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The sound intensity, '''''I''''', (acoustic intensity) is defined as the Sound Power '''''Pac''''' per unit area ''A''. The usual context is the measurement of sound Intensity in the air at a listener's location. The SI units are W/m2 ( Watt s per Square Metre ).

:
I = rac{1}{T} \int_{0}^{T}p(t) \cdot v(t)\,dt


For a spherical sound source, the intensity as a function of distance r is:
:
I_r = rac{P_{ac}}{A} = rac{P_{ac}}{4 \pi r^2}


The sound intensity ''I'' in W/m2 of a plane progressive wave is:
:
I = p \cdot v = rac{p^2}{Z} = Z \cdot v^2 = \xi^2 \cdot \omega^2 \cdot Z = rac{a^2 \cdot Z}{\omega^2} = E \cdot c = rac{P_{ac}}{A}


where:

The amplitude of sound intensity (not sound pressure!) decreases in the free field (direct field) with 1/r2 of the distance of a point source.

Sound Intensity Level , ''LI'', is the sound intensity, expressed in logarithmic units.
:L_I=10 \log_{10} rac {I}{I_o} (dB-SIL),
where ''Io'' is the reference intensity, 10-12 W/m2