= Compressional wave
{title2=$c_p=\sqrt{(\lambda+2\mu)/\rho}$}
A compressional wave is a <longitudinal wave> whose propagation produces changes of volume or density. In an isotropic elastic solid its <displacement> is parallel to the <wavevector>; substituting that polarization into the dynamic <Navier-Cauchy equation> gives the displayed speed. In a fluid without static shear stiffness the analogous speed is $\sqrt{K/\rho}$, with the appropriate compressional <bulk modulus>. Reflection at a rigid elastic boundary can convert part of its energy into a <shear wave>.
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