  We derive the phonon damping rate due to the four-phonon Landau-Khalatnikov
process in low temperature strongly interacting Fermi gases using quantum
hydrodynamics, correcting and extending the original calculation of Landau and
Khalatnikov [ZhETF, 19 (1949) 637]. Our predictions can be tested in
state-of-the-art experiments with cold atomic gases in the collisionless
regime.
