A point source spreads over a sphere, so doubling your distance quarters the intensity: −6 dB. A line of traffic spreads over a cylinder instead, and doubling your distance from it buys only 3 dB.
Intensity is power per unit area, and the area the sound has spread over grows as r² for a point and as r for a line. So the drill you walk away from obeys the inverse square law and the motorway does not — which is exactly why moving twice as far from a main road disappoints, and why a house 200 m from a motorway is barely quieter than one at 100 m. Ten times the distance is 20 dB from a point source and 10 dB from a line.
Free field only. Indoors, reflected sound stops the fall almost completely past a metre or two, so a room is always louder than this predicts. Outdoors, barriers, ground absorption, wind and temperature gradients all matter at long range, and air itself absorbs the high frequencies first — which is why distant sound is not just quieter but duller.
Estimate with the rule, then check it against the calculator that models it properly.
Open Sound Levels & Decibels →A point source spreads over a sphere, so doubling your distance quarters the intensity: −6 dB. A line of traffic spreads over a cylinder instead, and doubling your distance from it buys only 3 dB. Intensity is power per unit area, and the area the sound has spread over grows as r² for a point and as r for a line.