PROPAGATION OF TRAFFIC NOISE

Abstract

The effects of various traffic, ground cover, and geometric conditions on traffic noise propagation were evaluated in this study. There were two general methods of data collection. The first method consisted of using as many as four sound-level meters and graphic-level recorders to take simultaneous recordings of the traffic stream; the second method involved a constant noise source using a random noise generator. The L sub 10 noise level reduction per doubling of distance was found to increase substantially when the traffic volume was less than 1,000 vehicles per hour. Wind speed and direction were found to have a large effect on noise propagation. Ground cover was also found to have a definite effect. Data were taken on short grass, tall weeds, tall grass, average grass, pavement, gravel, smooth dirt, snow, and plowed field. The drop-off per doubling of distance was found to decrease from about 4.5 dBA for receiver heights of 10 feet (3 m) or below to 3.0 dBA for heights above 10 feet (3 m). At heights above 10 feet (3 m), the type of ground cover did not have a significant influence on the propagation loss. Noise attenuation per doubling of distance remained constant back to about 400 feet (122 m) where the drop-offs were influenced by the ambient noise level. Individual noise readings indicated that noise propagation was influenced by vehicle type and speed. Noise drop-off was larger for smaller percentage levels, but the differences decreased as volumes increased. Source height was also found to have an effect on noise propagation. (FHWA)