= Passive acoustic impedance
{title2=$\operatorname{Re}Z\geq0$}
With <pressure> amplitude $P=ZV$ and <velocity> $V$ directed into a boundary, the mean absorbed <acoustic energy flux> is $\tfrac12\operatorname{Re}(PV^*)=\tfrac12\operatorname{Re}Z|V|^2$. A passive boundary cannot supply net energy, so $\operatorname{Re}Z\geq0$ at a real frequency. Its reflection from a propagating <acoustic plane wave> has magnitude at most one. A pole continued to a complex incidence angle or frequency must be interpreted separately from real-frequency passive scattering.
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