An Investigation of the Sound Field Above a Surface With Periodically-Spaced Roughness

Stronach, Alex (2020). An Investigation of the Sound Field Above a Surface With Periodically-Spaced Roughness. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0001121b

Abstract

Outdoor audio-frequency acoustic signals can be amplified passively at selected frequencies by exploiting the interaction of incident sound with surfaces composed of periodically-spaced rectangular strips on an acoustically-hard base.

When sound is incident near grazing on acoustically-rigid ground with roughness composed from elements with periodic sub-wavelength spacing, air-borne acoustic surface waves are generated due to a high imaginary-component to the surface impedance as well as the formation and coupling of quarter-wavelength resonances in the gaps. This allows for passive amplification of acoustic signals at the surface wave frequency. This thesis provides a detailed, systematic study into the total sound field generated above surfaces with periodic roughness and how the topography and geometry affect the generation of air-borne acoustic surface waves.

Surfaces with a high number of scattering edges per wavelength result in strong surface wave generation due to high reactive component to the impedance. As the gap is increased thereby reducing the number of edges per wavelength, the gap resonances couple less strongly and the surface behaves as a rough surface. As the number of edges per wavelength approaches one, the signal enhancement is provided by Bragg diffraction. Through measurements and predictions, it is found that surface wave enhancement is not detected by a collocated geophone in sand via acoustic-seismic coupling since the sand is sufficiently absorbing so that no surface wave is detected. This systematic study provides a detailed insight into the formation of audio-frequency surface waves generated over periodically-rough surfaces.

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