The Sound energy flux reference article from the English Wikipedia on 24-Jul-2004
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Sound energy flux

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The sound energy flux is the average rate of flow of sound energy for one period through any specified area.

In a medium of density ρ for a plane or spherical free wave having a velocity of propagation c, the sound energy flux through the area S corresponding to an effective sound pressure p is

J = ( p2S / ρc ) cos θ

where θ = the angle between the direction of propagation of the sound and the normal to the area S.