SPL and Inverse Square Law

The sound level at a new distance from a source.

Run the calculator

Example

You enter

SPL at reference (dB) 110
Reference distance (ft) 1
Target distance (ft) 30
Mode free_field

You get

L2 db 80.46

Details, formula, and sources

L2 = L1 - 20 log10(d2/d1) with free-field / hemispherical / indoor mode adjustment.

L2 = L1 − 20 × log10(d2 / d1). Free-field (−6 dB per doubling of distance), hemispherical (ground-coupled), or indoor (less attenuation due to reflection).

Classical acoustics (inverse-square law); ISO 9613-2 (Acoustics - Attenuation of sound during propagation outdoors) by name.

Inverse-square principle free in physics texts; ISO 9613-2 licensed.

Estimate. Head rigger and manufacturer working-load-limit charts govern. Inspect every piece of hardware before the show.

Field names used by the API: L1_dB, d1, d2, mode, L2_dB

Mode adjustments free-field 0 dB / hemispherical −3 dB / indoor user-suppliedISO 9613-2 typical