Page 162 - The Master Handbook Of Acoustics

137
REVERBERATION

Measuring Reverberation Time

There are many approaches to measuring the reverberation time of a
room, and many instant-readout devices are on the market to serve
those who have only a casual interest in reverberation effects. For exam-
ple, sound contractors need to know the approximate reverberation
time of the spaces in which they are to install a sound-reinforcement
system, and measuring it avoids the tedious process of calculating it.
The measurements can also be more accurate because of uncertainty
in absorption coefficients. Acoustical consultants (at least the old-
fashioned ones) called upon to correct a problem space or verify a care-
fully designed and newly constructed space, generally lean toward the
method of recording many sound decays. These sound decays give a
wealth of detail meaningful to the practiced eye.

Impulse Sound Sources

The sound sources used to excite the enclosure must have enough
energy throughout the spectrum to ensure decays sufficiently above the
noise to give the required accuracy. Both impulse sources and those giv-
ing a steady-state output are used. For large spaces, even small cannons
have been used as impulse sources to provide adequate energy, espe-
cially in the lower frequencies. More common impulse sources are
powerful electrical spark discharges and pistols firing blanks. Even
pricked balloons have been used.
The impulse decays of Fig. 7-5 for a small studio have been
included to show their appearance. The sound source was a Japanese
air pistol that ruptures paper discs. This pistol was originally intended
as an athletic starter pistol but failed to find acceptance in that area. As
3
reported by Sony engineers, the peak sound-pressure level at 1 meter
distance is 144 dB, and the duration of the major pulse is less than
1 millisecond. It is ideal for recording echograms, in fact, the decays of
Fig. 7-5 were made from impulses recorded for that very purpose.
In Fig. 7-5, the straight, upward, traveling part on the left is the
same slope for all decays because it is a result of machine limitation
(writing speed 500 mm/sec). The useful measure of reverberation is
the downward traveling, more irregular slope on the right side. This
slope yields a reverberation time after the manner of Fig. 7-4. Notice