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Decibel Distance Calculator

Enter the reference level and distances to find the SPL at the target distance.

How it is calculated

The tool gives the level at a new distance for a point source in open space: SPL2 = SPL1 − 20 × log10(d2 ÷ d1). SPL1 is the measured sound pressure level in dB at distance d1, and d2 is the distance where you want the level. The distances can be in any unit as long as both use the same one, since only their ratio counts. It shows the new level, the change in dB (negative when you move away, positive when you move closer) and the distance ratio d2 ÷ d1. Both distances must be greater than zero. Each doubling of distance lowers the level by about 6 dB, and a tenfold distance lowers it by 20 dB. The formula ignores reflections, walls, air absorption, wind and several sources, so indoor and long-distance results will differ.

Worked example

A speaker measures 94 dB at 1 m and you want the level at 7 m. Distance ratio: 7 ÷ 1 = 7. Change: −20 × log10(7) = −20 × 0.845 = −16.9 dB. New level: 94 − 16.9 = 77.1 dB. For comparison, moving from 1 m to 2 m would cost only 6.0 dB, and from 1 m to 10 m exactly 20.0 dB.

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Common mistakes

  • Taking the level and the distance from different measurements, so the reference pair (SPL1, d1) does not belong together.
  • Applying the formula inside a room. Reflections keep the level higher than a free-field calculation predicts.
  • Mixing units between d1 and d2, such as feet for one and metres for the other, which distorts the ratio.
  • Adding the decibel change from a second source instead of combining sources. Two equal sources raise the level by about 3 dB, not double it.
  • Expecting the result to apply to a line source (traffic, a long pipe), where the level falls more slowly than for a point source.

About this calculator

Sound intensity falls off with distance according to the inverse-square law — doubling the distance from a point source quarters the sound intensity, which works out to a drop of 6 dB per doubling of distance in free field conditions. The formula is SPL2 = SPL1 − 20·log10(d2/d1), where SPL1 is the known sound pressure level at reference distance d1.

This calculator applies that formula to find the SPL at any new distance, which is standard practice in audio engineering, noise assessment, and live sound design — for example, checking whether a PA system will still be within safe or code-compliant noise limits at the edge of a venue, or estimating how much quieter a machine will sound from twice as far away.

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  • Editorial team

    You can run the formula backwards to find a distance for a target level. From 100 dB at 1 m, 85 dB is reached at d1 × 10^((SPL1 − target) / 20), which is about 5.6 m, and you can confirm it by typing 5.6 as the target distance. This is the free-field point-source case only. Are you checking a speaker, a machine or something else?

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