Inverse Square Law
For every doubling of the distance from the noise source, the sound pressure levels - Lp, will be reduced by 6 decibels
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For a free field every doubling of the distance from the noise source, the sound pressure levels - Lp, will be reduced by 6 decibels. The inverse square law can be expressed as in the diagram below.
Where
Lp1 = sound pressure level at location 1 (dB)
Lp2 = sound pressure level at location 2 (dB)
R1 = distance from source to location 1 (ft, m)
R2 = distance from source to location 2 (ft, m)
A "free field" is defined as a flat surface without obstructions.
Example - Shot of a Rifle and Decibel Level
| Distance (feet) |
Sound Pressure Lp (decibel) |
| 1.25 | 134 |
| 2.5 | 128 |
| 5 | 122 |
| 10 | 116 |
| 20 | 110 |
| 40 | 104 |
| 80 | 98 |
| 160 | 92 |
| 320 | 89 |
| 640 | 80 |
| 1280 | 72 |
| 2560 | 66 |
| 5120 | 60 |
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