Page 154 - The Master Handbook Of Acoustics

CHAPTER
7

Reverberation

ressing the gas pedal of an automobile results in acceleration of the
Pvehicle to a certain speed. If the road is smooth and level, this speed
will remain constant. With this accelerator setting the engine produces
just enough torque to overcome all the frictional losses and a balanced
(steady-state) condition results.
So it is with sound in a room. When the switch is closed, a loud-
speaker arranged to emit random noise into a room will produce a
sound that quickly builds up to a certain level. This is the steady-state
or equilibrium point at which the sound energy radiated from the
loudspeaker is just enough to supply all the losses in the air and at
the boundaries of the room. A greater sound energy radiated from the
loudspeaker will result in a higher equilibrium level, less power to
the loudspeaker will result in a lower equilibrium level.
When the loudspeaker switch is opened, it takes a finite length of
time for the sound level in the room to decay to inaudibility. This
“hanging-on” of the sound in a room after the exciting signal has been
removed is called reverberation and it has a very important bearing on
the acoustic quality of the room.
In England, a symphony orchestra was recorded as it played in a
large anechoic (echo-free) chamber. This music, recorded with almost
no reverberation for research purposes, is of very poor quality for nor-
mal listening. This music is even thinner, weaker, and less resonant
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