Page 172 - The Master Handbook Of Acoustics

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REVERBERATION

The combined decay departs somewhat from a straight line.

If one room has a very short reverberation time, the combined
reverberation time will be very close to the longer one.

If the reverberation time of each of the two rooms alone is the
same, the combined reverberation time is 20.8% longer than one
of them.
The character and quality of the sound field transmitted by a
stereo system conforms more closely to the mathematical assump-
tions of the above than does a monaural system.
The first five items can be applied to the case of a studio linked
to an echo chamber as well as a studio linked to a listening room.

Decay Rate

The definition of reverberation time is based on uniform distribution of
energy and random directions of propagation. Because these conditions
do not exist in small rooms, there is some question as to whether what
we measure should be called reverberation time. It is more properly
termed decay rate. A reverberation time of 0.3 of a second is equivalent
to a decay rate of 60 dB/0.3 sec = 200 dB per second. The use of decay
rate instead of reverberation time would tell the experts that we are
aware of the basic problems. Speech and music sounds in small rooms
do decay even though the modal density is too low to hang the official
“reverberation time” tag on the process.

Eliminating Decay Fluctuations

The measurement of reverberation time by the classical method that
has been described involves the recording of many decays for each con-
dition and much work in analyzing them. Schroeder has published a
new method by which the equivalent of the average of a great number
of decays can be obtained in a single decay. One practical, but clumsy,
5
method of accomplishing the mathematical steps required is to:
1. Record the decay of an impulse (noise burst or pistol shot) by the
normal method.
2. Play back that decay reversed.
3. Square the voltage of the reversed decay as it builds up.