music Stations that retail for the same approximate price of $599: the Harman Kardon MS 100, the Bose SoundDock 10, and the Bowers & Wilkins Zeppelin (see slide 2). All three products provide Ipod docking playback capability and an auxiliary input for external sources such as CD player,etc. The latter was used in these tests to reproduce a CD-quality stereo test signal fed from a digital sound source.

Listening Test Method
The Music Stations were evaluated in the Harman International Reference Listening Room (slide 4) described in detail in a previous blog posting. Each Music Station was positioned on a shelf attached to the Harman automated in-wall speaker mover, which provides the means for rapid multiple comparisons among three products designed to be used in, on, or near a wall boundary. The music stations were level-matched within 0.1 dB at the listening position by playing pink noise through each unit and adjusting the acoustic output level to produce the same loudness measured via the CRC stereo loudness meter .

All tests were performed double-blind with the identities of the products hidden via an acoustically transparent, but visually opaque screen. The listening panel consisted of 7 trained listeners with normal audiometric hearing. Each listener sat in the same seat situated on-axis to the Music Stations positioned at seated ear height, approximately 11 feet away (slide 5).

The Music Stations were evaluated using a multiple comparison (A/B/C) protocol whereby listeners could switch at will between the three products before entering their final comments and ratings based on overall preference, distortion, and spectral balance. This was repeated using four different stereo music programs with one repeat (4 programs x 2 observations = 8 trials). In total, each listener provided 216 ratings, in addition to their comments. The typical length of the test was between 30-40 minutes. The presentation order of the music programs and Music Stations were randomized by the Harman Listening Test software to minimize any order-related biases in the results.

Results: Overall Preference Ratings For the Music Stations
A repeated measures analysis of variance was used to statistically establish the effects and interactions between the independent variables and the different sound quality ratings. The main effect was related to the Music Stations with no significant effects or interactions observed between the program material and Music Stations. Note that in the following discussion, the brands/models of the Music Stations have removed from the results since this information is not relevant to the primary purpose of the research and this article. Instead, the Music Station products have been assigned the letters A,B and C in descending order according to their mean overall preference rating.

The mean preference ratings and upper 95% confidence intervals based on the 7 listeners are plotted in slide 7. Music Station A received a preference rating of 6.8, and was strongly preferred over the Music Stations B (4.58) and C (4.08).

Individual Listener Preference
The individual listener preference ratings and upper 95% confidence intervals are plotted in slide 8. The intra and inter listener reliability in ratings were generally quite high. All seven listeners rated Music Station A higher than the other two products, although some listeners, notably 55 and 64, were less discriminating and reliable than other the listeners. Both these listeners had significantly less training and experience than the other listeners, which has been demonstrated in previous studies to be an important factor in listener performance.

Distortion Ratings
Nonlinear distortion includes audible buzzes, rattles, noise and other level-dependent distortions related to the performance of the electronics, transducers, and mechanical integrity of the product’s enclosure. In these tests, the average playback level was held constant (78 dB(B) slow), and listeners could not adjust it up or down. Under these test conditions, some listeners still felt there were audible differences in distortion (slide 9) with Music Station A (distortion rating = 7.19) having less distortion than Music Stations B (5.5) and C (4.94).

Some of these differences in subjective distortion ratings could be related to a “Halo Effect," a scaling bias wherein listeners tend to rate the distortion of loudspeakers according to their overall preference ratings - even when the distortion is not audible. An example of “Halo Effect” bias has been noted in a previous loudspeaker study by the author [1]. Reliable and accurate quantification of nonlinear distortion in perceptually meaningful terms remains problematic until better subjective and objective measurements are developed.

Spectral Balance Ratings
Listeners rated the spectral balance of each Music Station across seven equally log-spaced frequency bands using a ± 5-point scale. A rating of 0 indicates an ideal spectral balance, positive numbers indicate too much emphasis within the frequency band, and negative numbers indicate a deemphasis within the frequency band. Rating the spectral balance of an audio component is a highly specialized task that requires skill and practice acquired through using Harman’s “How to Listen” listener training software application. In a previous study [1], it has been shown that spectral balance ratings are closely related to the measured anechoic listening window of the loudspeaker, although may vary with changes in the directivity and the ratio of direct/reflected sound at the listening location.

The mean spectral balance ratings averaged across all programs and listeners are plotted in slide 10. Listeners felt Music Station A had the flattest or most ideal spectral balance, with the exception of a need for more upper/lower bass, and less emphasis in the upper treble. Music Station B was judged to have too much emphasis in the upper bass (88 Hz), and too little emphasis in the upper midrange/treble. Music Station C was rated to have a slight overemphasis in the upper bass, and a very uneven balance throughout the midrange with a peak centered around 1700 Hz.

Listener Comments
Listeners provided comments that described the audible difference among three Music Stations. The frequency or number of times a specific comment was used to describe each product is summarized in slide 11. The correlation between the product’s preference rating and each descriptor is indicated by correlation coefficient (r) shown in the bottom row of the table. The same table data shown in slide 11 are plotted in graphical form in slide 12.

The most common three descriptors applied to the Music Station A were neutral (16), bright (9), and thin (9). These descriptors generally confirm the perceived mean spectral balance ratings summarized in slide 10.

The three most frequent descriptors applied to Music Station B were colored (13), boomy bass (10), and uneven mids(6). The “boomy bass” is clearly suggested in spectral balance ratings (see the large 88 Hz peak) in slide 10.

The three most frequent descriptors used to describe the sound quality of Music Station C were colored (19), uneven mids (9), and harsh (6). All three descriptors have a high negative correlation with the overall preference rating, and may explain the low preference rating this product received. The coloration and unevenness of the midrange are confirmed in the spectral balance rating in slide 10. The harshness is most likely related to the perceived spectral peak perceived around 1700 Hz.

Conclusions
This article summarized the results of a controlled, double-blind listening test performed on three comparatively priced Ipod Music Stations using seven trained listeners with normal hearing. The results provide evidence that the sound quality of Music Station A was strongly preferred over Music Stations B and C. There was strong consensus among all seven listeners who rated Music Station A highest overall. The Music Station preference ratings can be largely explained by examining the perceived spectral balance ratings of the products, which are in turn closely related to listener comments on the sound quality of the products.

The most preferred product, Music Station A, was perceived to have the flattest, most ideal spectral balance, and solicited frequent comments to its neutral sound quality. As the spectral balance ratings deviated from flat or ideal, the products received frequent comments related to coloration, boomy bass, and uneven midrange. While the distortion ratings were highly correlated with preference, more investigation is needed to determine the extent to which the distortion ratings are related to a possible scaling bias known as the “halo effect."

In part 3 of this article, I will present the objective measurements of these products - both anechoic and in-room acoustical measurements - to see if they can reliably predict the subjective ratings of the products reported here.

References
[1] Sean E. Olive, “ A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part I - Listening Test Results,” presented at the 116th AES Convention, preprint 6113 (May 2004).

