Page 223 - Master Handbook of Acoustics

reverberation time.
In small and relatively dead rooms such as many studios, control rooms, and home listening rooms,
the direct sound from the source usually dominates. A true reverberant field may be below the

ambient noise level. However, the reverberation time equations have been derived for conditions that
exist only in the reverberant field. In this sense, then, the concept of reverberation time is
inapplicable to small, relatively dead rooms. We measure something very much like the reverberation
time in large, more live spaces. But in small, dead rooms, more accurately, what we measure is the
decay rate of the normal modes of the room.
Each axial mode decays at its own rate determined by the absorbance of a pair of walls and their

spacing. Each tangential and oblique mode has its own decay rate determined by distance traveled,
the number of surfaces involved, the variation of the absorption coefficient of the surfaces with angle
of incidence, and so on. Whatever average decay rate is measured for an octave of random noise will
be representative of the average decay rate at which that octave of speech or music signals would die
away. Although the applicability of computing reverberation time from the equations based on
reverberant field conditions might be questioned because of the lack of reverberant field, the

measured decay rates most certainly apply to this space and to these signals.

Acoustically Coupled Spaces

The shape of a reverberation decay can reveal acoustical problems in the space. One common effect

that alters the shape of the decay is due to acoustically coupled spaces. This is quite common in large
public gathering spaces but is also found in offices, homes, and other small spaces. The principle
involved is illustrated in Fig. 11-11. In this example, the main space, perhaps an auditorium, is
acoustically quite dead and has a reverberation time corresponding to slope A. An adjoining hall with
hard surfaces opens into the main room and has a reverberation time corresponding to slope B. A
person seated in the main hall near the connecting opening could very well experience a double-slope

reverberation decay. After the sound level in the main room falls to a fairly low level, the main room
reverberation would be dominated by sound fed into it from the slowly decaying sound in the
adjoining hall. Assuming the reverberation time described by slope A is acoustically correct for the
main room, persons subjected to slope B would hear degraded sound.