Sound Propagation in Inhomogeneous Media

Abstract

Outdoor sound propagation is of interest in community noise problems, for example noise prediction. The interaction of sound with the ground is also of interest in remote surveillance by acoustic and seismic sensors. In this thesis, previous work concerned with sound propagating in a layered refracting atmosphere, and with acoustic-to-seismic coupling is reviewed and extended. The thesis makes contributions in several areas concerned with these topics. Specifically - methods of predicting propagation in a variable sound velocity gradients are reviewed and a new formulation is advanced; - a program FFLAGS (Fast Field program for Layered Air Ground Systems) is reviewed and revised to improve numerical efficiency and enable wider application; - the Biot theory of wave propagation in poro-elastic media is reviewed and contrasted with a recent theory advanced by de la Cruz and Spanos, particularly in respect of boundary conditions; - the influence of porosity on wave propagation in fluid-saturated porous elastic media is investigated using predictions based on Biot's theory. It is found that there are large difference between predictions for air-filled soils and water-saturated sediments. It is also predicted that porosity plays an important role in determining the wave speed, especially in water-saturated sediments; - the influence of porosity on acoustic-to-seismic coupling is investigated. It is found that a) it is possible to obtain improved agreement with data on the frequencies of maximum acoustic-to-seismic coupling, and, b) it is possible to obtain tolerable agreement with new data on acoustic-to-seismic coupling spectra; - a new method of measuring excess attenuation spectrum of sound using MLSSA (Maximum length Sequence System Analyser) is described and used to investigate the field due to a dipole source over an impedance plane; - directivity effects over an impedance or poroelastic plane are explored using the FFP formulation, asymptotic approximations and laboratory experiments. The over all theme of this thesis is the study of the influence of inhomogeneities in the medium on sound wave propagation. Because of the multiplicity of topics, relevant literature reviews are included in each of the chapters.