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

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

[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] 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

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).

Evaluating the Sound Quality of Ipod Music Stations: Part 1

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.