A Method For Training Listeners and Selecting Program For Listening Tests

2010-03-11T10:40:00.000-08:00

The benefits of training listeners for subjective evaluation of reproduced sound are well documented [1]-[3]. Not only do trained listeners produce more discriminating and reliable sound quality ratings than untrained listeners, but they can report what they perceive in very precise, quantitative and meaningful terms.

One of the unexpected byproducts of listener training is that it identifies which music selections are most sensitive to distortions commonly found within the audio chain [4]. This is exactly what was found in a series of listener training experiments the author reported in a 1994 paper entitled, “A method for training listeners and selecting program material for listening tests.” The following sections summarize the findings of those early experiments, which helped establish an objective method for training and selecting listeners and program material used for listening tests at Harman International over the past 16 years. A slide presentation summarizing the paper can be downloaded here, and will be referred to throughout the following sections.

Matching the Sound of Spectral Distortions to Their Frequency Response Curve

A computer-based training task was designed where listeners were required to compare different spectral distortions added to programs and then match the frequency response curve of the filter that generated the distortion (see slides 4-5). This was repeated using eight different equalizations and twenty different music selections digitally edited into short 10-20 s loops.

The equalizations included ±3 dB shelving filters at low (100 Hz) and high (5 kHz) frequencies, and ±3 dB resonances (Q = 0.66) centered at 500 Hz and 2 kHz (slide 6). An equalized version of the program (Flat) was always provided as a reference. The twenty music selections include classical, jazz and pop/rock genres with instrumentations that varied from solo instruments, speech and small combos to rock/combos and orchestras (slide 7). Pink noise was also included since this continuous broadband signal has been found to produce the lowest detection thresholds of resonances in loudspeakers [5],[6].

Eight untrained listeners with normal hearing participated in the training exercises, which were conducted over five separate listening sessions consisting of 32 trials each (slides 8 and 9). The presentation order of the equalizations, trials, and programs were randomized to prevent any order related biases. The listener’s performance was based on the percentage of correct responses given over the course of the five training sessions.

The Results

The training results were statistically analyzed using a repeated measures analysis of variance (ANOVA) to determine the effect the different music programs, equalizations, and trials had on the listeners’ performance in correctly identifying the different equalizations (slide 11).

Listener Performance Is Strongly Influenced by Program Selection

The single largest effect on the listener’s performance was the program selection. Slide 13 plots the mean listener performance scores for each of the twenty programs averaged across all eight equalizations. The percentage of correct responses ranged from a high of 88% (pink noise) to a low of 54% (jazz piano trio). Listeners performed the task best when using broadband, spectrally dense continuous signals like pink noise or pop/rock selections like Tracy Chapman, Little Feat, and Jennifer Warnes. Listeners performed worse on programs featuring solo instruments, small combos and speech that produced more discontinuous and narrow band signals. More about this later.

Equalization Context Influences Listener Performance

The effect of equalization on listener performance was surprisingly small (slide 14). There was a tendency for listeners to correctly identify the spectral distortions that occurred at low and high frequency regions versus the midband equalizations. The explanation for this can be found by examining the interaction effect between equalization * trial, indicating that listener performance depended on which combinations of equalizations were presented within a trial. In other words, the context in which an equalization was presented influenced listener performance (slide 15). These contextual effects can be summarized as follows:

Listeners gave more correct responses when the presented equalizations were more separated in frequency.

Listeners gave more correct responses when presented spectral boosts versus notches; spectral notches were often confused with spectral peaks located at slightly higher frequencies.

Low frequency boosts were often confused with high frequency cuts (and vice versa).

Low frequency cuts were often confused with high frequency boosts (and vice versa)

Greater frequency separation between different equalizations would produce more distinctive tonal or timbral differences that would help improve identification. The second observation confirms previous research that has found spectral notches are more difficult to detect than spectral peaks of similar bandwidth [5]. The one exception is broadband dips, which have similar detection thresholds as resonance peaks with equivalent bandwidth[6]. Observations c) and d) are related to each other, and are more difficult to explain. On first glance, it seems implausible that boosts and cuts separated five octaves apart can be confused with one another. A possible explanation is that listeners are using information across the entire bandwidth to judge the perceived perceive balance of the bass and treble. In this case, the slope or shape of the spectra must be an important factor (slide 16). Since a boost or cut of similar magnitude at opposite ends of the audio bandwidth produce similar broadband shapes or slopes, this might explain why listeners might confuse the two with each other.

Program and EQ Interact to Influence Listener Performance

There was also a significant interaction between program and equalization that affected listener performance. This interaction effect was most apparent for the programs presented in training session 3 where listener performance varied significantly depending on the combination of programs and equalization presented to the listener (slide 18). It seems plausible that these differences were related to differences in the spectra of the programs, which was confirmed by plotting the average 1/3-octave spectra of the four programs (slide 19). The largest listener response errors tended to occur when the equalization fell in a frequency range where there was little spectral energy in the programs (e.g. Programs P10 (Stan Getz) and P19 (Canadian Brass)). It makes sense that listeners cannot easily analyze the spectral distortions if the program material does not contain signals that make them audible.

Not All Listeners Are Equal to the Task

No amount of training will make me eligible for the Canadian Olympic hockey team - even if I were 25 years younger. Some people simply lack the innate mental and physical raw material to perform a highly specialized task. This is also true for critical listening as illustrated by the average performance scores of eight listeners after 5 listening sessions (slide 20). The range of individual listener performances range from 82% (listener 4) to 31% (listener 3). All listeners had normal hearing. Therefore, the reason for this large inter-listener variance in performance is related to other factors such as listener motivation, attentiveness, and their listening (and general) intelligence. Training data such as this, can provide an objective quantifiable metric for selecting the best listeners for audio product evaluations.

Practice Makes Perfect

The success of any listener training task that it can lead to measurable improvement in performance with repetition. Slide 21 show shows listener performance measured over five training sessions based on the eight listeners tested. The graph shows monotonic improvement in performance from 65% correct responses to 80% after five training sessions. Additional training sessions would most likely realize further gains in performance for some subjects. In other words, the training works!

Programs With Wider and Flatter Spectrums Improve Listener Performance (Why Tracy Chapman is as Good as Pink Noise)

Spectrum analysis was performed on the different program selections to see if this could explain the strong effect of program on listener performance. The 1/3-octave spectrum of each program was plotted based on a long-term average taken over the entire length of the loop. When we looked at the spectrums of the programs it became clear that this was a significant predictor of how well listeners would perform their task.

Slide 22 plots the average spectrum of four groups of program (5 programs in each group) rank ordered (from highest to lowest) according to the listener performance scores they produced. It clearly shows that the programs with the flattest and most extended spectrums (e.g. pink noise, pop/rock, full orchestra) were better suited for identifying spectral distortions. After pink noise, Tracy Chapman (program 2 in the above graph) had among the widest and flattest spectrums measured, and along with pink noise (program 1) registered the highest listener performance scores. Programs that had narrow band spectra with limited energy above and below 500 Hz (speech, solo instruments, small jazz and classical ensembles) concentrated in group 4 were less suited for identifying spectral distortions. While these groupings had some of the most musically entertaining selections, in the end, they were not good signals for detecting and characterizing spectral distortions in audio components.

