  A recent Letter has reported that sound waves can carry gravitational mass. I
analyze this effect in a Hooke's law solid, considering a wave packet moving in
the $z$ direction with an amplitude that is independent of $x$ and $y$. The
analysis shows that, at second order in an expansion around small amplitude
vibrations, there is a small net motion of material, and thus mass, in the
direction opposite to the wave packet propagation. This is a straightforward
consequence of Newton's laws.
