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.
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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Meteorological tower (45 ft) used during field measurements.
Wind speed and direction were measured at the top of the tower and temperature
was measured at eight positions (5 foot increments) on the tower.
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