  Sound propagation in a Fermi liquid with impurities is studied using the
Landau-Boltzmann equation and the result is compared with a recent experiment
in liquid ${}^3$He in aerogel by Northwestern University group. The sound
absorption calculated using the fixed impurity model is a few orders of
magnitude larger than the experiment. We take into account the simultaneous
motion of aerogel molecules and propose a model in which the momentum loss of
${}^3$He quasi-particles during the collisions with the aerogel is converted to
a drag force that acts on the aerogel molecules. This collision drag model
gives a reasonable description for the temperature and the pressure dependence
of observed sound velocity and absorption.
