Geluidsintensiteit

The loudness of a sound is related to the perception it creates on the human ear. It depends upon how energetically the source of sound is vibrating.

The loudness is described by a physical quantity, the intensity. A source's intensity is defined as the power emitted by the source per unit area perpendicular to the direction of propagation.

Generally, the power fluctuates with time. Hence, the intensity is defined as the average power per unit perpendicular area.

The SI unit of power is watt and that of area is meter squared. Hence, the SI unit of intensity is watt per meter squared.

If the source emits the sound waves uniformly in all directions, the area perpendicular to the propagation is that of a sphere covering the source. Hence, the intensity falls off as the square of the distance.