ABSTRACT
Given the many influences on outdoor sound including discontinuous ground, meteorology, diffraction by barriers and topography, it is convenient to use as straightforward a description of the acoustical properties of a ground surface as possible. The acoustical properties of porous ground depend primarily on the ease with which air can move in and out of the surface. This chapter explains why it is undesirable to use the empirical one parameter models to represent outdoor ground impedance, despite their comparative simplicity. Any physically admissible impedance model can be transformed by using Pade approximations to enable implementation in the time domain, but, since it has a straightforward time-domain implementation, it has been common to use the C. Zwikker and C. Kosten phenomenological model for time-domain computations. The Zwikker and Kosten model is, essentially, a high flow resistivity adiabatic approximation of identical pore models.