Conclusions

A listener training method has been described that teaches listeners how to identify spectral distortions according to their frequency response curve. Experimental evidence was shown indicating listeners improved their performance in this task after 5 training sessions, although not all listeners are equal in their performance.

Statistical analysis of the training data revealed that the program selections are the largest factor influencing listener performance in this task: programs with continuous broadband spectra (e.g. pink noise, Tracy Chapman,etc) provide the best signals for characterizing spectral distortions whereas programs with narrow band spectra (e.g. speech, solo instruments) provide poor signals for performing this task. Furthermore, listeners seem to confuse certain types of spectral distortions with others when the distortions presented share similarities in their frequency, bandwidth, and broadband spectral slope or shape.

Finally, it is important to remember that the training methods and programs discussed in this study focussed on perception and analysis of spectral distortions. While these types of distortions are the most dominant ones found in loudspeakers, microphones and listening rooms, there are other types of distortions for which a different set of programs are likely better suited for revealing their audibility and subjective analysis. The current Harman listener training software “How to Listen” includes training tasks on spectral distortion as well as spatial, dynamic and various types of nonlinear distortions for which we hope to discover the optimal programs for detecting and analyzing their audibility. Stay tuned.

References

Olive, Sean E., "Differences in Performance and Preference of Trained Versus Untrained Listeners in Loudspeaker Tests: A Case Study,” J. Audio Eng. Soc. Vol. 51, issue 9, pp. 806-825, September 2003. Download for free here, courtesy of Harman International.

Bech, Soren, “Selection and Training of Subjective for Listening Tests on Sound-Reproducing Equipment,” J. Audio Eng. Soc., vol. 40 no. 7/8 pp. 590-610 (July 1992).

Toole Floyd E. "Subjective Measurements of Loudspeakers Sound Quality and Listener Performance," J. Audio Eng. Soc., vol. 33, pp. 2-32 (1985 Jan./Feb.).

Olive, Sean E., “A Method for Training Listeners and Selecting Program Material for Listening Tests” presented at the 97th AES Convention, preprint 3893, (November 1994).

Toole, Floyd E. and Sean E. Olive, “The Modification of Timbre by Resonances: Perception and Measurement,” J. Audio Eng. Soc., Vol. 36, pp. 122-142 (March 1998).

Olive, Sean E.; Schuck, Peter L.; Ryan, James G.; Sally, Sharon L.; Bonneville, Marc E. “The Detection Thresholds of Resonances at Low Frequencies,” J. Audio Eng. Soc., Vol. 45, Issue 3, pp. 116-128 (March 1997).

Olive Sean E., “Harman’s How to Listen - A New Computer-based Listener Training Program,” May 30,2009.

Evaluating the Sound Quality of Ipod Music Stations: Part 1

2010-02-05T14:34:00.000-08:00

For many consumers, an iPod Music Docking Station may be the primary audio device through which they experience most of their recorded music and infotainment. These ubiquitous devices offer a convenient, low cost, portable and easy-to-use solution for enjoying an Ipod through loudspeakers -- but what about their sound quality? What sonic compromises are made in order to achieve this level of convenience and portability? Do certain models or brands of Ipod Music Stations offer better sound than others, and if so, how can consumers identify which ones they are? These are legitimate questions that consumers should be asking when purchasing an Ipod Music Station. Unfortunately, the answers are not readily found.

Choosing an Ipod Music Station based on sonic performance quality is a daunting task for consumers. There are dozens of models to choose from that vary in price from $80 to as high as $3000 for a model designed by Ferrari. Competent in-store demonstrations and reviews of these products are difficult to find, and the technical specifications on the packaging provide no clear indication of how good they sound. For traditional loudspeakers, it is already possible to quantify their sound quality, but the audio industry continues to withhold this information from consumers. Without meaningful performance specifications in place, consumers cannot make sound purchase decisions, nor can manufacturers be easily held accountable for delivering products that sound “ not good enough.”

This article describes a listening test method used at Harman International for evaluating the sound quality of Harman and competitors’ Ipod Music Stations. The goal is to provide subjective ratings of Ipod Music Stations that are accurate, reliable and scientifically valid. From this data, a set of technical performance specifications can be developed that quantify how good the products sound.

Designing Listening Tests For Ipod Music Stations

Fortunately, there already exists a large body of scientific knowledge on how to design accurate, reliable and valid listening tests on loudspeakers. A key ingredient is careful control of listening test nuisance variables: these are psychological, electro-acoustical and experimental factors not directly related to the product(s) under test but nonetheless influence and bias the results (click on the figure below). Some of the more significant nuisance variable controls that should be in place but often are ignored by audio manufacturers and reviewers are:

Double-blind conditions (this removes the effects of sighted biases related to brand, price,etc)
Trained listeners with normal hearing (trained listeners are up to 20 times more discriminating and reliable than untrained listeners, yet their overall sound quality preferences are similar to those of untrained listeners)
Quiet listening room with acoustics that are representative of average homes (important for hearing low level sounds and the quality of the loudspeaker's off-axis radiated sounds)
Loudness matching between products (the perception of timbre, spatial and dynamic attributes are level dependent)
Selection of well-recorded music selections that are revealing of sound quality differences
Multiple comparisons among products which are more discriminating and reliable compared to single stimulus presentations

These important nuisance variable controls are essential for obtaining accurate, reliable and valid sound quality ratings of Ipod Music Stations.

Including the Acoustical Effects of the Wall and Desktop in the Listening Test

If audio products are not tested under similar conditions for which they were designed and intended to be used, the ecological validity (as well as the external validity) of the test may be compromised: in other words, the test results will be of little value or relevance to how the product is typically used in the real world.

Most Ipod Music Stations are intended to be placed on a desktop surface or bookshelf located near a wall, which will cause acoustical reinforcement and cancellation at certain audio frequencies. Below 500 Hz, there will be a gradual increase in sound pressure level that unless compensated for in the design of the product can make vocals and bass instruments sound tubby and boomy. Diffraction effects or reflections from the desktop/bookshelf may also produce audible effects that should be included in the listening test. For these reasons, listening tests on Ipod Music Stations are best done on a desktop/wall boundary.

A Video On How We Evaluate the Sound Quality of Ipod Docking Stations

The video shown at the top of the page illustrates how Ipod Music Stations are currently evaluated in the Harman International Reference Listening Room. The acoustical properties and features of the room have been described in detail in a previous posting.

In the video you see a trained listener comparing three different Ipod Music Stations situated on our automated in-wall speaker mover configured with a removable shelf and desktop. An acoustically transparent, visually opaque screen is placed between the listener and the products under test, so that the test is double-blind (note: the term double-blind implies that neither the listener nor the experimenter know the identities of the products currently selected since the computer controls and randomly assigns the letters A/B/C to the products in each trial.)

The listener can switch between the different products at will and enter their responses via a wireless PDA equipped a custom listening test software (LTS) client application. Sound quality ratings are given on a number of different pre-defined scales that include preference, spectral balance, distortion, auditory image size.This is repeated twice using four different programs.

