Page 110 - The Master Handbook Of Acoustics

85
SOUND WAVES IN THE FREE FIELD

d

2d

3d

4d

FIGURE 4-1
In the solid angle shown, the same sound energy is distributed over spherical surfaces
of increasing area as d is increased. The intensity of sound is inversely proportional to
the square of the distance from the point source.

d 2
L 2 L 1 20 log , decibels (4-1)
d 1
In other words, the difference in sound-pressure level between two
points that are d 1 and d 2 distance from the source is:

d 2
L 2 L 1 20 log , decibels (4-2)
d 1
For example, if a sound-pressure level of 80 dB is measured at 10 ft,
what is the level at 15 ft?