Another very important but little known acoustical phenomena is the Inverse Square Law. As a sound wave propagates spherically, the sound energy is distributed over the ever-increasing surface diameter of the wave front surface. The Inverse Square Law teaches us that for every doubling of the distance from the sound source in a free field situation, the sound intensity will diminish by 6 decibels.

Under ideal conditions a free field could be represented by a sound signal being generated from a mountain peak. In real life situations however, rooms bounded by walls, floors and ceilings will interrupt the inverse square law at a distance in tan average 30′ square room at approximately 10-12 feet from the sound source. Nevertheless it is important to accept the notion that sound will diminish in intensity with distance. For example, in a typical classroom with a teachers voice signal of 65 decibels at a three-foot distance from the teacher; at 6 feet away the sound intensity will be 59 decibels and at twelve feet it will diminish down to 53 decibels.

(Figure A) shows a segment of the sound wave front surface area increasing with distance.

Figure A


In the angle shown in Figure A, the same sound energy is distributed over the spherical surfaces of increasing areas as d is increased. The intensity of the sound is inversely proportional to the square of the distance of the wavefront from the signal source.

1d = 1
2d = 4
3d = 9
4d = 16