Sound Pressure Level (SPL) Distance Calculator
Calculate SPL at any distance using the inverse square law — every doubling of distance reduces SPL by 6 dB.
Results
What is it?
The inverse square law states that in a free field (no reflections), sound intensity decreases with the square of distance. This translates to a -6 dB reduction for every doubling of distance. The formula is: SPL₂ = SPL₁ − 20 × log₁₀(d₂ / d₁). This applies outdoors or in anechoic environments; real rooms with reflections reduce the attenuation rate.
How to use
Enter the known SPL at a reference distance (e.g. measured 1 m from a speaker), then enter the target distance. The result shows the predicted SPL at that distance. Both distances use the same unit selector — ensure they match.
Example scenario
A PA speaker measures 110 dB at 1 m. At 10 m: SPL = 110 − 20 × log₁₀(10) = 90 dB. At 100 m: 70 dB — quiet conversation level. Concert front-of-house positions typically see 90–100 dB from high-power rigs.
Pro tip
NIOSH safe exposure limits: 85 dB for 8 hours, 91 dB for 2 hours, 100 dB for 15 minutes, 112 dB for under 1 minute. A concert venue hitting 110 dB at FOH means crew near sub speakers may experience 116+ dB — always wear hearing protection on stage.