This report, Volume IV, presents the results (1961-1962) of several studies of the interaction between air flow and airborne sound in a duct. Three projects are discussed:(1) Part A:The effects of various sound fields on the excitation of the boundary layer of a flat plate are discussed relative to the stagnation point and the boundary layer itself. Boundary layer distortions are shown to be a function of the sound signal and flow velocity. (2) Part B:The change in the cross-sectional distribution of the sound pressure in an absorbent duct with air flow is examined with respect to the shape of the velocity profile of the flow. Even for wavelengths greater than the boundary layer thickness the sound pressure increased at the absorbing walls. (3) Part C:The dependence of the acoustic radiation impedance of a tube orifice during flow conditions is investigated as a function of the flow velocity and turbulence level. Results show that as the flow increases the radiation impedance decreases.
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