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In Reply to: RE: "interfering early reflections" posted by KlausR. on December 27, 2007 at 00:57:00
Dealing with various points, out of order at times.
"Everest is basing his book on available research. I'm afraid that he has not read everything known in the field."
First, show me someone who has based a book on research that wasn't available, unless it was the first publication of their own research. As for not having read everything known in the field, I doubt that any individual has. I think you're trying to damn with faint praise while implying at the same time that you're in possession of unavailable research and have read everything known in the field.
What you ignore is that Everest's book is not intended as an acoustics textbook so much as a basic primer on acoustics theory which includes sufficient details to enable non-technical readers to treat home listening rooms or recording studios themselves. In fact, the introduction to the third edition, my copy, states "The purpose of this book remains the same: to apply the principles of acoustics to the audio arts." It's intended as a practical rather than a theoretical text. Citing a text written for the purpose of explaining why acoustic treatment is beneficial and how to go about it as evidence for the 'fact' that there are no problems and that treatment is therefore unnecessary as you have done simply has to count as misrepresentation as far as I'm concerned.
I also believe that Everest is much more well-read than you suggest. What is quoted/referenced in the Master Handbook is what he thinks is most relevant to the practical issues of room treatment for listening rooms and studios. It's not intended as a theoretical overview of the whole field and criticising it for not being what it was never intended to be while ignoring its actual purpose is, in my view, quite disingenuous on your part .
"His recommendation for reflection treatment is based on Olive's findings exclusively, as far as I remember. Olive investigates single reflections, not a complex sound field, he does not investigate what happens when the reflection is attenuated by x dB, and he certainly does not qualify the reflection as detrimental just because it is above the image shift threshold."
Well, I quoted Everest about 2 responses ago as contradicting these claims about Olive in a statement which says ""How about the two-loudspeaker stereo installation? Olive and Toole investigated this and found that the effects obtained from a single loudspeaker are directly applicable to the stereo case. This means that the information presented earlier is immediately available for application to stereo listening situations such as the home music-reproduction room."
As for not investigating what happens when the reflection is attenuated by x dB, Olive and Toole state what is perceived above and below certain thresholds. It's not a matter of asking what happens if the reflection is attenuated by a given number of decibels. The issue is what happens when reflections above a certain threshold are attenuated sufficiently to reduce their level below that threshold and that information is actually there in the data, clearly stated in the changes that occur at each threshold level.
Whether or not a given perceptual change is detrimental does not depend on the nature of the change but rather on whether the change suits our purposes or not. What is detrimental for one purpose may well be beneficial for another. I'm not surprised the changes aren't categorised in terms of benefit or detriment.
"I add a short passage of my writeup:
"It has been found that normal reflections in typical living rooms do not interfere with perception of the recorded space (Olive et al. 1989) and that for music enjoyment reflective walls have better effect (Kishinaga 1979).""
Well, I don't see that as an accurate summary of the Olive and Toole paper which documents changes in spatial perception based on the length of delay for a first reflection and the level of the reflection relative to that of the direct sound, changes which when the reach the point of broadening and spreading images certainly do interfere with the perception of the recorded space as the recording engineer intended it to be heard. Everest also cites data from Mankovsky on the influence of reverberation time in the listening room on the perception of reverberation from the recording venue contained in recordings. The reverberation characteristics of the listening room do affect the perception of the reverberation contained in recordings. Since reverberation time is a reflection phenomena, I'd count that reference as contradicting your summary statement as well.
With respect to Kishinaga I would have to ask what better effect and which aspect of enjoyment is improved. I'd also like to know what proportion of subjects felt that there was an improvement and how much. I'm sorry but your summaries always tend to conceal the really interesting meat of the information for me. You seize on the single aspect that suits your argument and leave out the information about the nature of the effect, how much it tended to be preferred and by what proportion of people etc, presenting your summary as a complete statement of the result with the implication that it applies equally to everyone. Since enjoyment is related in part to the satisfaction of preferences I would expect there to have been some range of variation in the individual responses of subjects and for not all of them to have found reflective walls as beneficial as others and even for some to have found it not beneficial.
