Sound Level Variation with Distance
In a real medium, the sound level decreases as the distance from the source increases.
The rate of decrease depends on whether the source behaves as a point source or a linear source.
(a) For Point Source
L2 = L1 − 20 log10
(r2/r1)
Where:
L1 = Sound level at distance r1 (dB)
L2 = Sound level at distance r2 (dB)
r1, r2 = Distances from the point source
L1 = Sound level at distance r1 (dB)
L2 = Sound level at distance r2 (dB)
r1, r2 = Distances from the point source
Important:
For a point source, the sound level decreases by approximately
6 dB whenever the distance from the source is doubled.
(b) For Linear Source
L2 = L1 − 10 log10
(r2/r1)
Where:
L1 = Sound level at distance r1 (dB)
L2 = Sound level at distance r2 (dB)
r1, r2 = Distances from the linear source
L1 = Sound level at distance r1 (dB)
L2 = Sound level at distance r2 (dB)
r1, r2 = Distances from the linear source
Important:
For a linear source, the sound level decreases by approximately
3 dB whenever the distance from the source is doubled.
Note:
Point sources (machines, generators, loudspeakers, compressors, etc.)
exhibit spherical spreading of sound energy and follow the inverse-square law.
Linear sources (highways, railway lines, conveyor systems, etc.)
exhibit cylindrical spreading and therefore show a slower reduction in sound level with distance.
Disclaimer
This content is provided solely for educational and informational purposes.
Every effort has been made to ensure accuracy; however, users should verify
calculations, assumptions, and engineering applications with relevant standards,
textbooks, codes, and professional practices before implementation.
The author assumes no responsibility for any loss or consequences arising
from the use of this material.
© 2026 Mohan Dangi. All Rights Reserved.
Prepared for Environmental Engineering and Noise Pollution Studies.
Prepared for Environmental Engineering and Noise Pollution Studies.
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