Editing Sound power (section)

==Mathematical definition==
Sound power, denoted ''P'', is defined by<ref>Landau & Lifshitz, "Fluid Mechanics", Course of Theoretical Physics, Vol. 6</ref>
:<math>P = \mathbf f \cdot \mathbf v = Ap\, \mathbf u \cdot \mathbf v = Apv</math>
where
*'''f''' is the sound force of unit vector '''u''';
*'''v''' is the [[particle velocity]] of projection ''v'' along '''u''';
*''A'' is the area;
*''p'' is the [[sound pressure]].

In a [[Transmission medium|medium]], the sound power is given by
:<math>P = \frac{A p^2}{\rho c} \cos \theta,</math>
where
*''A'' is the area of the surface;
*''ρ'' is the [[mass density]];
*''c'' is the [[sound velocity]];
*''θ'' is the angle between the direction of propagation of the sound and the normal to the surface.
*''p'' is the [[sound pressure]].
For example, a sound at SPL = 85&nbsp;dB or ''p'' = 0.356 Pa in air (''ρ'' = {{val|1.2|u=kg.m-3}} and ''c'' = {{val|343|u=m.s-1}}) through a surface of area ''A'' = {{val|1|u=m2}} normal to the direction of propagation (''θ'' = 0°) has a sound energy flux ''P'' = {{val|0.3|u=mW}}.

This is the parameter one would be interested in when converting noise back into usable energy, along with any losses in the capturing device.