The Inverse Square Law holds true
when the sound field is sufficiently far from the source that intensity is proportional to
the square of the sound pressure. This field, known as the acoustic far-field,
is quickly developed in open, free-field circumstances.
Nevertheless, there exists a near
field very close to the sound source (see Figure 4), which is generally within one
wavelength of the lowest sound frequency produced by the source. Within this distance,
sound pressure level varies widely and measurements are not meaningful. The shaded areas
in Figure 4 illustrate the possible variation in sound pressure levels with position.
Figure 4 Variation in Sound
Pressure Level with Distance from Source