= Intensity
{disambiguate=physics}
{title2=$I=P/A$}
{wiki}
= Wave intensity
{synonym}
= Intensity
{synonym}
<Wave intensity> is the time-averaged <power> carried through unit <area> perpendicular to the transport direction. It has <SI> units $\mathrm{W\,m^{-2}}$. For a uniform beam carrying <power> $P$ through a perpendicular <area> $A$, $I=P/A$; a nonuniform beam instead requires integration of its local <energy flux>.
For a progressive plane <acoustic wave> in a lossless fluid,
$$
I=\frac{p_0^2}{2\rho c}=\frac{p_{\mathrm{rms}}^2}{\rho c},
$$
where $p_0$ is the peak <pressure> <amplitude>, $p_{\mathrm{rms}}$ its <root mean square>, $\rho$ the mean <mass density> and $c$ the <speed of sound>. Doubling the <wave amplitude> therefore quadruples the <wave intensity> in the same medium. A <standing wave> can have large local <pressure> <amplitude> with zero net averaged <sound intensity>, so the progressive-wave formula must not be applied to an arbitrary field.
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