Page 174 - The Master Handbook Of Acoustics

149
REVERBERATION

arbitrary level of 0 dB at time = 0, after which it decays according to the
reverberation time of the room, which is assumed to be 0.5 seconds (60
dB decay in 0.5 seconds). The “ck” consonant sound, peaking 0.32 sec-
onds later, is 25 dB below the “ba” sound peak. It too decays at the same
rate as the “ba” sound according to the assumed 0.5-second RT60.
Under the influence of the 0.5-second reverberation time, the “ck” con-
sonant sound is not masked by the reverberation time of “ba.” If the
reverberation time is increased to 1.5 seconds, as shown by the broken
lines, the consonant “ck” is covered completely by reverberation.
The primary effect of excessive reverberation is to impair the intelli-
gibility of speech by masking the lower level consonants. In the word
“back,” the word is unintelligible without a clear grasping of the “ck”
part. Understanding the “ck” ending is the only way to distinguish
“back” from bat, bad, bass, ban, or bath. In this oversimplified way, we
can grasp the effect of reverberation on the understandability of
speech and the reason why speech is more intelligible in rooms hav-
ing lower reverberation times.
Sound-reinforcement engineers have been assisted greatly by
6
7
the work of the Dutch investigators, Peutz and Klein. Because of their
work, it is a straightforward procedure to predict with reasonable
accuracy the intelligibility of speech in a space from geometrical fac-
tors and a knowledge of reverberation time.

Influence of Reverberation on Music

The effect of hall “resonance” or reverberation on music is intuitively
grasped but is not generally well understood. This subject has received
much attention from scientists as well as musicians, and the final word
has yet to appear. Beranek has made a valiant attempt to summarize
present knowledge and to pinpoint essential features of concert and
opera halls around the world, 8,9 but our understanding of the problem
is still quite incomplete. Suffice it to say that the reverberation decay
of a music hall is only one important factor among many, another being
the echo pattern, especially the so-called “early sound.” It is beyond the
scope of this book to treat this subject in any detail, but an interesting
point or two commonly overlooked are discussed briefly.
Normal modes have been considered in some detail because of their
basic importance (see Chap. 15). They are also active in music halls