The PDA client communicates with the LTS server application that performs the following functions:

A test wizard that defines of all experimental design and setup parameters (perceptual scales, presentation of stimuli, program, randomization of test objects, playback level,etc), which are then stored in a database
automation and administration of the listening test and its hardware (e.g. speaker mover, media player, DSP, audio switcher)
collection, storage and statistical analysis of listening test data
real-time monitoring of listener’s performance and ratings during the test

LTS makes conducting listening tests an efficient and repeatable process by minimizing human interaction and errors in the listening test setup, storage, and analysis of the results.

Conclusions

This article has described a listening test method used for evaluating Ipod Music Stations with the goal to provide accurate, reliable and valid sound quality ratings. In Part 2, I will show some results from a recent listening test conducted on different Ipod Music Stations, followed by some different acoustical measurements of the products in Part 3. By studying the relationship between well-controlled scientific listening tests and comprehensive acoustical measurements of Ipod Music Stations, a meaningful technical specification based on sound quality can be found.

The Effect of Whole-body Vibrations on Preferred Bass Equalizations of Automotive Audio Systems

2009-11-22T12:33:00.001-08:00

Binaural Room Scanning (BRS) is a technology that allows Harman scientists to binaurally capture, store, and later reproduce sound fields through a headphone-based auditory display that includes head tracking for accurate localization of sound sources [1]. BRS enables us to do controlled, double blind comparative evaluations of different automotive audio systems, home theatre systems or sound reinforcement systems that would otherwise not be practical or possible to do. However, current BRS systems do not typically capture and reproduce the whole-body vibrations that are associated with low frequencies reproduced by the audio system. Therefore, an important question related to the accuracy and ecological validity of BRS-based evaluations is whether whole-body vibrations play an important role in our perception of the quality and realism of the automotive audio system.

We recently presented a paper at the 127th Audio Engineering Society Convention that addressed this research question [2]. Three experiments were reported that measured the effects of both real and simulated whole-body vibration associated with the low frequency sounds reproduced by an automotive audio system on listeners’ preferred bass equalizations. A PDF of the side presentation can be found here.

In all three experiments, the same automotive audio system was used: a high-quality 17-channel audio system installed in a 2004 Toyota Avalon. The car was parked in our automotive audio research lab with the engine turned off so that the effects of the road and engine noise and vibration were not part of the experiment. The BRS system was calibrated for each individual listener to minimize errors related to headphone fit, etc. The differences in magnitude response measured at the listeners’ ear in situ versus through the BRS system were very small indeed (see slide 9).

The Effect of Real Vibration on Preferred Bass Levels

In the first experiment, listeners adjusted the level of bass equalization (see slide 10) while experiencing real whole-body vibration produced by the car audio system. The same task was also repeated via the BRS headphone-based system without the vibration present. This was repeated three times using three different music programs (see slide 7). On average, listeners adjusted the bass equalization 1.5 dB higher for the BRS playback condition where the low frequency vibration was not present (see slide 11). The preferred bass level was found to be program dependent due to the amount of bass present in the signal, and the resulting vibration it produced(see slides 12 and 13).

The Effect of Simulated Vibration on Preferred Bass Levels

In the second experiment, we simulated the whole-body vibration produced by the audio system by attaching an actuator to the driver's seat of the car. The actuator was driven by the low frequency portion of the audio signal below 100 Hz. Comprehensive whole-body vibration measurements performed prior to this experiment found that most of the whole-body vibration produced by the audio system occurs below 100 Hz (see slides 15 and 16) at the seat and floor. The level and frequency of the whole-body vibration varies with music program, and the weight of the listener.

Each listener sat in the driver’s seat of the car listening a virtual BRS rendering of the automotive audio system reproduced through headphones. Listeners adjusted the bass level in their headphones while experiencing four different levels of simulated whole-body vibration that varied from none, low (0 dB) medium (+4 dB) and high (+8 dB). The medium level corresponded to the measured vibration in experiment one, when the bass equalization of the automotive audio system was adjusted to its preferred level.

The experimental results indicated that the preferred level (dB) of bass equalization decreased 3 dB when the level of whole-body vibration increased 8 dB. (see slide 21), and varied with program. At the low vibration level, there was no effect on the preferred bass level, since the vibration level was near or below detection thresholds reported in the literature. At the highest level (8 dB), the vibration tended to be annoying, and listeners tended to turn the bass level down with the hope that the vibration would also be reduced. The effect of vibration on preferred bass level was somewhat dependent on the listener, which could be related to their weight (see slide 24).

The experimental results confirm those of a previous experiment conducted by Martens et al. where the vibration was simulated via a platform, and both head-tracking and individualized BRS calibrations were not employed [3]. The results from the Martens' et al. experiments and this one are plotted above in Figure 1. In spite of the methodological differences between the two experiments, there is good agreement between the two studies. This suggests that the effect of whole-body vibration on the preferred level of bass equalization is quite robust.

The Effect of Whole-body Vibration on the Similarity in Sound Quality between BRS and In Situ Reproductions

The third experiment, listeners sat in the car and rated the overall similarity in sound quality between the BRS headphone-based reproduction with and without the simulated vibration compared to the same audio system experienced in situ. Listeners could switch at will between the in situ and two BRS reproductions (with and without shaker). The two BRS treatments were presented double-blind, and repeated two times with four music programs (see slide 27).

The results (see slide 29) show that sound quality of the BRS reproduction system was significantly improved with the presence of whole-body vibration (shaker on).

Conclusions

From these experiments, it is clear that the whole-body vibration associated with the low frequency sounds of an audio system influences listeners’ perception of the quality and quantity of bass. When the vibration is absent from a stereo or binaural recording of music reproduced through headphones there may be a perceived lack of bass. A 4 dB increase in whole-body vibration produces about a 1.5 dB decrease in preferred level of bass equalization. However, there appears to be upper and lower threshold limits beyond which a change in vibration level will have no effect. Moreover, the amount of vibration and its effect on preferred levels of bass equalization will depend on the low frequency characteristics of the music and the individual listener (and possibly their weight).

Finally, adding simulated whole-body vibration to BRS reproductions can greatly enhance their perceived realism and fidelity when compared to the in situ experience, as long as the vibration levels are above the listener's detection threshold.

References

[1] Sean E. Olive and Todd Welti, “Validation of a Binaural Car Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems,” presented at the 36th International AES Automotive Audio Conference, (June 2-4, 2009).

[2] Germain Simon, Sean E. Olive, and Todd Welti, “The Effect of Whole-body Vibration on Preferred Bass Equalization in Automotive Audio Systems,” presented at the 127th Audio Eng. Soc. Convention, preprint 7956, (October 2009).

[3] William Martens, Wieslaw Woszczyk, Hideki Sakanashi, and Sean E. Olive, “Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Vibration,” presented at the AES 36th International Conference, Dearborn, Michigan (June 2-4, 2009).

