Page 204 - Master Handbook of Acoustics

CHAPTER 11

Reverberation

I f you press the accelerator pedal of an automobile, the vehicle accelerates to a certain speed. If

the road is smooth and level, this speed will remain constant. With a constant force on the

accelerator, the engine produces enough horsepower to overcome frictional and aerodynamic losses,
and a balanced (steady-state) condition results. If you take your foot off the accelerator, the car will
gradually slow, and come to a stop.
Sound in a room behaves similarly. When a loudspeaker is turned on, it emits noise in a room that
quickly grows to a certain level. This level is the steady-state or equilibrium point at which the sound
energy radiated from the loudspeaker is enough to overcome losses in the air and at the room

boundaries. A greater sound energy radiated from the loudspeaker will result in a higher equilibrium
level, whereas less energy to the loudspeaker will result in a lower equilibrium level.
When the loudspeaker is turned off, it takes a finite length of time for the sound level in the room to
decay to inaudibility. This aftereffect of the sound in a room, after the excitation signal has been

removed, is reverberation and it has an important bearing on the acoustical quality of the room.
A symphony orchestra recorded in a large anechoic chamber, with almost no room reverberation,
would yield a recording of very poor quality for normal listening. This recording would be even
thinner, weaker, and less resonant than most outdoor recordings of music, which are noted for the
emptiness of their sound. Clearly, symphonic and other music requires reverberation to achieve an
acceptable sound quality. Similarly, many music and speech sounds require a room’s reverberant

assistance to sound natural, because we are accustomed to often hearing those sounds in reverberant
environments.
Formerly, reverberation was considered the single most important characteristic of an enclosed
space for speech or music. Today, although it is still very important, reverberation is considered one
of several important and measurable parameters that define the sound quality of an acoustical space.

Growth of Sound in a Room

When a sound is initiated in a room, the room will contain the energy as it builds to a steady-state
value. The time required to reach this steady-state value is determined by the rate of growth of sound

in the room. In turn, this growth rate is determined by the level of energy at the source, and the room’s
acoustics.
Let us consider a source S and a listener L in a room, as shown in Fig. 11-1A. As source S is
instantaneously energized, sound travels outward from S in all directions. Sound travels a direct path
to the listener L and we shall consider zero time (Fig. 11-1B) as that time at which the direct sound

reaches the ears of the listener. The sound pressure at L instantly jumps to a value D less than that
which left S due to spherical divergence and small losses in the air. The sound pressure at L stays at