Tyco 579-769 Speaker Basics, ⎝ D2, Equation 2-8.The Inverse Square Law, 3.dB and Distance Chart

Speaker Basics

Inverse Square Law	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	⎞
⎜		⎟
Δ dBspl = 20 log ⎜		⎟
⎝ D2		⎠

D1 D2

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

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