The Subjective and Objective Evaluation of Room Correction Products

2009-11-01T15:52:00.001-08:00

In a recent article, I discussed audio’s circle of confusion that exists within the audio industry due to the lack of performance standards in the loudspeakers and rooms through which recordings are monitored. As a result, the quality and consistency of recordings remain highly variable. A significant source of variation in the playback chain occurs from acoustical interactions between the loudspeaker and room, which can produce >18 dB variations in the in-room response below 300-500 Hz.

In recent years, audio manufacturers have begun to offer so-called “room correction” products that measure the in-room response of the loudspeakers at different seating locations, and then automatically equalize them to a target curve defined by the manufacturer. The sonic benefits of these room correction products are generally not well known since, to my knowledge, no one has yet published the results of a well-controlled, double-blind listening test on room correction products. To what degree do room correction products improve or possibly degrade the sound quality of the loudspeaker/room compared to the uncorrected version of the loudspeaker/room? Can the sound quality ratings of the different room correction products be explained by acoustical measurements performed at the listening location?

A Listening Experiment on Commercial Room Correction Products

To answer these questions, we conducted some double-blind listening tests on several commercial room correction products [1]. I recently presented the results of those tests at the 127th Audio Engineering Society Convention in New York. A copy of my AES Keynote presentation can be found here.

A total of three different commercial products were compared to two versions of a Harman prototype room correction that will find its way into future Harman consumer and professional audio products. The products included the Anthem Statement D1, the Audyssey Room Equalizer, the Lyngdorf DPA1, and two versions of the Harman prototype product (see slide 7). Included in the test was a hidden anchor: the same loudspeaker and subwoofer without room correction. In this way, we could directly compare how much each room correction improved or degraded the quality of sound reproduction.

Each room correction device was tested in the Harman International Reference Room using a high quality loudspeaker (B&W 802N) and subwoofer (JBL HB5000) (slides 8 and 9). A calibration was performed for each room correction over the six listening seats according to the manufacturer’s instructions. Two different calibrations were performed with the Harman prototype: one based on a multipoint six-seat average, while the second calibration used a six-microphone spatial average focused on the primary listening seat. The different room corrections were level matched for equal loudness at the listening seat.

The Listener's Task

A total of eight trained listeners with normal hearing participated in the tests. Using a multiple comparison method, the listener could switch at will between the six different room corrections, and rate them according to overall preference, spectral balance, as well as give comments (see slide 14). The administration of the test, including the design, switching, collection and storage of listener responses, was computer automated via Harman’s proprietary Listening Test Software. A total of nine trials were completed using three different programs repeated three times. The presentation order of the program and room corrections was randomized.

Results: Significant Preferences For Different Room Corrections

The mean preference ratings and 95% confidence intervals are shown above in Figure 1 (or slide 17). The room correction products are coded from R1 through R6 in descending order of preference. The identities of the products associated with the results are not relevant for the purpose of this article. Three of the five room corrections (RC1-RC3) were strongly preferred over no room correction (RC4). However, one of the room corrections (RC5) was equally rated to the no correction treatment (RC4), and one of the room corrections (RC6) was rated much worse. Overall, the sound quality of R6 was rated "very poor" based on the semantic definitions of the preference scale.

Perceived Spectral Balance of Room Corrections

Listeners rated the perceived spectral balance of each room correction across seven equal logarithmically spaced frequency bands. The mean spectral balance ratings averaged across all listeners and programs are shown in slide 18. The more preferred room corrections were perceived to have a flatter, smoother spectral balance with extended bass. The less preferred room correction products (R5 and R6) were perceived to have too little bass, which made them sound thin and bright.

Listener Comments on Room Corrections

Listeners also gave comments related to the spectral balance of the different room correction products. Slide 19 shows the number of times a particular comment was used to describe each room correction. The bottom row indicates the correlation between preference rating and the frequency of the comment. The most preferred room corrections were described as "neutral" and "full," which corresponded to flatter, smoother and more bass extended spectral balance ratings. The least preferred room corrections (R4-R6) were described as colored, harsh, thin, and muffled, which corresponded to less flat, less smooth, and less bass extended spectral balance ratings. Slide 20 graphically illustrates the same information in slide 19.

Correlation Between Subjective and Objective Measurements

In-room acoustical measurements were made at the six listening seats using a proprietary 12-channel audio measurement system developed by the Harman R&D Group. Slides 23 and 24 show the amplitude response of the different room corrections spatially averaged for the six seats (slide 23), and at the primary listening seat (slide 24). The measurements are plotted from top to bottom in descending order of preference, each vertically offset to more clearly delineate the differences. A few observations can be made:

The six-seat spatially averaged curves (slide 23) of the room corrections do not explain listeners' room correction preferences as well as the spatially averaged curves taken at the primary seat (slide 24). This makes perfect sense since all of the listening was done in the primary listening seat.

Looking at slide 24, the most preferred room corrections produced the smoothest, most extended amplitude responses measured at the primary listening seat. The largest measured differences among the different room corrections occur below 100 Hz and around 2 kHz where the loudspeaker had a significant hole in its sound power response. The room corrections that were able to fill in this sound power dip received higher preference and spectral balance ratings.

A flat in-room target response is clearly not the optimal target curve for room equalization. The preferred room corrections have a target response that has a smooth downward slope with increasing frequency. This tells us that listeners prefer a certain amount of natural room gain. Removing the rom gain, makes the reproduced music sound unnatural, and too thin, according to these listeners. This also makes perfect sense since the recording was likely mixed in room where the room gain was also not removed; therefore, to remove it from the consumers' listening room would destroy spectral balance of the music as intended by the artist.

Conclusions

There are significant differences in the subjective and objective performance of current commercial room correction products as illustrated in these listening test results. When done properly, room correction can lead to significant improvements in the overall quality of sound reproduction. However, not all room correction products are equal, and two of the tested products produced results that were no better, or much worse, than the unequalized loudspeaker. Room correction preferences are strongly correlated to their perceived spectral balance and related attributes (coloration, full/thin, bright/dull). The most preferred room corrections produced the smoothest, most extended in-room responses measured around the primary listening seat.

More tests are underway to better understand and, if necessary, optimize the performance of Harman's room correction algorithms for different acoustical aspects of the room and loudspeaker.

References

[1] Sean E. Olive, John Jackson, Allan Devantier, David Hunt, and Sean Hess, “The Subjective and Objective Evaluation of Room Correction Products,” presented at the 127th AES Convention, New York, preprint 7960 (October 2009).

Audio's Circle of Confusion

2009-10-31T23:00:00.000-07:00

Audio’s “Circle of Confusion” is a term coined by Floyd Toole [1] that describes the confusion that exists within the audio recording and reproduction chain due to the lack of a standardized, calibrated monitoring environment. Today, the circle of confusion remains the single largest obstacle in advancing the quality of audio recording and reproduction.

