SPL and Inverse Square Law
The sound level at a new distance from a source.
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
Checked against published worked examples from: QSC.
Details, formula, and sources
ScopeL2 = L1 - 20 log10(d2/d1) with free-field / hemispherical / indoor mode adjustment.
Formula or tableL2 = L1 − 20 × log10(d2 / d1). Free-field (−6 dB per doubling of distance), hemispherical (ground-coupled), or indoor (less attenuation due to reflection).
Source and editionClassical acoustics (inverse-square law); ISO 9613-2 (Acoustics - Attenuation of sound during propagation outdoors) by name.
Free accessInverse-square principle free in physics texts; ISO 9613-2 licensed.
What governsEstimate. AHJ and licensed professional govern.
API field namesL1_dB, d1, d2, mode, L2_dB
- Mode adjustments free-field 0 dB / hemispherical +3 dB (half-space) / indoors +6 dB (a rough quarter-space approximation)ISO 9613-2 typical