  Liquid helium and spin-1/2 cold-atom Fermi gases both exhibit in their
superfluid phase two distinct types of excitations, gapless phonons and gapped
rotons or fermionic pair-breaking excitations. In the long wavelength limit,
revising and extending Landau and Khalatnikov's theory initially developed for
helium [ZhETF 19, 637 (1949)], we obtain universal expressions for three- and
four-body couplings among these two types of excitations. We calculate the
corresponding phonon damping rates at low temperature and compare them to those
of a pure phononic origin in high-pressure liquid helium and in strongly
interacting Fermi gases, paving the way to experimental observations.
