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Author Topic: Minimum phase and acoustics  (Read 14015 times)

Bogic Petrovic

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Re: Minimum phase and acoustics
« Reply #45 on: October 26, 2010, 11:16:36 pm »

AndreasN wrote on Tue, 26 October 2010 20:44

........
Another strange thing is that all the excess phase plots I've looked at, from real rooms, tends to be flatter in the high end than the low end. If one is to take this literally, EQ'ing high end should be better than EQ'ing low end.. We know it doesn't work that way.
........

I generally agree with you, and I believe that reason for this lies in fact that it's much easier to absorb and/or diffuse high frequencies than low frequencies. (when we talk about reflections)

From my experience classic absorption and diffusion in rooms is a way to make acoustical response in room with somewhat less non-minimum phase behavior...

Idea is: even if we can't get a flat response in that way, with classical acoustic treatment, we can "kill" most of non-minimum phase room behavior ... and rest of treatment can be done with IIR equalization (as we discuss in this thread)

I'll show you measurements from one tiny room we're doing recently. Measurements isn't done by REW but we imported wav impulses and doing analysis.

index.php/fa/15702/0/


Unfortunately I don't have measurements without acoustical treatment, but I'm sure that you can use your imagination and/or your experience for understanding what response you can get from empty room with dimensions 3.56x3.67x2.55m (i repeat, this is a dimensions of an empty room)

I hope this helps for an understanding of acoustical room behavior and importance of classical acoustic room treatment, where signal level room equalization, can continue to improve response, when classical acoustical methodologies reach his limits in very small rooms.



regards
boggy

p.s. i can't do analysis without smoothing... REW ignore my attempts to switch off smoothnig... best i can get is 1/48 octave smoothing

JohnM

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Re: Minimum phase and acoustics
« Reply #46 on: October 27, 2010, 04:01:42 am »

Bogic Petrovic wrote on Wed, 27 October 2010 04:16

p.s. i can't do analysis without smoothing... REW ignore my attempts to switch off smoothnig... best i can get is 1/48 octave smoothing
Boggy, in the REW Analysis Preferences uncheck the "Allow 96PPO Log spacing" box. That option allows REW to convert measurements to a 96PPO log spaced format if that reduces storage, but requires that the measurements be filtered to 48PPO. If you uncheck the box and just re-apply the IR windows you should get an unsmoothed response.
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Bogic Petrovic

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Re: Minimum phase and acoustics
« Reply #47 on: October 27, 2010, 07:02:12 am »

JohnM wrote on Wed, 27 October 2010 10:01

.........Boggy, in the REW Analysis Preferences uncheck the "Allow 96PPO Log spacing" box. That option allows REW to convert measurements to a 96PPO log spaced format if that reduces storage, but requires that the measurements be filtered to 48PPO. If you uncheck the box and just re-apply the IR windows you should get an unsmoothed response.


Thank you very much John, here are unsmoothed results:
index.php/fa/15703/0/

regards

Boggy

Geoff Emerick de Fake

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Re: Minimum phase and acoustics
« Reply #48 on: October 27, 2010, 01:29:07 pm »

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]
AndreasN wrote


JohnM wrote on Tue, 12 October 2010 17:10

AndreasN wrote on Tue, 12 October 2010 15:11

Quote:

Now we have something we can work with. Anywhere the excess group delay plot is flat is a minimum phase region of the response.
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.
"Flat" is a relative term, if you use an electron microscope is anything flat?


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.
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