Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area, also called the sound power density and the sound energy flux density. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m²). One application 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. Human hearing is 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 intensity level is a logarithmic expression of sound intensity relative to a reference intensity.

Sound intensity, denoted I, is defined by $${\displaystyle \mathbf {I} =p\mathbf {v} }$$ 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.

The average sound intensity during time T is given by $${\displaystyle \langle \mathbf {I} \rangle ={\frac {1}{T}}\int _{0}^{T}p(t)\mathbf {v} (t)\,\mathrm {d} t.}$$ For a plane wave , $${\displaystyle \mathrm {I} =2\pi ^{2}\nu ^{2}\delta ^{2}\rho c}$$ Where,

For a spherical sound wave, the intensity in the radial direction as a function of distance r from the centre of the sphere is given by $${\displaystyle I(r)={\frac {P}{A(r)}}={\frac {P}{4\pi r^{2}}},}$$ where

Thus sound intensity decreases as 1/r² from the centre of the sphere: $${\displaystyle I(r)\propto {\frac {1}{r^{2}}}.}$$