= Sound intensity
{wiki}
<Sound intensity> is the time-averaged <acoustic energy flux>, a <vector> measured in $\mathrm{W\,m^{-2}}$. In <linear acoustics>, $\mathbf I=\langle p'\mathbf u'\rangle$, where $p'$ is acoustic <pressure> and $\mathbf u'$ particle <velocity>. Its direction identifies net <energy> transport, and integrating its normal component over a surface gives the acoustic <power> crossing that surface.
A progressive plane <acoustic wave> satisfies $I=p_{\mathrm{rms}}^2/(\rho c)$, with mean <mass density> $\rho$ and <sound speed> $c$. This formula uses the relation between <pressure> and <velocity> for that particular travelling field. Two opposite equal <waves> instead form a <standing wave> with zero net <sound intensity>, despite nonzero <root mean square> <pressure>. A <microphone> alone measures <pressure>; inferring transported <power> from it therefore requires assumptions about the field, especially near reflecting boundaries.
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