Refraction and Attenuation of Sound by Wind and Thermal Profiles over a Ground Plane

Abstract

There is a significant excess attenuation of sound traveling downwind within a typical turbulent shear profile due to the downwind refraction and spreading of the sound rays. This attenuation is in excess of that due to absorption by turbulent eddies, spherical spreading, and ground reflection. When the source and receiver are both located on the ground, the excess attenuation of the directly transmitted sound due to refraction is predicted to be uniform at −11 dB beyond a short range downwind of the source until a shadow zone is reached at a much longer range. This predicted excess attenuation decreases markedly with the elevation of the source and receiver, particularly at short range. There can be an indefinitely large number of reflected rays transmitted to a receiver downwind of a ground-level source, and the shallow reflected rays which skip off the ground a large number of times may have little excess attenuation, since they travel through the laminar sublayer where the velocity gradient is constant. There are several caustics formed downwind of a ground-level source. The results show that wind can easily cause anomalous recording of the sound-pressure level from freeways, airports, and other urban noise sources.