Sound Level vs Distance Calculator
Sound spreads out as it travels, so the further you stand from a source the quieter it becomes. For a point source radiating freely into open space, the sound pressure level falls by 20 times the base-10 logarithm of the ratio of the two distances. The practical consequence, familiar to anyone who works with audio or noise, is that the level drops by about 6 decibels every time you double your distance from the source. This calculator applies that inverse-square relationship: enter the known sound level at a reference distance, then the reference distance and the new distance, and it returns the level you would expect at the new position along with the drop. It is handy for estimating how far back a loudspeaker, machine or noisy tool needs to be to reach a target level, or for sanity-checking a measurement. The rule assumes a free field with no reflecting surfaces, so it suits outdoor and open settings; indoors, reflections reduce the fall-off and the real level drops more slowly. Every figure here is computed deterministically from the 20 log(d2/d1) formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step.
Sound level falls with distance by the inverse-square rule: L2 = L1 - 20 log(d2 / d1). A level of 90 dB at 1 m drops by 12.04 dB at 4 m, to 77.96 dB, about 6 dB per doubling of distance.
Sound level vs distance formula
L2 = L1 - 20 x log10(d2 / d1)
L1 = sound level at the reference distance (dB)
d1 = reference distance, d2 = new distance
L2 = sound level at the new distance (dB)
The factor of 20 reflects working in sound pressure. Doubling the distance gives a drop of 20 log(2), about 6 decibels; quadrupling gives about 12 decibels.
Worked example
A source measures 90 dB at 1 metre. What level is expected at 4 metres in the open?
- Distance ratio: 4 / 1 = 4.
- Level change: 20 x log10(4) = 20 x 0.60206 = 12.04 dB.
- Level at 4 m: 90 - 12.04 = 77.96 dB.
The level at 4 metres is 77.96 dB. These are the calculator's default inputs, so the result above matches the widget exactly.
Drop with distance from a point source
| Distance ratio | Level change |
|---|---|
| 2x | -6.02 dB |
| 4x | -12.04 dB |
| 8x | -18.06 dB |
| 10x | -20.00 dB |
Acoustic measurement standards: US National Institute of Standards and Technology.
Sound level vs distance calculator: frequently asked questions
How does sound level change with distance?
For a point source radiating freely, sound pressure level falls by 20 times the base-10 logarithm of the ratio of the two distances. In practice this means the level drops by about 6 decibels each time the distance from the source doubles. The relationship comes from the inverse-square law, since sound energy spreads over a larger area as it travels.
What is the formula?
The new level equals the reference level minus 20 times log base 10 of the new distance divided by the reference distance. Written out: L2 = L1 minus 20 log(d2 / d1). If the new distance is larger, the level drops; if it is smaller, the level rises. The 20 comes from working in sound pressure rather than power.
Why does sound drop about 6 dB per doubling of distance?
Because 20 times the logarithm of 2 is about 6.02. Each time you double the distance from a point source in a free field, the ratio d2 over d1 is 2, so the drop is 20 log(2), which is roughly 6 decibels. Quadrupling the distance drops it by about 12 decibels, and so on.
Does this work indoors?
The 20 log(d2/d1) rule assumes a free field, meaning open space with no reflecting surfaces. Indoors, walls, floors and ceilings reflect sound and reduce the drop with distance, so the real level falls more slowly than the formula predicts. Use this calculator for outdoor or open-field estimates and treat indoor results as approximate.
Is loud noise a health concern?
Yes. Prolonged exposure to high sound levels can cause permanent hearing damage. Public health agencies including the US Centers for Disease Control and Prevention advise limiting exposure to loud noise and using hearing protection. Increasing your distance from a loud source is one of the most effective ways to reduce the level reaching your ears.
Official sources
- Acoustics and sound measurement standards: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.