How sound changes with distance
How much quieter will it be further away?
Sound generally reduces as it travels away from a source — but the rate of reduction depends on the type of source. A fixed item of mechanical plant behaves approximately like a point source, while a long road with continuous traffic may behave approximately like a line source. Use the calculator to compare the difference.
This represents extrapolation toward the source and may not be valid in the acoustic near field.
This calculator illustrates geometric spreading under idealised conditions. It does not include barriers, ground absorption, atmospheric absorption, reflections, building screening, terrain, meteorological effects, source directivity or changes in source operation. The barrier calculator above must be used separately — its attenuation is not automatically added to this result.
An ideal point source measured at 90.0 dB(A) at 1.0 m is predicted to be 70.0 dB(A) at 10.0 m, representing a reduction of 20.0 dB.
Illustrative geometric-spreading diagram, not to scale.
Point sources
A stationary motor or air-conditioning unit can often be approximated as a point source when the receiver is sufficiently far from the equipment. Under ideal free-field conditions, the sound level reduces by approximately 6 dB for every doubling of distance.
Line sources
A sufficiently long road with a continuous, evenly distributed flow of traffic can be approximated as a line source. Under ideal conditions, the sound level reduces by approximately 3 dB for every doubling of perpendicular distance from the road.
Comparing the two
| Distance change | Ideal point source | Ideal line source |
|---|---|---|
| Doubling | Approximately −6 dB | Approximately −3 dB |
| Four times farther | Approximately −12 dB | Approximately −6 dB |
| Ten times farther | −20 dB | −10 dB |
These two behave differently because a point source spreads sound energy over an expanding spherical area, while a line source approximates sound energy spreading over an expanding cylindrical area. Real sources may transition between these behaviours depending on their dimensions and the distance to the receiver.
Technical limitations
This calculator illustrates geometric spreading under idealised conditions. It does not include barriers, ground absorption, atmospheric absorption, reflections, building screening, terrain, meteorological effects, source directivity or changes in source operation.
- A real road is finite and will not behave as a perfect line source at every distance.
- Individual vehicle movements may behave more like moving point sources.
- A group of separate mechanical units may not behave as one simple point source.
- Near-field measurements should not be extrapolated without care.
- Measurements and predictions must use equivalent acoustic descriptors and operating conditions.
- The barrier calculator must be used separately — its attenuation must not automatically be added to this result.
- This calculator is educational and is not a substitute for a site-specific noise assessment or SoundPLAN model.
Related reading
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