An investigation of the sound field above a surface with periodic roughness

When audio-frequency sound is incident near grazing on acoustically-hard surfaces with which could be used to amplify acoustic signals and, therefore, improve detection ranges of, perimeter security systems. Experimental and numerical studies using the Boundary Element Method (BEM) of the sound field generated over periodicallyspaced rectangular strips also show several enhancements as a result of complex interactions between the sound field and the rough surface. Surface waves result in excess attenuation spectra with anomalous maxima greater than the 6.02 dB that would be expected from constructive interference above a smooth acoustically rigid surface. The enhancements are found to depend on the roughness spacing and can be attributed to effects due to the finite width and periodicity of the array, quarter-wavelength resonances in the gaps between elements and Bragg diffraction. Pressure maps of the total sound field over rough surfaces show the details of the sound field at the frequencies of interest. As well as being useful for amplifying frequencies arriving at a sensor array, detailed study of the enhancements provides understanding of the evolution of the sound field over rough surfaces.