Page 222 - Master Handbook of Acoustics

FIGURE 11-10 An example of a room’s reverberation characteristic before and after room treatment.

A significant rise in reverberation time in the upper bass and lower midrange is changed to a flatter
characteristic with a moderate increase in reverberation time at low frequencies.

Reverberation Time Variation with Position

In most rooms, there is enough variation of reverberation time from one physical position to another
to justify taking measurements at several positions. The average then gives a better statistical picture
of the behavior of the sound field in the room. If the room is symmetrical, it may be efficient to spot
all measuring points on one side of the room to increase the effective coverage with a given effort.

Decay Rate and the Reverberant Field

The definition of reverberation time is based on uniform distribution of energy and random directions

of propagation. These idealized conditions do not exist in small, absorptive rooms because of room
mode effects, so what we measure, technically, should not be called reverberation time. It is more
properly termed a decay rate. For example, a reverberation time of 0.3 second is equivalent to a
decay rate of 60 dB/0.3 sec = 200 dB/sec. Speech and music in small, absorptive rooms certainly
decay, even though the modal density is too low to meet the requirements of the definition of