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

Stronach, Alexander; Taherzadeh, Shahram and Attenborough, Keith (2019). An investigation of the sound field above a surface with periodic roughness. In: INTER-NOISE 2019 MADRID - 48th International Congress and Exhibition on Noise Control Engineering 2019, 16-19 Jun 2019, Madrid, Spain.



When audio-frequency sound is incident near grazing on acoustically-hard surfaces with periodic sub-wavelength roughness air-borne acoustic surface waves are generated 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 periodically-spaced 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.

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