Page 160 - The Master Handbook Of Acoustics

135
REVERBERATION

7-3B is the same thing except that the ver-
tical scale is plotted in decibels, i.e., to a Sound source
logarithmic scale. On Off
During the growth of sound in the room,
power is being fed to the sound source.
During decay, the power to the source is
cut off, hence the difference in the shapes of A
the growth and decay curves. The decay Sound pressure
of Fig. 7-3B is a straight line in this ideal-
ized form, and this becomes the basis for
measuring the reverberation time of an Time
enclosure.

Reverberation Time

Reverberation time is defined as that time
required for the sound in a room to decay Sound pressure level, dB B
60 dB. This represents a change in sound
intensity or sound power of 1 million Time
(10 log 1,000,000 = 60 dB), or a change of
FIGURE 7-3
sound pressure or sound-pressure level
of 1,000 (20 log 1,000 = 60 dB). In very The growth and decay of sound in a room. (A)
Vertical scale in linear sound pressure units. (B) The
rough human terms, it is the time required vertical scale in logarithmic units (decibels).
for a sound that is very loud to decay to
inaudibility. W. C. Sabine, the Harvard pioneer in acoustics who intro-
duced this concept, used a portable wind chest and organ pipes as a
sound source, a stopwatch, and a pair of keen ears to measure the time
from the interruption of the source to inaudibility. Today we have
better technical measuring facilities, but we can only refine our under-
standing of the basic concept Sabine gave us.
This approach to measuring reverberation time is illustrated in
Fig. 7-4A. Using a recording device that gives us a hardcopy trace of
the decay, it is a simple step to measuring the time required for the
60-dB decay. At least it is simple in theory. Many problems are
encountered in practice. For example, obtaining a nice, straight decay
spanning 60 dB or more as in Fig. 7-4A is a very difficult practical
problem. Background noise, an inescapable fact of life, suggests that a
higher source level is needed. This may occur if the background noise