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This report, Volume III, gives theoretical and experimental results on the interaction between sound and air flow in a duct for the following studies: (1) signal amplification as related to size, shape, and position of periodically spaced Helmholtz resonators mounted in the walls of a duct, (2) extension of flow velocity and signal frequency parameters (as employed in Vols. I and II) for a duct coated with a porous absorber, and (3) investigation of plate absorbers for their usefulness in wind tunnel applications. Results of these studies show, respectively, (1) the ratio of the Helmholtz resonator neck width to the spacing, for sound propagation in the flow direction, is an important factor; (2) the solution of a wave equation agrees with experimental measurements for frequencies having a wavelength greater than and comparable to the duct diameter; and (3) the attenuation is dependent on the type of plate absorber (pliable, rigid, or resilient) and the flow velocity. Also reported upon is the construction of a test duct for determining the sound field influence on the air flow boundary layer over a plate vibrating perpendicular to its surface. PUBLICATION REVIEWThis technical documentary report has been reviewed and is approved. M.QTASHNOCK

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