Audio's Circle of Confusion

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).

The Harman International Reference Listening Room

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!

Welcome to My Blog on The Science of Sound Recording and Reproduction

This blog is concerned with all matters related to the quality of recorded and reproduced sound. Some of the topics I hope to cover in upcoming posts include recording technology, listening tests, loudspeakers, headphones, automotive audio, and acoustical interactions between loudspeakers and listening rooms.

I am an audio scientist by profession, and in matters related to the sound quality, I prefer to make conclusions based on hard scientific evidence gathered through properly controlled listening tests and meaningful objective measurements. Unfortunately, most of the audio industry doesn't operate this way. Why not? Quality subjective and objective measurements require significant investments in time, facilities, and expertise, whereas opinions on sound quality cost almost nothing.  Sometimes you get what you pay for.

I'm particularly  interested in the psychoacoustics of audio (i.e. the relationship between the human perception and measurement of sound). Here, controlled listening tests play an important role  since they permit scientists to make accurate, reliable and valid correlations between listeners' preferences and the variables being tested (e.g. different loudspeakers, room treatments, etc). From these listening tests  will hopefully emerge  a set of measurement and design rules from which the audio chain can be consistently optimized to produce a quality listening experience. 

I hope the reader will find this blog educational and entertaining.

Note:  The above photograph shows a listener auditioning different loudspeakers in Harman International's Multichannel Listening Lab.  Loudspeaker positional effects are controlled by an automated speaker mover that shuffles each loudspeaker into the same exact position within 3 seconds. During the test, an acoustically transparent but visually opaque curtain (shown in the up position here) is dropped in front of the loudspeakers so that the listener is not biased by visual factors such as loudspeaker size, brand, price,etc.