The inverse square law
In open air, sound from a small source spreads over a sphere, so intensity falls with the square of distance: L₂ = L₁ − 20·log₁₀(r₂/r₁), or −6 dB per doubling of distance.
A long line source such as a busy road spreads over a cylinder instead, losing only about 3 dB per doubling. Indoors, reflections keep levels higher than these estimates.
Frequently asked questions
How far away is 85 dB safe?
It depends on the source: a 100 dB chainsaw at 1 m drops to about 86 dB at 5 m and 80 dB at 10 m in open air.