Inverse Square Law

The inverse square law states that the intensity of sound from a point source in free field decreases proportionally to the square of the distance from the source. This corresponds to a 6 dB reduction in sound pressure level each time the distance is doubled. It applies perfectly only in free-field conditions (no reflections), such as outdoors or in an anechoic chamber. In real rooms, the reverberant field creates a lower limit below which further distance provides no additional reduction. This limit is the reverberant level, reached at and beyond the critical distance. The inverse square law is fundamental for calculating direct sound levels from known source data, determining loudspeaker coverage, and understanding why sound insulation requirements increase for rooms close to noise sources. ISO 9613-1 uses it as the basis for outdoor sound propagation predictions.