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2-9
Speakers are essentially “point sources” of sound. Sound radiates outward in all directions,
creating a spherical sound pattern. The sound pressure is spread over an increasingly larger
surface area as the sound moves away from the source. This causes a drop in loudness per unit
area. The drop in SPL is referred to as the “Inverse Square Law,” and originates from the fact that
as the diameter of the sound-sphere doubles, the surface area increases by a factor of four. This
behavior of outwardly radiating sound causes a drop in SPL of –6 dB per doubling of distance.
You can calculate the change in SPL at any distance from a speaker as follows:
D1 D2
Δ dB
spl
= 20 log
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
2
1
D
D
Equation 2-8. The Inverse Square Law
The figure below illustrates how SPL decreases with distance as you move away from a speaker:
Figure 2-3. dB and Distance Chart
Continued on next page
Speaker Basics
Inverse Square Law