The sound intensity (I) is the sound power in relation to a sounded area. It decreases with the square of the distance (r) from the sound source. This relationship is called the reciprocal quadrature law and applies to the sound energy: `I = 1/r^2`. The basic unit for sound intensity is watts per square meter (`W/m2`).
In the considerations, it is assumed that the sound source propagates its sound energy uniformly in all directions. Under this condition, the sound intensity is reduced to a quarter of its original value when the distance is doubled. Expressed indecibels, the sound intensity in this case is reduced by 6 dB; if the distance from the sound source is increased tenfold, the sound intensity is reduced by 20 dB, which corresponds to one hundredth of the original value.