  The system of hydrodynamic-type equations, derived by two-side distribution
function for a stratified gas in gravity field is applied to a problem of
ultrasound propagation and attenuation. The background state and linearized
version of the obtained system is studied and compared with the Navier-Stokes
one at arbitrary Knudsen numbers. The WKB solutions for ultrasound in a
stratified medium are constructed in explicit form. The problem of a generation
by a moving plane in a rarefied gas is explored and used as a test while
compared with experiment.
