ATS recently completed a research project on long distance sound propagation in the Phoenix valley. The project was in response to resident complaints about high noise levels at distances of 1/4 mile or more from recently constructed freeways. Measurements confirmed that the complaints were valid and that maximum hourly Leq sometimes exceeded the FHWA/Arizona Noise Abatement Criterion at distances well beyond where sound walls could be an effective mitigation measure.

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The study included detailed noise and meteorology measurements in a representative neighborhood over a two week period in March 2004 and a one week period in October 2004. In addition, computer modeling using the Parabolic Equation method was used to investigate how local atmospheric conditions affect sound propagation. The measurements and the modeling demonstrated that prevailing atmospheric conditions that occur approximately 70% of the time in the Phoenix valley could result in 20 dB or greater sound level changes between early morning inversion conditions and afternoon lapse conditions. Further, short-term sound level fluctuations of as much as 10 dB could be caused by variations in the early-morning, down-slope winds that are typical in the Phoenix valley. These wind flows are not detectable at the ground surface, which means that there can be 10 dB sound level changes over a few minutes with no apparent change in traffic or ground-level meteorological conditions.

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The study also documented the benefits of the quiet pavements that have been widely applied to Phoenix area freeways. The particular treatment was an approximately 1 inch thick layer of asphalt rubber friction course (ARFC) that was applied to the concrete roadway surface. With our measurements we were able to demonstrate that the ARFC resulted in an 8 to 10 dB reduction in the A-weighted sound levels both at 100 feet from the edge of the roadway and in the community 1/4 to 1/2 mile from the roadway.