Decibel Distance Calculator

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.

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.