The circle of confusion is graphically illustrated in Figure 1. Music recordings are made with (1) microphones that are selected, processed, and mixed by (2) listening through professional loudspeakers, which are designed by (3) listening to recordings, which are (1) made with microphones that are selected, processed, and mixed by (2) listening through professional monitors...... you get the idea. Both the creation of the art (the recording) and its reproduction (the loudspeakers and room) are trapped in an interdependent circular relationship where the quality of one is dependent on the quality of the other. Since the playback chain and room through which recordings are monitored are not standardized, the quality of recordings remains highly variable.

Creating Music Recordings Through An Uncalibrated Instrument

A random sampling of ones own music library will quickly confirm the variation in sound quality that exists among different music recordings. Apart from audible differences in dynamic range, spatial imagery, and noise and distortion, the spectral balance of recordings can vary dramatically in terms of their brightness and particularly, the quality and quantity of bass. The magnitude of these differences suggests that something other than variations in artistic judgment and good taste is at the root cause of this problem.

The most likely culprits are the loudspeakers and rooms through which the recording were made. While there are many excellent professional near-field monitors in the marketplace today, there are no industry guidelines or standards to ensure that they are used. The lack of meaningful, perceptually relevant loudspeaker specifications makes the excellent loudspeakers difficult to identify and separate from the truly mediocre ones. To make matters worse, some misguided recording engineers monitor and tweak their recordings through low-fidelity loudspeakers thinking that this represents what the average consumer will hear. Since loudspeakers can be mediocre in an infinite number of ways, this practice only guarantees that quality of the recording will be compromised when heard through good loudspeakers [1]. This is very counterproductive if we want to improve the quality and consistency of audio recording and reproduction.

Another significant source of variation in the recording process stems from acoustical interactions between the loudspeaker and the listening room [1]-[3] Below 300-500 Hz, the placement of the loudspeaker-listener can cause >18 dB variations in the in-room response due to room resonances and placing the loudspeaker in proximity to a room boundary.

Evidence of acoustical interactions has been well documented survey of 164 professional recording studios where the same high-quality, factory calibrated monitored was installed [4]. Figure 2 shows the distribution of in-room responses measured at the primary listening location where the recordings are monitored and mixed. The 1/3-octave smoothed curves show a reasonably tight ± 2.5 dB variation above 1 kHz. However, below 1 kHz, variation in the in-room response gets progressively much worse at lower frequencies. Below 100 Hz, the in-room bass response can vary as much 25 dB among the different control rooms! You needn’t look any further than here to understand why the quality and quantity of bass is so variable among the recordings in your music library.

Evaluating Loudspeakers When the Recording is a Nuisance Variable

Loudspeaker manufacturers are also trapped in the circle of confusion since music recordings are used by listening panels, audio reviewers, and consumers to ultimately judge the sound quality of the loudspeaker. The problem is that distortions in the recording cannot be easily separated from those produced by the loudspeaker. For example, a recording that is too bright can make a dull loudspeaker sound good, and an accurate loudspeaker sound too bright [5]. A review of the scientific literature on loudspeaker listening tests indicates that recordings are a serious nuisance variable that need to be carefully selected and controlled in the experimental design and analysis of test results.

At Harman International, we try to minimize loudspeaker-program interactions in our loudspeaker listening tests by using well-recorded programs that are equally sensitive to distortions found in loudspeakers. Listeners become intimately familiar with the sonic idiosyncrasies of the different programs through extensive listener training and participation in formal tests. In each trial of a loudspeaker test, the listener can switch between different loudspeakers using the same program, which allows them to better separate the distortions in the program (which are constant), from the distortions in the loudspeaker.

Through 25+ years of well-controlled loudspeaker listening tests, scientists have identified the important loudspeaker parameters related to good sound, which can be quantified in a set of acoustical measurements [6],[7] By applying some statistics to these measurements, listeners’ loudspeaker preferences can be predicted [8]. The bass performance of the loudspeaker alone accounts for 30% the listener’s overall preference rating. Good bass is essential to our enjoyment of music, which unfortunately is a frequency range where loudspeakers and rooms are most variable (see Figure 2). Controlling the behavior of loudspeakers and rooms at low frequencies is essential to achieving a more consistent quality of audio recording and reproduction. Fortunately, there are technology solutions today that provide effective control of acoustical interactions between the loudspeaker and rooms.

Breaking the Circle of Circle of Confusion

As Toole points out in [1], the key in breaking the circle of confusion lies in the hands of the professional audio industry where the art is created. A meaningful standard that defined the quality and calibration of the loudspeaker and room would improve the quality and consistency of recordings. The same standard could then be applied to the playback of the recording in the consumer’s home or automobile. Finally, consumers would be able to hear the music as the artist intended.

References

[1] Floyd E. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, Focal press (July 2008).

[2] Floyd Toole, “Loudspeakers and Rooms: A Scientific Review,” J. Audio Eng. Soc., Vol. 54, No. 6, (2006 June). A free copy of this paper can be downloaded here

[3] Sean E. Olive and William Martens “Interaction Between Loudspeakers and Room Acoustics Influences Loudspeaker Preferences in Multichannel Audio Reproduction,” presented at the 123rd Convention of the AES, preprint 7196 (October 2007).

[4] Aki V. Mäkivirta and Christophe Anet, “The Quality of Professional Surround Audio Reproduction, A Survey Study,”19th International AES Conference: Surround Sound - Techniques, Technology, and Perception (June 2001).

[3] Todd Welti and Allan Devantier, “Low-frequency Optimization Using Multiple Subwoofers,” Audio Eng. Soc., Vol. 54, No. 5, (May 2006). A free copy of this paper can be downloaded here

[4] Sean E. Olive, John Jackson, Allan Devantier, David Hunt, and Sean Hess, “The Subjective and Objective Evaluation of Room Correction Products,” presented at the 127th AES Convention, New York, preprint 7960 (October 2009).

[5] Sean E. Olive,”The Preservation of Timbre: Microphones, Loudspeakers, Sound Sources and Acoustical Spaces,”8th International AES Conference: The Sound of Audio (May 1990)

[6] Floyd E. Toole, “Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 1,” J. Audio Eng. Soc., Vol. 34,No.4, pp.227-235, (April 1986). A free copy of this paper can be downloaded here

[7] Floyd E. Toole, “Loudspeaker Measurements and Their Relationship to Listener Preferences: Part 2,” J. Audio Eng. Soc., Vol. 34, No.5, pp. 323-348, (May 1986). A free copy of this paper can be downloaded here

[8] Sean E. Olive, “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part II - Development of the Model,” presented at the 117th Convention of the AES, preprint 6190 (October 2004).

Validation of a Binaural Room Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems

2009-06-14T12:24:00.000-07:00

In a previous posting on Audio Musings, I described Harman’s binaural room scanning (BRS) measurement and playback system. BRS is a powerful audio research and testing tool that allows Harman scientists to capture, store and later reproduce through a head-tracking headphone-based auditory display the acoustical signature of one or more audio systems situated in the same or different listening spaces. BRS makes it practical to conduct double-blind listening evaluations of different loudspeakers, listening rooms, and automotive audio systems in a very controlled and efficient way.

