|AndreasN wrote on Tue, 26 October 2010 13:44|
In any case, the situation is one where positive pressure is cancelled with negative pressure, and vice versa.
Not so...Attenuation of signal is different than active cancellation. [/quote]
|JohnM wrote on Tue, 12 October 2010 17:10|
"Flat" is a relative term, if you use an electron microscope is anything flat?
|AndreasN wrote on Tue, 12 October 2010 15:11|
I've also looked at this aspect in several different room measurement. The only way I can find any part of the excess group delay to "be flat" is by not looking hard enough. As I zoom in, the response observed is never flat in any region of any room responses.
Now we have something we can work with. Anywhere the excess group delay plot is flat is a minimum phase region of the response.
This situation is different. If looking at a pure minimum phase response, the excess phase plot is absolutely flat no matter how far one zooms in.
"Being sorta minimum phase" is obviously not the same as being minimum phase. At what point is the excess phase plot flat enough? 10 millisecond deviation? 1 millisec? 0.1 ms? It doesn't seem to hold up to scrutinous analysis.
As any phenomenon related to perception, there is not absolute standard. In addition, the hearing process ignores phase, although some effects of phase are perfectly audible (comb filtering, precedence effect, image shift...).
At what point is the deviation from flat response audible, 0.5dB, 1dB, 3dB? Even that is not simple. A 20dB cut of 1/100 octave is inaudible for 9 persons out of 10.
The whole idea is to break the room response in a number of factors, some being minimum-phase, the others being non minimum-phase, and to apply some EQ to counteract the minimum-phase ones. For the others, the solutions are not electronic.
I think you are taking a too extreme view of this issue. We know that it's impossible to perfectly cancel out the imperfections in a room, but there are two attitudes facing this dilemma:
one is to try and make the best of what is available, the other is, like you, say that it's impossible and do nothing about it.