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radiating similar spectra in all directions (i.e. relatively con- stant directivity). Consequently, it is logical that reflections of those sounds should not be spectrally altered by reflecting or scattering surfaces of room boundaries. In practical terms, this argues for areas of either full reflection or full absorption. At present I know of no spectrally neutral sound-attenuating device, although scattering/diffusing devices can approach this, but with other consequences. The widely used (and recommended in some standards) 1-inch absorbing panels are ill advised, certainly at first-reflection locations. At very high frequencies direct sound dominates, simplifying acoustic concerns.
The reverberation time (RT) target for home entertain- ment spaces, based primarily on speech intelligibility, is easy to hit: ≤ 0.5 s. This number applies also to cinemas and film production facilities (dialogue again), but music recording control rooms tend to aim for lower RTs, sometimes much lower. Even at 0.5 s, with relatively directional sound sources, there is nothing resembling a diffuse sound field, meaning that random incidence absorption coefficients are of limited use. The importance of first-reflected sounds suggests that it might be advantageous to know the angle-specific frequency- dependent absorption and scattering/diffusing properties of acoustical materials and devices.
There is evidence that the precedence effect deteriorates when the spectra of the direct and delayed sounds differ. It is plausible to think that similar effects extend to other aspects of perception, including spaciousness and timbre. Chapters 5 thru 10 in my book provide an overview of some of the fac- tors, but it is clear that we need more data elaborating the progression of perceptual effects for level and spectral varia- tions within isolated and multiple reflections. These data would ideally come from psychoacoustic experiments incor- porating delay and directional variables associated with real- istic listening circumstances. The result would be solid evi- dence supporting performance targets and tolerances for the off-axis performance of loudspeakers and the reflecting/absorbing/diffusing surfaces at which first reflec- tions occur in room. This could be an interesting collabora- tion between scientists with acoustical and psychoacoustical expertise. A global industry awaits guidance.
Subjectively it has been found that the effect of the room is greatest with a single loudspeaker (channel) with its effects diminishing as the active channel count increases. However, with a very high proportion of movie and TV sound emerg- ing from the front-center channel (a mono signal) the room cannot be ignored.
The inevitable question is: What constitutes an “ideal” listening room? Right now we don’t know, and given the ability of humans to adapt to differing rooms, it may matter less than some people would like us to think. However, there is a limit to what we can adapt to, and adaptation very likely utilizes a portion of our neural “horsepower” (causing fatigue?). So, perhaps that feeling of exquisite relaxation I get when I listen to a superb sound system is real, not a fig- ment of my imagination. If so, there is motivation for research by acoustical scientists, and work to do by compe- tent acoustical engineers.
Looking Ahead
Because of the science we have, and the abundance of affordable measurement tools, the standards of sound repro- duction in general have been elevated in homes and record- ing facilities. However, problems remain, in the form of loud- speakers that are less good than they could have been, flawed acoustical treatment practices and misguided attempts to “equalize” rooms. Right now, there is no assurance that repro- duced sound closely resembles what was heard at the time the art was created. This is a pity, because it is not possible to confidently attribute credit or blame for what we hear.
In the end, consumers, audio professionals and acousti- cal consultants need to be able to anticipate whether a play- back facility is likely to deliver a reasonable facsimile of an original performance, without exceeding the tolerances of normal adaptation. We certainly need more and better spec- ifications on loudspeakers, and manufacturers with the courage to publish them. Between here and there are many opportunities for challenging applied research projects, gen- erating new knowledge, and interminable committee meet- ings for those willing to undertake the standards work.
The technology to do much better exists. In the meantime, there are countless personal opinions to sort through, and a lot of adapting to get on with so that we can enjoy the abundance of music out there. Fortunately music is what it is, in spite of the acoustical variations and abuses we heap upon it.
References
Benade, A.H. (1984). “Wind instruments in the concert hall.” Text of an oral presentation at Parc de la Villette, Paris; part of a series of lectures entitled “Acoustique, Musique, Espaces”, 15 May 1984 (personal communication).
Bregman, A.S. (1990), “Auditory Scene Analysis, The Perceptual Organization of Sound,” MIT Press, Cambridge, MA.
Devantier, A. (2002). “Characterizing the Amplitude Response of Loudspeaker Systems,” 113th Convention, Audio Engineering Society, Preprint 5638.
Olive, S. E. and Toole, F. E. (1989). “The Detection of Reflections in Typical Rooms,” Journal of the Audio Engineering Society, 37, pp. 539-553. Available at: <http://www.harman.com/en-us/our- company/innovation/pages/scientificpublications.aspx>.
Olive, S. (2001). “A New Listener Training Software Application,” 110th Convention, Audio Engineering Society, Preprint No. 5384.
Olive, S. (2003). “Difference in Performance and Preference of Trained versus Untrained Listeners in Loudspeaker Tests: A Case Study,” Journal of the Audio Engineering Society, 51, pp. 806-825. Available at: <http://www.harman.com/en-us/ourcom- pany/innovation/pages/scientificpublications.aspx>..
Olive, S. (2004a). “A multiple regression model for predicting loud- speaker preference using objective measurements: part 1 – lis- tening test results,” 116th Convention, Audio Engineering Society, Preprint 6113.
Olive, S. (2004b). “A multiple regression model for predicting loud- speaker preference using objective measurements: part 2 – development of the model,” 117th Convention, Audio Engineering Society, Preprint 6190.
Shirley, B.G., Kendrick, P. and Churchill, C. (2007). “The effect of stereo crosstalk on intelligibility: comparison of a phantom stereo image and a central loudspeaker source,” Journal of the Audio Engineering Society, 55, pp. 852-863.
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