Decibel Shield
Sound distance calculator (inverse square law)
In open air a small sound source loses 6 dB every time the distance doubles, so 80 dB at 1 m is about 60 dB at 10 m and 40 dB at 100 m. Enter a level and a distance to see the level anywhere else, or the distance at which it falls to a level you choose.
Last updated:
60.0 dB at 10 m
100 m is where it falls to 40 dB
Free-field estimate: no walls, ground, barriers, wind or air absorption. Real readings are usually a few dB different, and higher-pitched sound fades faster with distance.
Decibel level by distance, point source
Each row is a level measured at 1 m from a small source, worked out for the other distances with the inverse-square law. Everything here is arithmetic, not measurement; see the assumptions below.
| At 1 m | 3 m | 10 m | 30 m | 100 m | 300 m | 1 km |
|---|---|---|---|---|---|---|
| 60 dB | 50 | 40 | 30 | 20 | 10 | 0 |
| 70 dB | 60 | 50 | 40 | 30 | 20 | 10 |
| 80 dB | 70 | 60 | 50 | 40 | 30 | 20 |
| 85 dB | 75 | 65 | 55 | 45 | 35 | 25 |
| 90 dB | 80 | 70 | 60 | 50 | 40 | 30 |
| 100 dB | 90 | 80 | 70 | 60 | 50 | 40 |
| 110 dB | 100 | 90 | 80 | 70 | 60 | 50 |
| 120 dB | 110 | 100 | 90 | 80 | 70 | 60 |
| 130 dB | 120 | 110 | 100 | 90 | 80 | 70 |
| 140 dB | 130 | 120 | 110 | 100 | 90 | 80 |
The inverse square law formula
For a point source in free field, the level at distance r₂ is the level at r₁ minus 20 log₁₀(r₂/r₁). Doubling the distance subtracts 6 dB, ten times the distance subtracts 20 dB. For a line source, such as a steady stream of traffic, the factor is 10 log₁₀, or 3 dB per doubling, because the sound spreads over a cylinder rather than a sphere. OSHA's technical manual and the FAA's noise basics both describe the same relationship.
Sources: OSHA Technical Manual, Section III Chapter 5; FAA, Fundamentals of Noise and Sound. The sound pressure level page explains why 6 dB is a quarter of the intensity.
What the calculator leaves out
- Ground and walls. A hard surface under the path reflects sound and can add up to 3 dB; a barrier that blocks the line of sight subtracts 5 dB or more.
- Air absorption. Over hundreds of metres the air soaks up high frequencies first, which is why distant traffic is a low rumble.
- Weather. Wind and temperature gradients bend sound; a source is often louder downwind and at night.
- Background. You hear a sound while it is above the level around you. Use the decibel meter to measure that level where you stand.
How far away can you hear 80 dB?
It depends on what is behind it. A point source at 80 dB one metre away drops to about 60 dB at 10 metres, 40 dB at 100 metres and 20 dB at a kilometre in free field. You stop hearing it when it falls below the background level, so in a quiet field at night that is much farther than on a city street at 60 dB.
Why does sound drop 6 dB when the distance doubles?
A small source spreads its energy over the surface of a sphere, and that surface grows with the square of the distance. Twice the distance means a quarter of the intensity, which is 6 dB less on the decibel scale. A long line source, such as a road, spreads over a cylinder instead, so it loses 3 dB per doubling.
Is the inverse-square law exact for noise outdoors?
No. It is the free-field starting point. Ground reflection can add up to 3 dB, barriers and buildings subtract, air absorbs high frequencies over long distances, and wind and temperature gradients bend sound up or down. Outdoor noise standards such as ISO 9613-2 add corrections for each of these.
How far from a 100 dB source is it safe without hearing protection?
NIOSH treats 85 dBA over an 8-hour day as the limit. A 100 dB point source measured at 1 m falls to 85 dB at about 5.6 m in free field, so beyond that distance an 8-hour exposure stays within the recommendation. Closer than that the safe time halves every 3 dB.