Room Acoustics Forum by Rives Audio

RE: applicability of the Olive and Toole paper to stereo situations

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In Reply to: RE: Olive and Toole specifically say in the paper – no, they don’t!!! posted by David Aiken on January 13, 2008 at 14:09:06

>Accepting that the tests in the paper did not involve stereo and that Olive and Toole did not research the stereo issue before writing the paper does not make Everest wrong. Everest wrote his book after the paper. My edition is 1994 and that would give them a 5 year window in which to have looked at it and reported on that situation while allowing Everest to be correct. You can't use a cut it off date of the original paper in this case.<

We are now 18 years after Olive’s paper and Toole has published his literature review in JAES 2006: had there been any evidence gathered by Toole and co-workers, he would have mentioned it in his AES paper. However, he did not mention anything at all, neither from himself nor from others. Of course it is possible that Toole & Co did some research on stereo systems, but how likely is it that they kept that in a drawer? In none of his numerous publications Toole mentions anything about spatial aspects in stereo systems. So frankly, I don’t give Everest the benefit of the doubt. I have the 2001 edition of his book, the passages w.r.t this issue haven’t changed and he still cites only Olive’s 1989 paper, nothing more.

>1- Everest is correct that the 1989 paper can be applied to stereo situations and there is evidence to support that view but it's not contained in a paper you've read. You've criticised Everest before for not citing as many documents as you've read but can you honestly say you've read every document there is? I don't think you can categorically state that Everest is wrong, just as I can't categorically state that he is right.>

If there is evidence then why on earth doesn’t Everest cite it?

As I said, if you look at the issue you will see that Everest is inherently wrong: with a single loudspeaker/single reflections situation the image will spread or shift towards that reflection when the reflection level is above threshold.

With a stereo system and two lateral reflections, towards what is the image supposed to shift?????

You have two phantom sources between the loudspeakers, one generated by the direct sound, one generated by the two lateral reflections. A very different situation. You only will have image shift when the reflection phantom source is not identical with the direct sound phantom source. So Everest’s phrase that Olive’s findings are directly applicable to stereo systems is plain wrong in this general sense. It may be correct under very specific conditions.

Image spread or sense of spaciousness depends on how similar/dissimilar the left and right ear signals are: the more they are dissimilar the better is the sense of spaciousness and for this there is evidence.

Just happened to find one piece of evidence that counters the general recommendation.

Ando, “Architectural Acoustics”, Springer 1998: In the chapter entitled “The IACC of a two-channel loudspeaker reproduction system” Ando writes:

“In order to realize a smaller value of the IACC (interaural cross-correlation coefficient) in an actual sound field, additional loudspeakers and/or reflectors in the listening room can be taken into consideration.

IACC: A high interaural cross-correlation is equivalent to high binaural similarity which results in low subjective preference (Schroeder “Binaural dissimilarity and optimum ceilings for concert halls: More lateral sound diffusion”, J. of the Acoustical Society of America, vol. 65, no. 4, p.958). Conversely, for more dissimilar ear signals (high binaural dissimilarity) the preference is high. Acoustic asymmetry lowers interaural cross-correlation which results in an increased sense of spaciousness and generally in increased preference (Ando 1977“Subjective preference in relation to objective parameters of music sound fields with a single echo, J. of the Acoustical Society of America, vol. 62, no.6, p.1463; Toole “Loudspeakers and rooms for sound reproduction – a scientific review”, J. of the Audio Engineering Society 2006, p.451)
.
I did not read every document there is but I started with those cited by Toole and I have dug deeper. I have searched journals like J. Audio Engineering Society, J. Acoustical Society of America, J. Sound and Vibration, J. Applied Acoustics, J. Acoustical Society Japan, J. Building Acoustics, Transactions of German Broadcast Engineers, IEEE Transactions on Speech and Audio Processing, Acoustical Science and Technology, I am currently browsing Acustica.

> Second, I consider the reasonableness of the claim. It seems reasonable to assume that things will behave similarly in a stereo situation and that the data can be used in that situation. I know of nothing to suggest that things won't work that way so while I may not have confirming research, I don't know of any that would disconfirm it.>

The simple fact that two loudspeakers generate a first phantom source and two lateral reflections generate a second phantom source, something which is not present in the single loudspeaker/single reflection situation, should be enough to disconfirm the assumption “that things will behave similarly in a stereo situation”. Feel free to ignore the obvious.

>On the other hand, I also have seen statements from a number of people who have followed Everest's recommendation and who found it worked in practice. I therefore tried it myself and found that it worked in practice for me as well. Each individual practical report may be anecdotal but they all support Everest and that counts for something too.<

Did any of these people examine in detail whether or not there were changes in spatial aspects like those described by Olive and Toole, i.e. image shift from a first to a second position? I do not doubt that absorbing reflections will have audible consequences, after all you are removing sound pressure level and you are probably also modifying the overall spectrum.

>In my view the expert opinion of a professional backed by consistent anecdotal reports should always outweigh the opinion of a well read amateur with no practical experience in the field and who seems to be the only person holding his particular view.<

Am I really the only person who is holding this view? I suggest that you read Floyd Toole’s 2006 AES paper. I think that when you look for experts in this discipline, Toole is THE man. And Toole says that reflections are not problems. All I did was read his paper and then look up the evidence he presents, and some more. All I do here is confirm the conclusion drawn by a man who has spent his entire life investigating these matters and who hence has first-hand experience. When asked to choose between Everest and Toole, I honestly believe that Toole is the better choice.

Klaus

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