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Re: simulated reverberation

Dear John,

It is difficult to obtain and silence "real" rooms for testing, and I have
used simulated room impulse responses.  Still, I find myself always
referencing back to the real room where a subject can sit and listen to
real reverberation.  I feel the real room is still the best.

But simulations offer much more flexibility in testing and I've found the
image method, Allen (1979), to give approximately equivalent speech
intelligibility results to a real room (no additive noise).  I simplified
with an omnidirectional source, did not use frequency specific absorption,
nor did I include the digitization correction as proposed by Peterson
(1986).  There is a slightly artificial quality to the speech, but not too
bad, I've considered adding a bit of randomization to the impulses to try
to get rid of this artificial sound, but haven't tryed it yet.  The
frequency specific absorption might also help.

Still, this may be more accurate than your test requires.  Depending on
what you are interested in it may be acceptable to use exponentially
decaying noise for the impulse response (or at least beyond the first 30
ms, depending on your room's dimensions).  I believe Pierce's book on the
fundamentals of acoustics has echo density as a function of time.  It has
even been suggested that weighted masking noise simulates reverberation
for speech, see Gelfand (1979), but this is likely an oversimplification.
If speech is your interest the idea of noise replacing reverberation is
further addressed in Nabelek's (1989) work.

As for the moving air, this will have a minimal contribution provided the
air velocity is much less than the speed of sound.  Perhaps you have heard
that the motion of the speaker or listener will drastically change the
impulse response, which is true. As will temperature and humidity changes
(this is problem for signal processes trying to cancel reverberation.) In
the above cases I would propose that the impulse response in nonstationary
or time dependent, but still linear.

No matter what method, it will never be perfect.  In my mind the question
is, what aspects need to be included given what the test is trying to
determine [intelligibility, quality, localization, music, environmental
sounds?] The following may or may not need to be considered.

-reverberation time
-room dimensions
-listener position, velocity, head rotation, clothing
-absorption coefficients four walls, floor, ceiling. (freq. dependent?)
-source position (inside/outside critical distance)
-source directivity
-objects within the room
-temperature humidity
-time dependence

Hope this will stimulate some thoughts,
Brad Libbey
Georgia Institute of Technology

Allen, J. B., and Berkley (1979).  "Image Method for Efficiently
Simulating Small-Room Acoustics." J. Acoust Soc. Am. 65:  943-950.

Gelfand, Stanley A. and Shlomo Silman. (1979).  "Effects of small room
reverberation upon the recognition of some consonant features."  J.
Acoust. Soc. Am., 66: 22-29.

Nabelek, Anna K. , Tomasz R. Letowski, and Frances M. Tucker. (1989).
"Reverberant overlap- and self-masking in consonant identification."  J.
Acoust. Soc. Am., 86: 1259-1265.

Peterson, P. M. (1986). "Simulating the Response of Multiple Microphones
to a Single Acoustic Source in a Reverberant Room." J. Acoust Soc. Am. 80:

Pierce, Allen D. (1991). Acoustics, and Introduction to Its Physical
Principles and Applications, Woodbury, NY: Acoustical Soc. of America.