How does sound level decrease with distance?

For a point source in free field, sound pressure level decreases by 6 dB for each doubling of distance from the source (inverse square law).

What is the inverse square law for sound?

L2 = L1 − 20·log10(d2/d1). This means SPL drops proportionally to the square of the distance ratio.

It applies in the direct field only. Indoors, reflections create a reverberant field where level stays roughly constant beyond a certain distance.

Line sources follow L2 = L1 − 10·log10(d2/d1), giving only 3 dB reduction per doubling of distance, not 6 dB.

You need a 20 dB reduction = 10× the reference distance. If 90 dB at 1 m, you need 10 m to reach 70 dB (free field, point source).

Free Engineering Tool #215

Calculate sound pressure level at any distance from a point source. Uses the inverse square law for free-field propagation.

L₂ = L₁ − 20·log₁₀(d₂/d₁)Point SourceFree Field

Level at Target Distance (L₂)

—

Attenuation

—

Distance Ratio

—

For a point source in a free field, SPL drops 6 dB for each doubling of distance.

Example — Machine at 90 dB @ 1 m

L₂ = 90 − 20·log₁₀(10/1) = 90 − 20 = 70 dB at 10 m