Classical theories for a point source in air above a surface with an impedance having an imaginary part much larger than the real part (such as that of a thin porous layer over a hard boundary) predict the occurrence of an airborne surface wave near grazing incidence [1]. Raspet and Baird [2] demonstrated that the surface wave arising from a point source above an impedance plane is an independently propagating wave. The propagation of audio-frequency sound waves over hard surfaces is affected significantly if they are rough. Tolstoy [3,4] and Twersky [5] have derived theories for surface wave generation above randomly and periodically rough boundaries. Consistent with the expected behaviour, surface waves due to a point source in air over a rough surface are characterised by cylindrical spreading with increasing range in the horizontal plane, exponential decay with increasing height above the plane, and a reduced phase velocity v < c, where c is the speed of sound in air.
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