Work out sound intensity instantly with clear inputs, formula shown and shareable results.
Sound from a point source spreads over a sphere, so intensity is power divided by 4 pi r^2. A 50 mW source at 3 m gives 4.4e-4 W/m2, about 86 dB. Doubling the distance quarters the intensity, which is the 6 dB per doubling rule used in noise assessment.
Intensity from power
I = P / (4 pi r^2)
Level
L = 10 log10(I / 1e-12)
It is the nominal threshold of human hearing at 1 kHz, so 0 dB marks the quietest audible sound.
Only close to the source. Room reflections build a reverberant field that stops the inverse square law from applying beyond the critical distance.