Page 112 - The Master Handbook Of Acoustics

87
SOUND WAVES IN THE FREE FIELD

walls would be to make the change for a doubling of the distance
something less than 6 dB.
An awareness of the inverse square law is of distinct help in esti-
mating acoustical situations. For instance, a doubling of the distance
from 10 to 20 feet would, for free space, be accompanied by the same
sound-pressure level decrease, 6 dB, as for a doubling from 100 to 200
feet. This accounts for the great carrying power of sound outdoors.

Inverse Square in Enclosed Spaces

Free fields exist in enclosed spaces only in very special and limited
circumstances. The reflections from the enclosing surfaces affect the
way sound level decreases with distance. No longer does the inverse
square law or the inverse distance law describe the entire sound field.
For example, assume that there is an installed loudspeaker in an
enclosed space that is capable of producing a sound-pressure level of
100 dB at a distance of 4 ft. As shown in the graph of Fig. 4-3, free field

100
Inverse square
(6 dB/dist. double) distance
Critical
Sound-pressure level - dB 90 Direct sound Reverberant

sound level

80

70
4 10 20 30 50 100
Distance - ft
FIGURE 4-3
Even in an enclosed space the inverse square law is followed close to the source. By
definition, the critical distance is that distance at which the direct sound pressure is
equal to the reverberant sound pressure.