Page 178 - The Master Handbook Of Acoustics

153
REVERBERATION

4

Reverberation time - seconds 3 2 Auditoriums recording Recording studios
Speech and

for speech
1

Television studios

0
0 10,000 100,000 1,000,000
Volume - cubic feet
FIGURE 7-15

Spaces designed for speech and music recording require shorter reverberation times.

especially at low frequencies. Adjustment of reverberation time at high
frequencies is easily accomplished by adding or removing relatively
inexpensive absorbers. At low frequencies, the situation is quite different
as absorbers are bulky, difficult to install, and sometimes unpredictable.
Researchers at the British Broadcasting Corporation observed that
subjective judgments seemed to indicate a tolerance for a certain amount
of bass rise of reverberation time. Investigating this in controlled tests,
Spring and Randall 11 found that bass rise to the extent indicated in
Fig. 7-16 was tolerated by the test subjects for voice signals. Taking the
1 kHz value as reference, rises of 80% at 63 Hz and 20% at 125 Hz were
found to be acceptable. These tests were made in a studio 22 × 16 × 11
feet (volume about 3,900 cu ft) for which the midband reverberation
time was 0.4 second (which agrees fairly well with Fig. 7-15).
Bass rise in reverberation time for music has traditionally been
accepted to give “sonority” to the music in music halls. Presumably,
somewhat greater bass rise than that for speech would be desirable in
listening rooms designed for classical music.