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Thread: Audio Science in the Service of Art

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    WBF Technical Expert [Technical Expert] tonmeister2008's Avatar
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    Audio Science in the Service of Art

    “Audio Science in the service of art” is a philosophy in sound reproduction where the goal is to use science and technology to faithfully reproduce the art as the musical artist intended [1]. The art is the music, its performance, and the process of capturing it on the recording. The audio system is not part of the art, and should neither add, remove or editorialize the artist’s message. Audio components should not sound like musical instruments: Beethoven never wrote parts for loudspeaker and amplifier, so you shouldn’t be hearing them when listening to recordings of his music. The perfect audio system has no sonic personality, no musical qualities, and is the system that you notice the least.

    Many audio companies do not subscribe to this scientific approach towards sound reproduction. Some companies will admit they simply cannot afford the infrastructure required to conduct proper scientific-based objective and subjective measurements. Anechoic chambers, dedicated listening rooms, speaker movers, listening test software, trained listening panels, and educated and trained scientific/engineering staff require significant long-term financial commitment to R&D. Other companies view sound reproduction as an artistic or marketing driven opportunity to screw with the art, and perform cosmetic surgery on the music: a little bass augmentation here, a little midrange tuck there, lift the treble here, and the facelift is complete. The problem with this approach is that recordings can be flawed in an infinite number of different ways. A cure-all bandaid solution will ultimately do more harm than good for most recordings, especially the good ones, which is a disservice to the art.

    Faithful reproduction of the art assumes that the listening conditions in which the art was created are well defined. While there exists many accurate monitor loudspeakers in the marketplace today (e.g. the JBL LSR professional monitor series are designed to the same targets as our consumer loudspeakers) there is nothing that guarantees the artist will use them. Without meaningful loudspeaker and room calibration standards common to the professional and consumer audio industries, recordings and their reproduction remains a poorly controlled, highly variable process. For example, a survey of 164 professional recording studios in Europe using the same factory-calibrated 3-way monitor found up to 25 dB variations below 100 Hz measured at the mixing position. It’s no wonder the bass is so variable among different recordings!

    Both the consumer and professional audio industries are trapped in a codependent relationship whereby the perceived sound quality of one’s product is interdependent on the others’. Known as the circle-of-confusion, this unfortunate state of affair can only be solved through a common standard that defines the performance of the loudspeaker and its acoustical interaction with the room.

    Faithful reproduction of the art requires a thorough scientific understanding of the relationship between the perception and measurement of sound so that the important variables can be identified. Most audio scientists agree that the circle-of-confusion problem will not be solved until we optimize the performance of the loudspeaker and its acoustical interaction with the room acoustics. Our scientific understanding of what makes a loudspeaker sound accurate and neutral is already well understood.

    This research question was studied at the Canadian National Research Council (NRC)[2],[3]and more recently, at Harman International [4]-[6], the parent company of loudspeaker brands Infinity, Harman Kardon, JBL and Revel. Using scientific-based, double-blind loudspeaker listening tests, scientists studied which physical parameter of loudspeaker performance were most related to listeners’ sound quality ratings, and overall preference. To eliminate the effects of sighted biases (e.g. brand, price, size, reputation) the tests were performed double-blind with other known listening test nuisance variables carefully controlled. The loudspeaker positional effects in comparative loudspeaker tests were solved by an automated speaker shuffler that positions each speaker into the exact same position.The tests were performed using trained listeners with normal hearing. More recent tests with untrained listeners indicate they also prefer the same loudspeakers as trained listeners,but give less consistent and discriminating ratings.

    The results of this research found that the preferred loudspeakers in the listening tests were also the most accurate ones,based on a set of a comprehensive anechoic measurements. The measurements used high frequency resolution (48 points per octave), and employed spatial averaging to separate resonances from diffraction/acoustic interference effects. The frequency response curves were then spatially averaged into a family of curves, based on a survey of user's set ups in their rooms, rooms that represent the quality of the direct, early and late reflected sounds heard in the room. A mathematical preference model based on these measurements has been recently developed and can predict the loudspeaker preference with a correlation of r = 0.86 (the agreement between the predicted and measured ratings of 70 different loudspeakers). The model tells us that both the quality of the direct and reflected sounds produced by the loudspeaker are almost equally as important, and the bass quality accounts for about 30% of a listeners’ loudspeaker preference. This suggests that the low frequency interaction between the loudspeaker and room acoustics is something that cannot be ignored.

