Two parallel semi-infinite plates are joined together at their common end, with vacuum in the infinitesimal gap between them and acoustic fluid outside. The configuration is an idealization of a realistic structural member such as the bow or keel of a ship where the structural elements are joined at an acute angle. The exact solution for acoustic diffraction from the parallel double plate junction is obtained using thin plate theory and the Wiener-Hopf technique. The diffraction coefficient is

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... trongly dependent upon frequency, unlike the analogous coefficients for edge diffraction from hard or soft screens. However, the relative simplicity of the general solution permits analytical comparisons with these idealized boundary conditions. At low frequencies the diffraction simulates the response from a soft or pressure release screen. At high frequencies, or more precisely under light fluid-loading conditions, the hard screen approximation is more appropriate. Numerical examples illustrate the transition from soft to hard diffraction, and an approximate but simple uniform diffraction coefficient is proposed which has the correct limiting behavior at low and high frequency.