Acoustic Wave Propagation in a Non-Gray Radiating Atmosphere

Abstract

An analysis based on radiative decay times allows the quantitative calculation of the effects of thermal radiation on acoustic wave propagation in an infinite non-gray atmosphere. Computations for the earth's troposphere, Mars, Venus, and ammonia in the laboratory show that in no case is measurable alteration of the propagation velocity predicted, but radiation is the main damping mechanism at low frequencies. Calculated damping is consistent with atmospheric observations. Laboratory demonstration of this effect is probably not possible in air, but appears feasible in low pressure ammonia. Acoustic wave propagation at high altitudes (low pressures) is shown to be adiabatic, contrary to a suggestion by Golitsyn. An approximate treatment to include the effects of the finite depth of the atmosphere shows that the results are somewhat altered in detail. An important finding is that for this problem a non-gray atmosphere cannot be usefully approximated by one or more gray coefficients. A simple approximation to the effective absorption coefficient is suggested.