Review of physical mechanism and computational models [outdoor sound propagation]

Abstract

Propagation of noise in the atmosphere is governed by a number of interacting physical mechanisms including geometrical spreading, molecular absorption, reflection from a porous ground, curved ray paths due to refraction, diffraction by ground topography and scattering by turbulence. Accurate predictions of noise levels from a distant source must somehow account for all of these phenomena simultaneously. Although this goal is still beyond current capabilities, developments in computational tools for predicting sound propagation through the atmosphere have increased dramatically during recent years. The computational techniques now include analytical solutions for selected atmospheric profiles, ray tracing techniques which include interaction with the ground and meteorological conditions, and more sophisticated numerical solutions to the full wave equation; the fast field program (FFP) and the parabolic equation (PE). All noise prediction models include the attenuation due to geometrical spreading and, if required, molecular absorption. Where the empirical based models differ from computational models is in the incorporation of the other attenuation mechanisms. The empirical models tend to rely on general tendencies found in experimental databases. They often work well as long the specific situation of interest falls within the bounds of the databases. Computational models on the other hand rely on our mathematical ability to describe real-life situations. The paper summarizes their limitations, their advantages, and shows a benchmark comparison of predictions