From these equations we can see that

- (4.27) gives that the velocity is parallel to the
wavevector, . This means that the motions are aligned with
the direction of propagation. Thus, the wave is
*longitudinal*. - If
is non zero, then
(4.26) and (4.28) show that
and are non zero and we say that the motion is
*compressive*. Compression is a characteristic property of sound waves.

and then using (4.26) and (4.28) we get

Therefore,

as before.