I also pointed out that all binaural recording/playback systems contain errors that require proper calibration for their removal. However, removing all BRS errors can become very expensive and impractical, so some compromise is necessary. This precipitates the need to experimentally validate the performance of the BRS system to ensure that the remaining errors after calibration do not significantly change listeners’ perceptual ratings of audio systems evaluated through the BRS system as compared to in situ evaluations.

To this end, Todd Welti, Research Acoustician at Harman International, and I recently presented the results of a series of BRS validation tests performed using different equalizations of a high quality automotive audio system [1]. You can view the Powerpoint presentation of the conference paper here. For more detailed information on this experiment, you can view the proceedings from the recent 36th AES Automotive Conference in Dearborn, Michigan, when they become available in the AES e-library .

To assess the accuracy of the BRS system, a group of trained listeners gave double-blind preference ratings for different equalizations of the audio system evaluated under both in situ (in the car) and BRS playback conditions. For the BRS playback condition, the listener sat in the same car listening to a virtual headphone-based reproduction of the car's audio system. The purpose of the experiment was to determine whether the BRS and in situ methods produced the same preference ratings for different equalizations of the car's audio system.

Listeners gave preference ratings for five different equalizations using 4 different music programs reproduced in mono (left front speaker), stereo (left and right front channels) and surround sound (7.1 channels). The three playback modes were tested separately to isolate potential issues related to differences in how the BRS system reproduced front versus rear, and hard versus phantom-based, auditory images.

The listening test results showed there were no statistically significant differences in equalization preferences between the in situ and BRS playback methods. This was true for mono, stereo and multichannel playback modes (see slides 21-23). An interesting finding was that these results were achieved using a BRS calibration based on a single listener whose calibration tended to work well for the other listeners on the panel. This suggests that individualized listener calibrations for BRS-based listening tests may not be necessary, so long as the calibration and listeners are carefully selected.

In conclusion, this validation experiment provides experimental evidence that a properly calibrated BRS measurement and playback system can produce similar preferences in automotive audio equalization as measured using in situ listening tests.

Reference

[1] Sean E. Olive, Todd Welti, “Validation of a Binaural Car Scanning Measurement System for Subjective Evaluation of Automotive Audio Systems,” presented at the 36th International AES Automotive Audio Conference, (June 2-4, 2009).

Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Equalization

2009-06-11T11:43:00.000-07:00

Last week I attended the AES Automotive Audio Conference in Dearborn, Michigan where about 70-odd (pun intended) audio scientists and engineers gathered to discuss the latest scientific and technological developments in automotive audio. A detailed description of the program can be found here.

This article focuses on a paper I co-authored and presented called “Whole-body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Equalization" [1]. The work was a joint effort between three researchers, Drs. William Martens, Wieslaw Woszcczyk, and Hideki Sakanashi, from the CIRMMT at McGill University in Montreal, and myself, at Harman International. A copy of our Powerpoint presentation given at the conference can be viewed here.

It is well established that human perception is a multimodal sensory experience [2]. For example, both auditory and visual cues associated with a sound source and its acoustic space are integrated and interrogated by high level cognitive processes that determine our spatial perception of the source based on the plausibility, strength and agreement between the visual and auditory cues. Bimodal sensory interactions have been reported in studies where the video quality of the picture influences listeners’ judgment of the audio system’s sound quality and vice versa (although the audio quality has much less influence on the perceived quality of video than vice versa) [3].

However, little is known regarding how low frequency (below 100 Hz) whole-body vibration produced by the audio system influences our perception of the quality and quantity of bass. Perhaps the most related study is Rudmose’s “case of the missing 6 dB” where the perceived loudness of low frequency signals reproduced through headphones was reported to be approximately 6 dB lower than that of loudspeakers producing the equivalent sound pressure levels at the ears [4]. Rudmose showed that the absence of tactile stimulus in headphone reproduction could, in part, account for why headphones sound less loud than loudspeakers when producing equivalent sound pressure level at the ears (the rest of the missing 6 dB was due to experimental factors, and the increased physiological noise in the ear canal introduced by the coupling of the headphone to the ear).

A Tactile-Auditory Bimodal Sensory Experiment

To shed more light on this mystery, an experiment was conducted at McGill University. A total of 6 trained tonmeisters listened through calibrated headphones to binaural recordings of a virtual high-quality automotive audio system. Each listener adjusted the low frequency boost applied to different multichannel music reproductions according to their taste while experiencing a high and low level whole-body vibration. This was generated by a programmable motion platform driven by the low frequency portion (below 80 Hz) of the music signal. In this way, vibration was delivered to both the feet and body of the listener through the chair (see slide 5). The virtual automotive audio system was based on a binaural room scan (BRS) of the audio system installed in our research vehicle located at the Harman International Automotive Audio Research Lab in Northridge, California (see slide 3). For more information on how BRS works, please refer to my previous BRS blog postings, Part 1 and Part 2.

Whole-Body Vibration Influences Preferred Equalization of the Audio System

The researchers found that the preferred bass equalization of music reproduced through the virtual automotive audio system was significantly influenced by the level of whole-body vibration experienced. While the amount of preferred bass boost varied with music program and listeners, the listeners always preferred less bass for the high vibration condition than for the low vibration one, which was 12 dB lower: on average, listeners preferred 6 dB less bass boost in their headphones moving from the low to high vibration conditions (see slide 10). In other words, there was a bimodal sensory interaction effect between the auditory and tactile senses that influenced listeners' preferred bass equalization of music reproduced through the headphones.

It is important to note that the 6 dB effect reported here may not be the same as observed in an automobile where the level and other physical characteristics of the vibration observed may be different from what was tested here. Under driving conditions, listeners experience additional sources of vibration (and acoustic noise) from the road and engine of the vehicle that may partially mask the whole-body vibration effects produced by the audio system. More research is currently underway to study how real and simulated whole-body vibration in vehicles influences listeners' perception of the audio system and its sound quality.

References
[1] William Martens, Wieslaw Woszczyk, Hideki Sakanashi, and Sean E. Olive, “Whole-Body Vibration Associated with Low-Frequency Audio Reproduction Influences Preferred Vibration,” presented at the AES 36th International Conference, Dearborn, Michigan (June 2-4, 2009).

[2] Multimodal Integration, Wikipedia.
[3] Beerends, John G; De Caluew, Frank E. “The Influence of Video Quality on Perceived Audio Quality and Vice Versa,” JAES, Vol. 45, (5), pp. 355-362 (May 1999).
[4] Rudmose, Wayne, “The case of the missing 6 dB,” J. Acoust. Soc. Am. Volume 71, Issue 3, pp. 650-659 (March 1982)

Harman's "How to Listen" - A New Computer-based Listener Training Program

2009-05-30T14:52:00.000-07:00

Trained listeners with normal hearing are used at Harman International for all standard listening tests related to research and competitive benchmarking of consumer, professional and automotive audio products. This article explains why we use trained listeners, and describes a new computer-based software program developed for training and selecting Harman listeners.

Why Train Listeners?

