Page 227 - Master Handbook of Acoustics

unsuitable for another type of music. In any case, the optimal reverberation time for music is both
variable and subjective. This topic has received much attention from scientists as well as musicians.
Beranek attempted to summarize knowledge and to pinpoint essential characteristics of concert and
opera halls around the world, but our understanding of the problem is still incomplete. Suffice it to

say that the reverberation decay of a music hall is an important factor among many, another being the
echo pattern of the early sound of a hall. It is beyond the scope of this book to treat this subject in
great detail, but some commonly overlooked points are discussed briefly.
Normal modes have great basic importance in any room’s acoustical response, and they are also
influential in concert halls and listening rooms. An interesting phenomenon is pitch change during
reverberant decay. For example, 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
factors have been suggested: shift of energy between normal modes and the perceptual dependence of
pitch on sound intensity. Balachandran demonstrated the physical (as opposed to psychophysical)
reality of the effect using the fast Fourier transform (FFT) technique on a reverberant field created by
2-kHz pulses. In his case study, he showed the existence of a primary 1,992-Hz spectral peak, and
curiously, another peak at 3,945 Hz. Because a 6-Hz change would be just perceptible at 2 kHz, and a

12-Hz change perceptible at 4 kHz, we see that the 39-Hz shift from the octave of 1,992 Hz would
give an impression of pitch change. The reverberation 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, seemingly 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 as in a masonry cathedral. There is great
disagreement as to what the optimal value is in any given room because it is a subjective problem,
with diverse cultural and aesthetic expectations; thus differences in opinions must be expected. The

optimal value depends not only on the person making the judgment but also on the type of sound
sources being considered.
In general, long reverberation times yield a lack of definition in music and loss of intelligibility in
speech. Spaces for speech particularly require shorter reverberation times than for music because of
the importance of the clarity provided by direct sound. In dead spaces in which reverberation time is
very short, loudness and tonal balance in music may suffer. It is not possible to specify precisely

optimum reverberation times for different services, but Figs. 11-13 to 11-15 show approximate
recommendations given by experts who do not always agree with each other.