Tutorial on sound propagation outdoors

Abstract

Concerns about noise in the community date back to the dawn of recorded history. Then, after centuries of relatively little activity, scientific interest grew during the 17th century and social concerns were again voiced during the 19th century. Many of the wave-propagation mechanisms relevant outdoors were understood at least qualitatively by the late 1800s. Today, knowledge of sound propagation phenomena is of great economic and social importance because of environmental and other concerns. Reality is more complicated than geometrical spreading above flat ground. Some grounds are acoustically hard like concrete, and others soft as snow. Corresponding reflection coefficients are complex and vary with angle. Grounds may not be flat, leading to shadow zones or alternatively multiple reflections at the ground. Gradients of wind or temperature refract waves either upwards (upwind or in a temperature lapse) or downwards (downwind or in a temperature inversion). Atmospheric turbulence causes fluctuations in the acoustical effects. Many of these features mutually interact. Measured sound pressure levels owe as much to near-surface weather and to ground shape and impedance as to acoustical factors such as source and receiver heights and their separation. This tutorial paper emphasizes field measurements and simple physical interpretations.