Sound pressure drops 6 dB for every doubling of distance from a point source in free field

In a free (unobstructed, non-reverberant) field, the intensity of sound from a point source decreases with the square of the distance. Because dB is a logarithmic scale, a 4× power reduction equals –6 dB. Doubling the distance from a source reduces intensity by a factor of 4 (inverse-square law), so SPL drops by 6 dB. This applies only outdoors in free field conditions; reflective surfaces and reverberant fields alter the relationship at greater distances. At twice the distance you lose 6 dB; at 10× the distance you lose 20 dB. This rule is essential for estimating loudspeaker coverage area and amplifier power requirements.

Examples

A loudspeaker measuring 100 dB SPL at 1 m will measure 94 dB at 2 m, 88 dB at 4 m, 80 dB at 10 m (inverse square law, free field). At 100 m it would theoretically be 60 dB — but real-world air absorption further reduces high frequencies.

Assessment

A speaker cluster reads 110 dB SPL at 10 feet. Estimate the SPL at 20 feet, 40 feet, and 80 feet. At what distance does it drop below 90 dB?