"When you apply reflection treatment you are changing the spectrum of the sound at the listening position, and maybe the spatial impression. If you like a different spectrum, that's ok but what does that prove? If you like a different spatial impression, that's ok but what does it prove?"
Once again you slant things. Yes, reflection treatment CAN change the spectrum of the sound at the listening position. It need not. Reflections themselves change the spectrum depending on the absorption/reflection characteristics of the surface and no surface reflects exactly the same spectrum of sound as that of the direct sound. Depending on the absorption characteristics of the reflection treatment, the spectrum of the reflected sound may be closer to that of the direct sound than that of the untreated wall, closer to that of the wall than of the direct sound, or different to both. The end result will vary depending on the characteristics of the reflection treatment but it need not change the spectrum of the sound and, if it is sufficiently broad band, it may well be closer to the spectrum of the direct sound and preserve the tonal character of the recording more than the sound without reflection treatment does.
You continually present the data in a way that suggests that reflections in normal rooms don't change the character of the sound when they invariably do. They may not change it sufficiently to cause colouration so gross as to be objectionable but they do still colour the sound and appropriate treatment can certainly reduce that colouration.
Olive and Toole's paper shows that reflections can certainly degrade spatial aspects of the sound, causing images to broaden and spread. Treatment can prevent or reduce that.
So we get to your questions "If you like a different spectrum, that's ok but what does that prove? If you like a different spatial impression, that's ok but what does it prove?" Certainly it's ok to like a different spectrum or spatial impression if we want to look at things purely from a matter of preference as many do, but what if someone wants to hear the spectrum and spatial impression captured on the recording rather than the spectrum and spatial impression generated by their room, especially if the colouration and spatial shifts introduced by their room are noticeable?
Just the way you frame the question implies that the room is neutral and that anyone who wants something different than the sound heard in their untreated room is after has to be aiming to get a sound that is more personally satisfying while deviating from the character of the original sound in some way. For some people that is certainly the case but for some it is not and I suspect that for those who have heard the benefits that can be obtained with treatment, heard sound that is much closer to that captured on the recording than is available in most untreated rooms, what is being aimed for is not simply an individual preference but rather a more accurate representation of the character of the recording.
You routinely ignore the fact that each room is different and imparts its own character to the sound. Most rooms are "reasonable" in that they certainly don't produce unacceptably objectionable results on most music but some rooms certainly aren't as benign as others and some rooms certainly do produce objectional results. One would never know that from your summary and one would also never know that every room does colour the sound in some way.
One reads your summary and wonders why someone would consider acoustic treatment for their room because one never finds a mention of the fact that the room always alters the sound and that some rooms do so more than others. One never finds a mention that treatment can produce beneficial results. It's always the same old refrain "reflections don't cause problems" with no mention of what they do cause, that the degree of the effect varies from room to room, and that it can be reduced to enable the listener to hear something that is much closer to the sound captured on a recording than what they do hear in an untreated room.
As I said, I think you seriously misrepresent Everest when you simply say that there is no evidence that reflections cause problems and ignore the fact that the whole book is based around achieving beneficial results in home listening rooms and in recording studios through acoustic treatment. You misrepresent Olive and Toole when you say there is no evidence that reflections cause problems and ignore their data showing that the audible effects of reflections vary with level and the length of delay and can, in the most extreme case, be perceived as echoes which are certainly problems though, thankfully, it's rare to actually perceive an echo in a listening room. Broadening and spreading of images can most certainly be experienced in many rooms, however, and is certainly problematic for some listeners.
Klaus, I'm calling it quits. You continually oversimplify the issues in what I'm increasingly coming to view as a significant misrepresentation of the data. I know you don't agree with me on that but that's the conclusion I've slowly reached over a lengthy period of debating some of these issues with you. I've reached it because it almost always seems to me that what is concealed by your summaries or omitted from them is always more interesting and more vital than what is revealed.
I'm closing this discussion from my end.
David Aiken
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