Relaxation effect on sound propagation in partially ionized gases

The effect of ionization-recombination processes on sound propagation in a partially ionized gas was investigated by employing the fluid model under the assumption that the ionization-recombination frequency is much lower than the collision frequency of heavy particles, and the sound speed and the absorption coefficient were obtained as functions of frequency. It is found that for low frequencies small disturbances propagate with the equilibrium sound speed, and that the sound speed increases monotonically with frequency and approaches the frozen sound speed for sufficiently high frequencies. It is found also that the sound is absorbed as it propagates in the medium and the absorption coefficient increases monotonically with the frequency. (author)