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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: RE: How do you explain the audibility of sound effects based on making comb filters? posted by morricab on December 7, 2007 at 08:03:31:
>It doesn't matter if it is added to the signal before it comes out of the speakers or as a result of constructive/destructive interference of waves generated by the speakers because all of that is happening before it reaches your ears.<
Apparently it does make a difference otherwise any sound in any room, including the best concert halls, would be quite unpleasant. In any case, you cannot use an electronic flanger with changing delay to prove that naturally occuring comb filters with fixed delay produce distortion. You cannot use your case of the Acoustats because your only hear the effect when you move so that effect is not part of the spectral envelope.
>In a symmetrical room you should get basically the same pattern from reflections for both the left and right channels should you not?<
Yes. The issue with early reflection comb filters is that each reflection interacts with the direct sound and with the other reflections. Human hearing integrates all of this, uses auditory filters, binaural decoloration etc. A single reflection is more audible than a one reflection among others. The more reflections you have the less audible is colorration (with artificial signals). The less regular spaced the reflections are on the time scale the less audible is coloration (for artificial signals).
When you use microphones to measure the comb filters you will get two different responses at the ear positions. These two responses are not perceived as two auditory events but are combined and processed in combination by human hearing. THAT makes a huge difference as compared to how two microphones work.
>Tell me then how the comb filter you are describing is different from the one generated by my Acoustats?<
The "comb filters" of your speakers are only audible when you move your head, aren't they? Comb filters I'm talking about are present without moving you head, they are part of the power spectrum contour (Seki et al. (2003), “Coloration perception depending on sound directionâ€, IEEE Transactions on Speech and Audio Processing, vol. 11, no.6, p.817).
>My only point is that a comb filter IS audible and if one cannot hear such a thing that is generated from wall reflections then perhaps it is a red herring and there is no appreciable comb filter generated in the first place!<
Some comb filters like flangers (with changing delay) are audible and some like early reflections are not. Your Acoustat comb filters are not spectral comb filters because you have to move your head. Standing wave patterns are no spectral comb filters because you only hear them when you move around the room.
You can measure early reflection combs quite easily, Ethan Winer shows graphs on his site "to prove the case" while forgetting very conveniently that humans have two ears and that microphones don't work like human hearing. Floyd Toole concludes in one of his papers that early reflection comb filters are a measurement issue only. In this particular case measurements are misleading. Like you said, a red herring.
>I might believe that early reflections do not generate any appreciable comb filter effect but if there is evidence that early reflections do indeed make deep nulls in the frequency response then I would say that the effect at least COULD be audible depending on what frequencies those nulls occur.<
Comb filters occur when a sound (direct sound) is mixed with its repetition (reflection). Peak and dip frequencies of the comb structure depend on the delay. With artifical signals you can easily measure and obtain a nice graph of a comb. What will happen when you play a piece of music, will you still get a clean comb structure? And still, there's that difference between what you can measure and what you can hear!
Salomons (“Coloration and binaural decoloration of sound due to reflectionsâ€, Thesis, Delft University 1995) has determined thresholds of audibility taking into account auditory filters. She did this for noise, not music.
To the best of my knowledge there is no evidene whatsoever that reflection based comb filters produce audible coloration when playing music through loudspeakers. If there was such evidence don't you think that the proponents of the "early reflections produce audible comb filters" theory would happily base their case on such evidence? Fact is that they don't and prove their case with non relevant stuff such as flangers or measurements that do not take into account human hearing mechanisms.
You are welcome to read my draft.
Klaus
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Follow Ups
- RE: How do you explain the audibility of sound effects based on making comb filters? - KlausR. 22:59:25 12/07/07 (2)
- RE: How do you explain the audibility of sound effects based on making comb filters? - morricab 02:16:20 12/10/07 (1)
- RE: How do you explain the audibility of sound effects based on making comb filters? - KlausR. 03:52:02 12/10/07 (0)
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