Acoustic propagation near porous and elastic boundaries

Tooms, Stephen (1991). Acoustic propagation near porous and elastic boundaries. PhD thesis The Open University.

Abstract

A model for predicting the response of a system composed of a fluid halfspace, overlying a porous ground layer, resting on an elastic halfspace, to incident plane waves, is developed to include further porous elastic layers within the ground. The dynamic properties of the porous elastic layers are modelled using a modified Biot formulation.

Using the same boundary conditions a Fast Field algorithm for Layered Air Ground Systems (FFLAGS) is formulated to predict acoustic propagation and acoustic-seismic coupling in the model layered system due to a point source in a horizontally stratified atmosphere. This is a full wave solution. Results are compared to those of existing propagation prediction methods. FFLAGS has been used to predict (i) the effects of temperature gradients on short range propagation over an asphalt like surface, (ii) sensitivity of received sound pressure levels to ground parameters for various atmospheric conditions, and (iii) the influence of ground parameters on acoustic-seismic coupling. Predictions of acoustic surface waves in the presence of an upward refracting atmosphere using Creeping wave theory and the FFP method have been shown to agree.

Dispersion equation based predictions of surface wave types have been assessed. It has been shown that the high velocity surface waves predicted by dispersion equation solutions on porous and elastic ground surfaces are not predicted to be excited by a point source. However several other surface wave modes have been predicted in layered systems, similar to those predicted in visco-elastic media.

The influence of ground elasticity on received sound pressure levels is examined. Measurable effects of elasticity of the surface are predicted for low density materials, and measured over a low density polyester foam. Controlled experiments have been performed to study the effect of soil wetting on acoustic to seismic coupling. It is found that the observed effects can be modelled using FFLAGS.

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