There are many compelling reasons for training listeners. First, trained listeners produce more discriminating and reliable judgments of sound quality than untrained listeners [1]. This means that fewer listeners are needed to achieve the same statistical confidence, resulting in considerable cost savings. Second, trained listeners are taught to identify, classify and rate important sound quality attributes using precise, well-defined terms to explain their preferences for certain audio systems and products. Vague audiophile terms such as “chocolaty”, “silky” or “the bass lacks pace, rhythm or musicality” are NOT part of the trained listener's vocabulary since these descriptors are not easily interpreted by audio engineers who must use the feedback from the listening tests to improve the product. Third, the Harman training itself, so far, has produced no apparent bias when comparing the loudspeaker preferences of trained versus untrained listeners [1]. This allows us to safely extrapolate the preferences of trained listeners to those of the general untrained population of listeners (e.g. most consumers).

Harman's “How to Listen” Listener Training Program

Harman’s “How to Listen” is a new computer-based software application that helps Harman scientists efficiently train and select listeners used for psychoacoustic research and product evaluation. The self-administered program has 17 different training tasks that focus on four different attributes of sound quality: timbre (spectral effects), spatial attributes(localization and auditory imagery characteristics), dynamics, and nonlinear distortion artifacts. Each training task starts at a novice level, and gradually advances in difficulty based on the listeners’ performance. Constant feedback on the listener's responses is provided to improve their learning and performance. A presentation of the training software can be viewed in parts 1 and 2

Spectral Training Tasks

There are two different spectral training tasks. In the Band Identification training task, the listener compares a reference (Flat) and an equalized version of the music program (EQ), and must determine which frequency band is affected by the equalization (see slide 5 of part 2). The types of filters include peaks, dips, peak and dips, high/low shelving and low/high/bandpass filters. The task is aimed at teaching listeners to identify spectral distortions in precise, quantitative terms (filter type, frequency, Q and gain) that directly correspond to a frequency response measurement.

At the easiest skill level, there are only 2 frequency band choices, which are easily detected and classified. However, as the listener advances, the audio bandwidth is subdivided into multiple frequency bands making the audibility and identification of the affected frequency band more challenging.

The Spectral Plot training exercise takes this one step further. The listener compares different music selections equalized to simulate more complex frequency response shapes commonly found in measurements of loudspeakers in rooms and automotive spaces. The listener is given a choice of frequency curves which they must correctly match to the perceived spectral balances of the stimuli. This teaches listeners to graphically draw the perceived timbre of an audio component as a frequency response curve. Once trained, listeners become quite adept at drawing the perceived spectral balance of different loudspeakers, and these graphs closely correspond to their acoustical measurements [2], [3].

Sound Quality Attribute Tasks

The purpose of this task is to familiarize the listener with each of the four sound quality attributes (timbre, spatial, dynamics and nonlinear distortion) and their sub-attributes, and measure the listener's ability to reliably rate differences in the attribute's intensity. For example, in one task the listener must rank order the relative brightness/dullness of two or more stimuli based on the intensities of the brightness/dullness of the processed music selection. As the difficulty of the task increases, the listener must rate more stimuli that have incrementally smaller differences in intensity of the tested attribute. Listener performance is calculated using Spearman’s rank correlation coefficient which expresses the degree to which stimuli have been correctly rank ordered on the attribute scale.

Preference Training

In this task, the listener enters preference ratings for different music selections that have had one or more attributes (timbre, spatial, dynamics and nonlinear distortion) modified by incremental amounts.

By studying the interrelationship between the modification of these attributes and the preference ratings, Harman scientists can uncover how listeners weight different attributes when formulating their preferences. From this, the preference profile of a listener can be mapped based on the importance they place on certain sound quality attributes. The performance metric in the preference task is based on the F-statistic calculated from an ANOVA performed on the individual listeners’ data. The higher the F-statistic, the more discriminating and/or consistent the listeners’ ratings are --- a highly desirable trait in the selection of a listener.

Other Key Features

Harman’s “How to Listen” training software runs on both Windows and Mac OSX platforms, and includes a real-time DSP engine for manipulating the various sound quality attributes. Most common stereo and multichannel sound formats are supported. In “Practice Mode”, the user can easily add their own music selections.

All of the training results from the 100+ listeners located at Harman locations world-wide are stored on a centralized database server. A web-based front end will allow listeners to log in to monitor and compare their performances to those of other listeners currently in training. Of course, the identifies of the other listeners always remain confidential.

Conclusion

In summary, Harman’s “How to Listen” is a new computer-based, self-guided software program that teaches listeners how to identify, classify and rate the quality of recorded and reproduced sounds according to their timbral, spatial, dynamic and nonlinear distortion attributes. The training program gives constant performance feedback and analytics that allow the software to adapt to the ability of the listener. These performance metrics are used for selecting the most discriminating and reliable listeners used for research and subjective testing of Harman audio products.

References

[1] Sean. E Olive, "Differences in Performance and Preference of Trained Versus Untrained Listeners in Loudspeaker Tests: A Case Study," J. AES, Vol. 51, issue 9, pp. 806-825, September 2003. Download for free here, courtesy of Harman International.

[2] Sean E. Olive, “A Multiple Regression Model for Predicting Loudspeaker Preference Using Objective Measurements: Part I - Listening Test Results,” presented at the 116th AES Convention (May 2004).

[3] Floyd E. Toole, Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms, Focal press (July 2008). Available from Amazon here

The Harman International Reference Listening Room

2009-05-23T11:45:00.000-07:00

Last week I returned from the AES Munich Convention where I gave a paper entitled ”A New Reference Listening Room for Consumer, Professional, and Automotive Audio Research.” It describes the features, scientific rationale, and acoustical performance of a new reference listening room designed and built for the purposes of conducting controlled listening tests and psychoacoustic research for consumer, professional, and automotive audio products. The main features of the room include quiet and adjustable room acoustics, a high-quality calibrated playback system, an in-wall loudspeaker mover, and complete automated control of listening tests performed in the room. A copy of my Munich AES presentation is available here.

The first prototype reference room was built at the Harman Northridge campus in 2007. Additional reference listening rooms have since been built at Harman locations in the UK, Germany, with the fourth one being constructed in Farmington Hills, Michigan. We are in the process of measuring and calibrating the performances of the different rooms using acoustical measurements and binaural room scans, which will be evaluated for their perceptual similarity in sound quality.

With a standardized listening room and playback system, Harman scientists can conduct listener training, psychoacoustic research and product testing at different Harman locations throughout the world. The results from these different locations can be compared or pooled together since the room, playback system, and trained listeners are a constant variable. With this brings greater testing efficiency, flexibility, and new opportunities in the kinds of product research and listening tests Harman is able to do in the future. Already, we are using the unique features of these rooms to conduct very controlled listening tests on consumer in-wall speakers, and to research and benchmark the performance of various commercial and prototype loudspeaker-room correction devices.

You will hear a lot more about the Harman International reference listening rooms in the near future because of the pivotal role they will play in the research, testing and subjective benchmarking of new Harman consumer, professional and automotive audio products. Just thinking about these research possibilities makes me truly excited!