Molecular simulations of sound waves propagating in a dilute hard sphere gas have been performed using the direct simulation Monte Carlo method. A wide range of frequencies is investigated, including very high frequencies for which the period is much shorter than the mean collision time. The simulation results are compared to experimental data and approximate solutions of the Boltzmann equation. It is shown that free molecular flow is important at distances smaller than one mean free path from the excitation point.
Alejandro Garcia and N. Hadjiconstantinou. "Molecular simulations of sound wave propagation in simple gases" Physics of Fluids (2001): 1040-1046. doi:http://dx.doi.org/10.1063/1.1352630