Page 175 - The Master Handbook Of Acoustics

150 CHAPTER SEVEN

and listening rooms. An interesting phenomenon is pitch change dur-
ing reverberant decay. In reverberant churches, organ tones have been
observed to change pitch as much as a semitone during decay. In
searching for an explanation for this phenomenon, two things have
been mentioned: shift of energy between normal modes, and the per-
ceptual dependence of pitch on sound intensity. There are problems in
both. Balachandran has demonstrated the physical (as opposed to psy-
chophysical) reality of the effect 10 using the Fast Fourier Transform
(FFT) technique on the reverberant field created by 2-kHz pulses. He
revealed the existence of a primary 1,992-Hz spectral peak, and curi-
ously, another peak at 3,945 Hz. Because a 6-Hz change would be just
perceptible at 2 kHz, and a 12-Hz change at 4 kHz, we see that the 39-
Hz shift from the octave of 1,992 Hz would give a definite impression
of pitch change. The reasons for this are still under study. The rever-
beration time of the hall in which this effect was recorded was about 2
seconds.

Optimum Reverberation Time

Considering the full range of possible reverberation times, there must
be an optimum time between the “too dry” condition of the outdoors
and anechoic chambers, and the obvious problems associated with
excessively long reverberation times in a stone cathedral. Such an opti-
mum does exist, but there is usually great disagreement as to just what
it is because it is a subjective problem and some differences in opinions
must be expected. The optimum value depends not only on the one
making the judgment, but also on the type of sounds being considered.
Reverberation rooms, which are used for measuring absorption coef-
ficients, are carefully designed for the longest practical RT60 to achieve
the maximum accuracy. The optimum here is the maximum attainable.
The best reverberation time for a space in which music is played
depends on the size of the space and the type of music. Slow, solemn,
melodic music, such as some organ music, is best served by long rever-
beration time. Quick rhythmic music requires a different reverberation
time from chamber music. No single optimum universally fits all types
of music, the best that can be done is to establish a range based on sub-
jective judgments of specialists.
Recording studios present still other problems that do not conform
to simple rules. Separation recording in which musical instruments are
recorded on separate tracks for later mixdown in general require quite