GROUND CHARACTERIZATION BY SHORT-RANGE PROPAGATION MEASUREMENTS

In recent years, short-range measurements of excess attenuation from a point source have been used together with semiempirical formulas for frequency dependence of impedance to enable the acoustical characterization of ground surfaces, including snow, in terms of a single parameter. This has then been advocated as a basis for predicting ground and propagation effects at longer ranges. An alternative method is described for determining acoustical properties of ground surfaces including sands, soils, and snow from iterative least-squares fitting of the level difference spectrum obtained between a pair of vertically separated microphones within 2 m of a broadband point source. The method is based on a three-parameter model for the surface normal impedance as a function of frequency, together with well-established formulations for propagation from a point source above either local or extended reaction surfaces. The three parameters are porosity, effective flow resistivity, and tortuosity. Independent (nonacoustic) measurements of porosity compare tolerably well with the acoustically determined values for soils that are homogeneous to several centimeters depth. For such soils, fitting comparisons reveal the superiority of the three-parameter impedance model to the single-parameter semiempirical model. Where there are obvious surface crusts, a double-layer model based upon a two-parameter approximation for the characteristic impedance of each layer is found to give better agreement with short-range propagation measurements than the three-parameter homogeneous approximation. © 1990, Acoustical Society of America. All rights reserved.