Page 232 - Master Handbook of Acoustics

FIGURE 11-16 Permissible bass rise of reverberation time for voice studios derived by subjective

evaluation in controlled tests by BBC researchers. (Spring and Randall.)

Bass rise in reverberation time for music performance has traditionally been accepted to add
sonority and warmth to the music in concert halls. A bass rise may help overcome the ear’s relative
insensitivity at low frequencies, or may simply be a cultural preference. Presumably, a somewhat
greater bass rise than that for speech would be desirable in listening rooms designed for classical
music. One metric used to define a bass boost is the bass ratio (BR), where BR = (RT 60/125 +

RT 60/250 )/(RT 60/500 + RT 60/1,000 ). In other words, the sum of RT at 125 and 250 Hz is divided by the
60
sum of RT at 500 and 1,000 Hz; a value greater than unity shows that reverberation time is longer at
60
low frequencies. Some designers recommend a bass ratio of 1.1 to 1.45 for halls with a reverberation
time less than 1.8 seconds, and a bass ratio of 1.1 to 1.25 for halls with a longer reverberation time.
A bass ratio of less than 1.0 is not recommended. This is explored more fully in Chap. 26.

Initial Time-Delay Gap

One important characteristic of natural reverberation in concert halls was revealed by Beranek’s
study of halls around the world. At a given seat, the direct sound arrives first because it follows the

shortest path. Shortly after the direct sound, the reverberant sound arrives. The time between the two
is called the initial time-delay gap (ITDG), as shown in Fig. 11-17. If this gap is less than about 40
msec, the ear integrates the direct and the reverberant sound successfully. In addition to all of the
reflections responsible for achieving reverberation density, the initial time-delay gap is another
important delay that must be considered. In particular, this gap is important in concert-hall design

(and in artificial reverberation algorithms) because it is the cue that gives the ear information on the
size of the hall.

FIGURE 11-17 The introduction of an initial time-delay gap plays an important role in room
reverberation. That time gap between arrival of the direct sound, and the first reflected sounds, helps