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 used 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 L10 noise level reduction per doubling of distance increased substantially when the traffic volume was less than 1,000 vehicles per hour. Wind speed and direction had a large effect on noise propagation. Ground cover also had 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 decreased from about 4.5 dBA for receiver heights of 10 feet (3m) or below to 3.0 dBA for heights above 10 feet (3 m). At heights above 10 feet (3m), the type of ground cover did not have a significant influence on the propagation loss. Noise attenuation per doubling of distance remained constant 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 also had an effect on noise propagation.

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