High frequency sound attenuation in short flow ducts

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Abstract
Traditionally, sound propagation in flow ducts has been analyzed using modal theory. However, for high-frequency (duct height divided by free-space wavelength much greater than unity) sources near (the order of the duct height or less) the duct termination, mode theory may be unnecessarily burdensome and seems to obscure the basic simplicity of the propagation phenomenon. In this paper, geometric acoustics is employed to develop absorbent liner design procedures for the case of high frequency sources in short ducts. Point and line sources in a two-dimensional parallel plate duct are utilized to develop basic concepts, which are then extended to the point and distributed sources in a cylindrical duct. In each case, an optimal impedance distribution (or segmentation) is found for subsonic plug flow. The applicability of this approach to the design of inlet and exhaust noise suppressors for aircraft turbomachinery is discussed.

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