However, practical sound sources are not infinitely small points. With an ideal point source this relation holds for any distance. The sound energy per unit area (called sound intensity) is inversely proportional to the square of the distance from the source, so this relationship has come to be called the “inverse-square law.” The sound level correspondingly falls rapidly, at a rate of −6 dB/double-distance). The sound energy is distributed uniformly over a spherical surface that, as a function of distance, experiences a rapid increase in area over which the sound energy is distributed. Figure 10.9 shows the situation for point sources and combinations of “point-like” sources: loudspeaker drivers in a box.įigure10.9a shows an ideal point source radiating sound equally in all directions in free space.
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