ABSTRACT
Attenborough et al.1 (and in this volume, Chapter 6) have described the phenomenon of ground effect over hard and rough surfaces and its representation by excess attenuation spectra. The excess attenuation at a given receiver may be calculated from knowledge of the surface impedance and the source-receiver geometry. Most naturally occurring outdoor surfaces are porous. As a result of being able to penetrate the porous surface, groundreected sound is subject to a change in phase as well as having some of
its energy converted into heat. In earlier outdoor noise prediction schemes, ground surfaces were considered as either “ acoustically hard,” which means that they are perfectly reecting, or “acoustically soft,” which implies that they are perfectly absorbing. According to ISO 9613-2,2 any ground surface of low porosity is “acoustically hard” and any grass-, tree-, or potentially vegetation-covered ground is “acoustically soft.” Although this might be an adequate representation in some circumstances, it oversimplies a considerable range of properties and resulting effects. Even the category of ground known as grassland involves a wide range of ground effects.3
