Sound intensity

Sound intensity also known as acoustic intensity is defined as the sound power per unit area. The SI unit of sound intensity is the watt per square metre (W/m²). The usual context is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.

Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone. Sound energy passing per second through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.

Mathematical definition

Sound intensity, denoted I, is defined by

where

Both I and v are vectors, which means that both have a direction as well as a magnitude. The direction of sound intensity is the average direction in which energy is flowing.