    The acoustical interaction between the loudspeaker and the room is the remaining problem that must be solved to close the loop between the creation and reproduction of the art. At low frequencies (below 200-300 Hz), all listening rooms contain a natural set of resonances or room modes that can significantly boost and attenuate low frequencies below 200-300 Hz [7]. The level and frequency of these resonances will depend on the room’s dimensions, geometry and absorption characteristics, as well as the locations of the loudspeakers and listeners.

    Fortunately, there are solutions today that can deal with these low frequency variations that occur between the loudspeaker and its acoustical interaction with the room. Bass in rooms can be tamed by judiciously placing the loudspeakers and listeners in locations where the room modes have the least effect [8]. In rectangular rooms, placing multiple (2 to 4) subwoofers in the room’s corners, wall midpoints,or at 25% and 75% along the wall dimension can cancel order modes via constructive interference, and not excite others. This solution has the benefit of reducing the spatial variance in bass quality across the listening area. Finally, equalization at single or multiple seating locations avoid exiting othersreduce some of the most deleterious effects. However, not all commercial room correction solutions are equal: some models can actually make the audio system sound worse than without correction.

    In summary, our scientific understanding of the relationship between the measurement and perception of loudspeakers and rooms is quite mature. Measurements exist today that can accurately and reliably predict loudspeaker sound quality, and there are practical and effective solutions for dealing with their acoustical interaction with listening rooms at low frequencies.

    It's time for the audio industry to finally close the loop between the recording and playback chains - to break out of the circle of confusion. A meaningful standard that defines the performance of the playback chain where the art is both created and reproduced would certainly be good place to start. Work on a new loudspeaker standard based on the NRC and Harman loudspeaker measurements is underway within the CEA and CEDIA standards groups. When completed, consumers will have access to product specifications that identify the excellent loudspeakers from the ones that are duds. Hopefully, the professional audio industry will adopt a similar standard so that consumers hear the music as it was intended by the artist. That would be the ultimate reward for audio science in the service of art.


    References

    [1] Floyd E. Toole. "Science in the service of art", Harman International white paper.

    [2] 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).

    [3]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).

    [4] 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).

    [5} Sean E. Olive Sean E. Olive, "A Multiple Regression Model for Predicting Loudspeaker Preferences using Objective Measurements: Part 1 -Listening Test Results," presented at the 116th AES Convention, May 2004.

    [6] Sean E. Olive "A Multiple Regression Model for Predicting Loudspeaker Preferences using Objective Measurements: Part 2 - Development of the Model", presented at the 117th AES Convention, October 2004.

    [7] Floyd E. Toole " Loudspeakers and Room - A Scientific Review", J. Audio Eng. Soc., Vol. 54, No. 6, June 2006 (download for free here courtesy of Harman International)

    [8] Todd Welti and Allan Devantier,"Low Frequency Optimization Using Multiple Subwoofers", J. Audio Eng. Soc., Vol. 54, No. 5, May 2006 (download for free, courtesy of Harman International).
    Last edited by tonmeister2008; 06-29-2010 at 10:41 AM.
    Cheers,
    Sean Olive
    Audio Musings

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    Site Founder And Administrator Steve Williams's Avatar
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    Sean.

    Great first contribution. A most interesting read.

    Can you explain about the 48 points per octave and how it is done.
    Steve Williams
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    [WBF Founding Member] audioguy's Avatar
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    Certainly as it pertains to loudspeakers, your quote of: "Other companies view sound reproduction as an artistic or marketing driven opportunity to screw with the art and perform cosmetic surgery on the music:" probably applies to 75% of all loudspeaker companies. Besides your company, and two or three others (the now deseased Dunlavy being one), virtually none use chambers. Some probably/may, use computer modeling and manufacturer provided driver specs to develop crossovers and ultimately the speaker, but that is about as far as it goes. And I would suggest that even some very well known and reasonably large (in the context of high end audio) speaker companies "voice" their speakers by ear (primarily).

    Your research notwithstanding, my experience suggests that even when you develop the ability "so that consumers can easily differentiate between accurate loudspeakers and ones that are not", most consumers will not pick "accurate". I would think that accuracy implies a standard to which the reproduced sound can be compared. VERY few audiophiles attend live concerts and even fewer attend live concerts of un-amplified music. Given that assumption, how does a person determine accurate if they have no standard against which they can measure.

    Great post, by the way !

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    WBF Technical Expert [Technical Expert] tonmeister2008's Avatar
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    Thanks Steve. Sure, we use a log sweep to measure our loudspeakers in the anechoic chamber. The sweep gives us 48 points per octave that are equally log-spaced in frequency from 10 Hz to 40 kHz.

