Influence of Fluid Motion Past a Plane Boundary on Sound Reflection, Absorption, and Transmission

It is shown that the effect of fluid motion past a plane boundary on the reflection and absorption of sound is equivalent to an increase of the normal acoustic impedance of the boundary by a factor (1 + M sinφ), where φ is the angle of incidence of the sound wave, and M is the Mach number of the flow velocity component in the incidence reflection plane of the wave. Similarly, the acoustic energy flux perpendicular to the boundary and the flow is shown to be increased by the same factor. Reflection and transmission coefficients of a thin solid interface between a fluid in motion and one at rest are given. Furthermore, some comments on the problem of transmission in duets are given. For propagation between two plane parallel boundaries with the same acoustic admittance, we find, for sufficiently small values of the admittance, that the sound pressure attenuation constant of the fundamental mode is modified approximately by the factors (1 + M)−2 and (1 − M)−2 for downstream and upstream propagation, where M is the flow Mach number.