  For direct detection of sub-MeV dark matter, a promising strategy is to
search for individual phonon excitations in a crystal. We perform an analytic
calculation of the rate for light dark matter (keV $<m_{DM}<$ MeV) to produce
two acoustic phonons through scattering in cubic crystals such as GaAs, Ge, Si
and diamond. The multiphonon rate is always smaller than the rate to produce a
single optical phonon, whenever the latter is kinematically accessible. In Si
and diamond there is a dark matter mass range for which multiphonon production
can be the most promising process, depending on the experimental threshold.