    The log sweep gives us much better frequency resolution at low frequencies, better signal-to-noise and is faster compared to what we used to use: a Maximum Length Sequence (MLS), which is a pseudo-random noise.
    Cheers,
    Sean Olive
    Audio Musings

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    Site Founder And Administrator Steve Williams's Avatar
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    Quote Originally Posted by tonmeister2008 View Post
    Thanks Steve. Sure, we use a log sweep to measure our loudspeakers in the anechoic chamber. The sweep gives us 48 points per octave that are equally log-spaced in frequency from 10 Hz to 40 kHz.

    The log sweep gives us much better frequency resolution at low frequencies, better signal-to-noise and is faster compared to what we used to use: a Maximum Length Sequence (MLS), which is a pseudo-random noise.
    thanks Sean
    Steve Williams
    aka oneobgyn
    There's ALWAYS another Steve Williams BUT there's only "oneobgyn"
    International Distributor of Center Stage Feet and owner of PitchPerfect Sound (www.pitchperfectsound.com)
    Dealer Lamm Electronics
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  6. #6
    WBF Technical Expert [Technical Expert] tonmeister2008's Avatar
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    Quote Originally Posted by audioguy View Post
    Certainly as it pertains to loudspeakers, your quote of: "Other companies view sound reproduction as an artistic or marketing driven opportunity to screw with the art and perform cosmetic surgery on the music:" probably applies to 75% of all loudspeaker companies. Besides your company, and two or three others (the now deseased Dunlavy being one), virtually none use chambers. Some probably/may, use computer modeling and manufacturer provided driver specs to develop crossovers and ultimately the speaker, but that is about as far as it goes. And I would suggest that even some very well known and reasonably large (in the context of high end audio) speaker companies "voice" their speakers by ear (primarily).

    Your research notwithstanding, my experience suggests that even when you develop the ability "so that consumers can easily differentiate between accurate loudspeakers and ones that are not", most consumers will not pick "accurate". I would think that accuracy implies a standard to which the reproduced sound can be compared. VERY few audiophiles attend live concerts and even fewer attend live concerts of un-amplified music. Given that assumption, how does a person determine accurate if they have no standard against which they can measure.

    Great post, by the way !
    I know some loudspeaker companies like PSB, Paradigm, Axiom and Definitive Technology, B&W own or rent anechoic chambers to test their loudspeakers. Surprisingly, some companies have chambers but only do a few measurements on and off-axis, which is frankly inexcusable. That is not enough measurements to fully characterize the sound radiated by the loudspeaker. We do measurements every 10 degrees, 360 degrees in both horizontal and vertical orbits, for a total of 70 measurements.

    It doesn't surprise me that some companies don't measure their speakers or tune them by ear.I've tested some of those speakers over the last 20 years, and it's obvious they were not properly evaluated or measured.

    The CEDIA/CEA loudspeaker standards will be based on comprehensive anechoic measurements that allow easy interpretation of their sound quality. Eventually there may be an overall sound quality rating based on those measurements.

    If a consumer is told that speaker Brand A has a sound quality of 90/100 and speaker brand B is rated 50/100 -- do you really think the consumer will choose the one that is rated 50/100 (the one that sounds less neutral)?

    When I shop at Costco the wines rated over 90 by Wine Spectator are usually almost sold out, and the ones rated less than 85 are usually still plentiful. Consumer purchases are highly influenced by simple ratings like that, particularly if the ratings are meaningful and come from a trusted source.

    Probably luxury goods like high-end audio and wine are less influenced by ratings and real quality versus perceived quality. For some people, it's just about price, exclusivity and the status that comes with ownership.
    Last edited by tonmeister2008; 06-27-2010 at 11:51 PM.
    Cheers,
    Sean Olive
    Audio Musings

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    Sean, can you please explain what will be used in CEDIA/CEA speaker measurement standards?

    Thanks,

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    WBF Technical Expert [Technical Expert] tonmeister2008's Avatar
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    Quote Originally Posted by amirm View Post
    Sean, can you please explain what will be used in CEDIA/CEA speaker measurement standards?

