Acoustical Surveying of Porous Soils

Abstract

Studies of outdoor sound propagation, particularly at near grazing-incidence and of the phenomenon of acoustic-to-seismic coupling have motivated a search for suitable models for the acoustical characteristics of outdoor ground surfaces. In many instances of outdoor propagation, it has been found possible to obtain sufficient accuracy by assuming both that the ground is locally-reacting, so that it may be described by its normal surface impedance (Z_s), and, by calculating Z_s from empirical power law relationships for the characteristic impedance of fibrous absorbents¹. These relationships introduce a single parameter, the flow resistivity of the surface. The range of validity of these empirical relationships was stated by their authors to be 0.01 < (fΩ_o/R_s) < 1; where f is the frequency in Hz, Ω_o is the equilibrium density or air in Kg m ^-3 and R_s is the flow resistivity in N s m^-4. Moreover the relationships were derived from data for dry fibrous materials with porosities near unity and flow resistivities ranging from 5000 to 80000 N s m^-3. Despite this, the relationships have been used successfully in combination with well-established theory for propagation from a point source above an impedance boundary to describe excess attenuation, up to 1000 Hz, over short ranges encompassing a wide variety of ground surfaces outdoors ². The single parameter that, consequently, describes the acoustical behaviour of the ground has been called the effective flow resistivity (R_se).