SPL at Distance Calculator
Sound pressure level from a point source falls off predictably with distance in a free field: every time the distance doubles, the level drops by 6 decibels. This calculator applies the inverse-square law to project a known SPL measured at one distance out (or in) to any other distance. It is useful for estimating loudspeaker coverage, noise levels at a property boundary, and how far a sound must travel before it falls below a target threshold. Enter your reference level and distance, then the distance of interest, to see the predicted level and the change in dB.
SPL at distance formula
Level change = -20 * log10(d2 / d1)
SPL at d2 = SPL at d1 + level change
(Each doubling of distance gives -6.02 dB)
This is the free-field inverse-square law for a point source. Sound intensity falls as 1 over distance squared, and sound pressure level is 20 times the base-10 log of the pressure ratio, giving the 20 * log10 distance term.
Where the inverse-square law applies
- Valid for a compact point source radiating into an unobstructed free field with no reflections.
- Level drops about 6 dB per doubling of distance for a point source, about 3 dB per doubling for a line source.
- Indoors, reflections make the level flatten out past the critical distance, so the prediction underestimates real level.
- Air absorption adds extra high-frequency loss over long distances and is not included here.
- The same distance correction applies to half-space (on-ground) sources; only the absolute offset differs.
SPL at distance: frequently asked questions
What is the inverse-square law for sound?
For a point source radiating into a free field, sound pressure level drops by 6 dB for every doubling of distance. The exact relation is SPL2 = SPL1 - 20 * log10(d2 / d1), where d1 is the reference distance and d2 is the new distance. This follows from sound intensity falling off as 1 over distance squared, and SPL being proportional to 20 times the log of pressure.
Why does level drop 6 dB per doubling and not 3 dB?
Sound intensity drops 1 over distance squared, which is a 6 dB reduction per doubling for a point source. The 3 dB per doubling figure applies to a line source (such as a long road or a row of sources) where energy spreads cylindrically rather than spherically. This calculator models a point source in a free field.
Does this account for room reflections or air absorption?
No. The inverse-square law assumes a free field with no reflective surfaces and negligible air absorption. Indoors, reflections raise the level beyond the free-field prediction once you pass the critical distance. Over long outdoor distances, air absorption (which rises with frequency, humidity and temperature) adds further loss not modeled here.
What reference distance should I use?
Manufacturer loudspeaker sensitivity is usually quoted at 1 meter. Many noise measurements use a stated reference distance on the source datasheet. Enter the distance at which your known SPL was measured as d1, then enter the distance of interest as d2.
Can I use this for a hemispherical source on the ground?
Yes. The distance dependence is identical for a source radiating into a half space (such as on hard ground): level still drops 6 dB per doubling. The absolute level differs by about 3 dB versus full-space radiation, but that constant offset is captured in your measured reference SPL, so the distance correction is unchanged.
Official sources
- NIOSH (CDC): Noise and Occupational Hearing Loss.
- OSHA: Occupational Noise Exposure.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.