Piecewise continuous distribution function method and ultrasound at half   space
  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. 
M.A. Solovchuk, S. B. Leble