Appendix A Noise Study Report - Laserfiche WebLink

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Chapter 3. Fundamentals of Traffic Noise 3.8. Sound Propagation When sound propagates over a distance, it changes in level and frequency content. The manner in which noise reduces with distance depends on the factors described below. 3.8.1. Geometric Spreading Sound from a localized source (i.e., a point source) propagates uniformly outward in a spherical pattern. The sound level attenuates (or decreases) at a rate of 6 dB for each doubling of distance from a point source. Highways consist of several localized noise sources on a defined path and hence can be treated as a line source, which approximates the effect of several point sources. Noise from a line source propagates outward in a cylindrical pattern, often referred to as cylindrical spreading. Sound levels attenuate at a rate of 3 dB for each doubling of distance from a line source. 3.8.2. Ground Absorption The propagation path of noise from a highway to a receptor is usually very close to the ground. Noise attenuation from ground absorption and reflective wave canceling adds to the attenuation associated with geometric spreading. Traditionally, the excess attenuation has also been expressed in terms of attenuation per doubling of distance. This approximation is usually sufficiently accurate for distances of less than 200 ft. For acoustically hard sites (i.e., sites with a reflective surface between the source and the receptor [e.g., a parking lot or body of water]), no excess ground attenuation is assumed. For acoustically absorptive or soft sites (i.e., sites with an absorptive ground surface between the source and the receptor [e.g., soft dirt, grass, or scattered bushes and trees]), an excess ground attenuation value of 1.5 dB per doubling of distance is normally assumed. When added to the cylindrical spreading, the excess ground attenuation results in an overall drop-off rate of 4.5 dB per doubling of distance. 3.8.3. Atmospheric Effects Receptors located downwind from a source can be exposed to increased noise levels relative to calm conditions, whereas locations upwind can have lowered noise levels. Sound levels can be increased at large distances (e.g., more than 500 ft) from the highway due to atmospheric temperature inversion (i.e., increasing temperature with elevation). Other factors (e.g., air temperature, humidity, and turbulence) can also have significant effects. Fairview Street Improvements from 9th Street to 16th Street 15 and Bridge Replacement Project Noise Study Report