Page 34 - The Master Handbook Of Acoustics

9
FUNDAMENTALS OF SOUND

Sound in Free Space
The intensity of sound decreases as the distance to the source is
increased. In free space, far from the influence of surrounding objects,
sound from a point source is propagated uniformly in all directions.
The intensity of sound decreases as shown in Fig. 1-8. The same sound
power flows out through A1, A2, A3, and A4, but the areas increase as
the square of the radius, r. This means that the sound power per unit
area (intensity) decreases as the square of the radius. Doubling the dis-
tance reduces the intensity to one-fourth the initial value, tripling the
1
1
distance yields 9, and increasing the distance four times yields 16 of

A4
A3
A2
A1

r

2r

3r

4r

FIGURE 1-8
In the solid angle shown, the same sound energy is distributed over spherical surfaces
of increasing area as r is increased. The intensity of the sound is inversely proportional
to the square of the distance from the point source.