Sound Propagation Illustrations
Sound Paths and Effects of Wind and Temperature
Outdoor sound propagation in an open area near the earth involves three distinct sound paths:
Path 1 - The direct, unimpeded path;
Path 2 - The path reflected by the ground surface, and
Path 3 - The path that is refracted and scattered by the sky.
Path 1 is the most direct path and has the greatest sound energy, followed by Path 2 and 3; each making less of a contribution to the total sound exposure at the receiver than Path 1. Paths 1 and 2 usually determine the sound level at the receiver. Path 3 is made up of sound that is refracted (bent) or scattered back to ground by inhomogeneous air of varying temperature, speed, direction, density, etc. Field studies show that when sound from Paths 1 and 2 are virtually eliminated, there still remain sound levels that are about 20 to 25 dB below the Path 1 and 2 sound levels. This implies that noise barriers will have a maximum attenuation of 20 to 25 dB.
The effect of changes in the ambient temperature gradient (the variation of temperature with altitude) on sound propagation are described in the above illustration. Part A describes a negative temperature gradient with cooler air above. The cooler, denser air causes an upward bending of the sound waves. Part B describes a positive temperature gradient with warmer air above. The warmer air above causes a downward bending of the sound waves.
The effect of wind blowing in the direction of sound propagation is shown in the above illustration. The sound traveling with the wind, Path 1B, is bent downward and adds more sound energy. In this illustration it is filling in some of the energy lost by the traveling through the vegetation on Path 1A. The net effect is sound levels downwind can be increased.
The effect of wind blowing against the direction of sound propagation is shown in the above illustration. The sound traveling against the wind is bent upward and diffracted into the atmosphere. The sound may never reach distant areas under these circumstances. The net effect is sound levels upwind can be decreased.