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Near grazing incidence, the sound field due to point sources over porous or comb-like boundaries can include an airborne surface wave. Analytical expressions for the phase and group speeds of this surface wave are obtained using (i) a thin air layer approximation for the impedance of a comb-like surface, (ii) two approximate models for the acoustical properties of porous media and (iii) a model for a surface consisting of identical parallel slanted slits above a hard plane. Comparisons of model predictions indicate that the approximate model, derived initially to predict the acoustical properties of porous asphalt, is the least accurate. Predictions of surface wave attenuation and group velocity are compared with data obtained over arrays of parallel aluminium strips. [ABSTRACT FROM AUTHOR]

Previous studies of airborne surface waves have been carried out at audio-frequencies over periodically rough surfaces formed by lighting lattices and bricks and at ultrasonic frequencies over mmscale compound gratings involving grooves of the same depth but different widths. In this paper, a modal model, which has been found to give comparable predictions to BEM while being much faster to run is used, together with BEM, to investigate audio-frequency sound propagation over grooves of varying widths and depths in the order of cm. Predictions of excess attenuation spectra, pressure contours and zeroes in the effective reflection coefficient over grooves with multiple widths and depths are used to indicate the nature of the surface waves which involve overlapping quarter wavelength resonances. Predictions are extended to multiple depth grooved surfaces without and with porous infills. [ABSTRACT FROM AUTHOR]

Ground effect results from the interference between sound travelling directly from a point source to a receiver and that reflected coherently from the ground. The associated excess attenuation spectra are determined by the source-receiver geometry and the ground impedance. If the ground is hard and smooth then at grazing incidence the first destructive interference may occur at too high a frequency to be useful in noise control. The effective impedance of a rough hard surface is finite and destructive interference can occur at much lower frequencies than if the surface is smooth. If the roughness is spaced periodically then the phenomenon of diffraction assisted rough ground effect occurs. Instead of the single broad destructive minimum observed typically with randomly-spaced roughness elements, a cluster of two or three relatively narrow destructive interference peaks are observed. This phenomenon has been explored through laboratory experiments using semi-cylindrical, rectangular and wedge-shaped strips placed on a glass sheet. The resulting ground effect depends on the number of roughness elements and their spacing. Good agreement with data has been obtained using multiple scattering theory for semi-cylindrical roughness and either BEM or FEM simulations for other roughness shapes [work supported by European Community FP7 project number 234306]. [ABSTRACT FROM AUTHOR]