Page 61 - Master Handbook of Acoustics

FIGURE 3-1 In the solid angle shown, as radius r is increased, the same sound energy is distributed
over spherical surfaces of increasing area. The intensity of sound is inversely proportional to the
square of the distance from the point source.

This sound is of uniform intensity (power per unit area) in all directions. The circles represent

spheres having radii in simple multiples. All of the sound power passing through the small square
area A1 at radius r also passes through the areas A2, A3, and A4 at radii 2r, 3r, and 4r, respectively.
The same sound power flows out through A1, A2, A3, and A4, but an increment of the total sound
power traveling in this single direction is spread over increasingly greater areas as the radius is
increased. Thus, intensity decreases with distance. This decrease is due to geometric spreading of the
sound energy and is not loss in the strict sense of the word.

Sound Intensity in the Free Field