    Thanks,
    Amir,

    The CEA has a loudspeaker standards committee that includes representatives from different manufacturers including Harman (Floyd Toole and Allan Devantier are representing us). The final draft is not completed and approved, but I am told the loudspeaker measurements are likely to be similar to what we currently use at Harman:

    1) Anechoic frequency response made a 2 meters or greater with at least 1/20-octave resolution
    2) Repeated at every 10 degrees in horizontal and vertical orbits
    3) Calculate and plot the following spatial averages:

    a) On-axis response
    b) Listening window
    c) Early Reflections
    d)Total Radiated Sound Power
    e) Early Reflection Directivity Index
    f) Sound Power Directivity Index

    I've included few slides here that explain the measurements and an illustrate example of how the measurements correlate with listeners' preference ratings (last slide).
    Cheers,
    Sean Olive
    Audio Musings

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    [WBF Founding Member] audioguy's Avatar
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    What about measuring step response? While you may not agree with him, here is a quote from John Dunlavy in an interview with Stereophile: "Of all of the measurements that we take that come more close to predicting, or most close to predicting how a speaker is going to emulate a properly recorded live performance, it's step response. Everything is implicit if you know how to interpret a step response...if my life depended upon my describing what I thought a speaker was going to sound like, all other factors being equal, I would choose step response. And feel very confident that I would be spot-on."

    Your comment about people preferring a speaker who gets a 90 score versus a 50 score would certainly apply to that category who buys on specs today but I would suggest that many who purchase in the high end market do not fall into that group.

    I am a BIG believer in the concept of building truly accurate loudspeakers. And I'm all for the development of the standards you have mentioned. And I also believe that those standards will help drive sales for those companies whose products measure best -- for the mid-fi market on down.

    However, I also spent 10 years selling and installing digital room correction systems (primarily SigTech) and heard hundreds and hundreds of systems in audiophile homes (these were mostly mega buck systems). What I found interesting is that many, many audiophiles preferred certain coloration over what clearly would have measured much more accurately. Just read the threads in WBF and it is clear the there is no consensus on this whole accuracy subject. Or go read some of the threads on certain loudspeakers on other forums and you will note the same thing.

    Sign ME up as a believer. But on the subject of getting most "high enders" to purchase based on a set of standard accuracy measurements,, I guess you could also call me a major cynic. But I hope I'm wrong about that.
    Last edited by audioguy; 06-28-2010 at 06:45 AM.

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    WBF Technical Expert [Technical Expert] tonmeister2008's Avatar
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    Thanks Audioguy. The step response of the loudspeaker can be calculated from the impulse response and tells you something about the time-coherence of the drivers. The speaker is time-coherent if the woofer/mid/tweeter arrive at the microphone at the same time: it should be pointed out that this happens at only 1 point in space for 1 ear since the drivers are physically separated by some distance - move your ear, and all bets are off.

    Most loudspeakers are not time-coherent, and frankly I don't think this is very important in terms of the loudspeaker's perceived sound quality. At mid/high frequencies we are relatively insensitive to phase/group delay caused by the cross-over, or typical driver offsets.

    More important is how well the drivers sum together and how well matched their directivities are at the cross-over region. This can be better viewed by looking at the frequency response measured all around the loudspeaker.

    John Atkinson wrote a nice article about the step response over at Stereophile.. If you read his paper, he independently came to the same conclusion as we did after reviewing and measuring 350 loudspeakers over many years: the frequency response is the best indicator of how good the loudspeaker sounds. I quote him:

    Floyd Toole, now with Harman International but then with Canada's National Research Council, in his summary of research at the NRC into loudspeaker performance that is described in two classic 1986 papers [32, 33], concluded thusly: "The advocates of accurate waveform reproduction, implying both accurate amplitude and phase responses, are in a particularly awkward situation. In spite of the considerable engineering appeal of this concept, practical tests have yielded little evidence of listener sensitivity to this factor...the limited results lend support for the popular view that the effects of phase are clearly subordinate to amplitude response."

    This is also my view. Of the 350 or so loudspeakers I have measured, there is no correlation between whether or not they are time-coherent and whether or not they are recommended by a Stereophile reviewer. However, I feel that if other factors have been optimized—on-axis response, off-axis dispersion, absence of resonance-related problems, and good linearity—like a little bit of chicken soup, time coherence (hence minimal acoustic phase error) cannot hurt. In my admittedly anecdotal experience, a speaker that is time-coherent (on the listening axis) does have a small edge when it comes to presenting a stereo soundstage, in terms of image focus and image depth. But time coherence does not compensate for coloration, poor presentation of instrumental timbres, a perverse frequency balance, or high levels of nonlinear distortion.
    Last edited by tonmeister2008; 06-29-2010 at 04:26 PM.
    Cheers,
    Sean Olive
    Audio Musings

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