Work out noise propagation distance instantly with clear inputs, formula shown and shareable results.
A point source spreads spherically, so level falls 6 dB per doubling of distance; a line source such as a road spreads cylindrically and falls only 3 dB per doubling. Atmospheric absorption adds a small further loss that becomes significant over hundreds of metres, particularly at high frequency.
Point source
Lp = Lw - 20 log10(distance) - 8 for hemispherical radiation
Line source
Lp = Lw - 10 log10(2 pi x distance)
Noise limits and assessment methods are set by planning conditions, permits or occupational regulation. Use the prescribed method rather than a simplified estimate.
Cylindrical spreading loses only 3 dB per doubling instead of 6. A road audible at 100 m is still audible at 400 m where a single machine would have faded.
Ground absorption, meteorological conditions — downwind and temperature inversions increase levels substantially — screening by terrain and buildings, and reflections. Simple spreading models can be several decibels out.