Diffraction of evanescent waves, with applications to aerodynamically scattered sound and radiation from unbaffled plates

We discuss a slight modification of the classical problem of diffraction of acoustic waves off a semi‐infinite, thin, rigid surface. The modification is: the waves incident on the surface are evanescent rather than freely propagating. The solution for the diffracted sound wave in the farfield, and some variations on this solution, are considered with reference to the following problems: (1) sound diffracted by a turbulent boundary layer flow off the sharp edge of a large hard surface; (2) for a large plate undergoing flexural vibration, radiation efficiencies associated with unbaffled plate edges of different boundary conditions.