Sound Propagation Over Rigid Porous Layers

Howorth, Craig (1991). Sound Propagation Over Rigid Porous Layers. PhD thesis The Open University.



The work studies two related topic areas namely:
1. the measurement of the acoustical characteristics of rigid porous materials, that is the impedance and propagation constant;
2. the propagation of sound over the surface of a hard backed layer of such a material.

A review of impedance measurement leads to the selection of an indirect method which is employed successfully on a wide range of surfaces. A numerical comparison of impedance models follows including a one-parameter semi-empirical model, a phenomenological model and a microstructural model which relates several physical parameters of a material to the acoustical properties of a surface. The models differ in their prediction of the acoustical properties of a low porosity material. A numerical comparison of the solutions of point source propagation in the presence of a porous media indicates that t he ‘extended’ Weyl van der Pol approximation is reliable over short source receiver distances. The study is extended to examine the phenomenon of the acoustical surface wave. Three experimental techniques are used to produce new evidence for the existence of such a wave which shows good agreement with the theoretical predictions. The indirect method is used to obtain impedances and model parameters for a wide variety of surfaces varying from soils to fibreglass and which are compared with the results of an impedance technique and with standing wave tube measurements.

The indirect method of impedance measurement is employed together with the microstructural model and the propagation model examined earlier in studies of the acoustical properties of porous road surfaces. It proves possible to use the indirect method both to determine the microstructural parameters and to classify the acoustical properties of such a pervious surface when the sound source is either a loudspeaker point source or a vehicle. The results of the parameter determination are validated by a series of non-acoustical measurements. With regard only to the excess attenuation provided by these surfaces, optimum combinations of material parameters are suggested which should lead to the production of a low noise road surface for many types of vehicle and tyre tread patterns.

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