Modeling diffuse and specular reflection surfaces and their effect on broadband interior sound field level and intensity flow.

Abstract

For steady-state broadband sound fields in enclosures, surfaces may be modeled as diffuse reflectors, specular reflectors, or some combination. Modeling of reflection surfaces when using energy-intensity based boundary elements is reviewed, and the proper energy-intensity source characteristics are derived. These characteristics depend on local correlation effects, the angular distribution of the incident field, and the angular dependence of reflection coefficient. Specular reflection surfaces subjected to fairly random reverberant sound fields exhibit behavior similar to diffuse reflection surfaces to lowest order. Simple solutions are obtained to illuminate the basic difference in interior sound fields caused by different reflection types. For example, random incidence sound propagating down a long corridor with non-absorbing walls exhibits different behavior depending on whether the surfaces are diffuse or specular reflectors. The diffuse case exhibits a spatial gradient in mean-square pressure along the corridor, whereas the specular case gives a uniform field. These solutions are also used to clarify the relationship between the flow of intensity and the gradient of mean-square pressure. This relationship has been represented simplistically and incorrectly in some energy based methods. The importance of benchmarking energy based methods against analytical solutions, rather than experimental data, is stressed.