Sound propagation over mixed impedances

Heap, N. W. (1982). Sound propagation over mixed impedances. PhD thesis The Open University.



Outdoor propagation of surface transportation noise is strongly influenced by reflections from the ground surface due to the creation of an intereference pattern. The result is an attenuation with distance over and above that expected from simple spherical divergence and atmospheric absorption.

A review of literature shows that ground effects can be characterised by a spherical wave reflection coefficient and a normal surface impedance. Theoretical solutions of point to point propagation over three different ground surface models (local reaction, extended reaction and a rigid backed layer) are described and the first two are shown to give good estimates of the excess attenuation as a function of distance.

Methods for measuring the normal surface impedance are discussed and experimental measurements reported. It is concluded that an indirect method employing the local reaction theoretical solution gives reasonable estimates over the frequency range 1-70 kHz.

For practical applications the homogeneous ground surface is modified to include the effects of impedance discontinuities. The theoretical solutions are compared with the results obtained from experiments on laboratory models built to a notional scale of 1:100. In the case of the source over a rigid surface and receiver over an acoustically soft ground surface, it is shown that (a) for a low noise source (e.g. cars) the impedance discontinuity has negligible effect and (b) that for elevated sources (e.g. lorries) the excess attenuation is increased relative to that of a homogeneous surface.

The double discontinuity introduced by a rigid strip in an acoustically soft ground surface is also examined. Experimental results indicate the effect of the strip is insignificant for strip widths less than 10% of the source receiver separation distance. For wider strips it is found that prediction can be improved by postulating an increased average surface impedance.

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