OBSERVATIONS OF HIGHWAY TRAFFIC NOISE MEASUREMENTS BEHIND BARRIERS AND COMPARISONS TO FHWA'S TRAFFIC NOISE MODEL

In 1998, the United States Federal Highway Administration (FHWA) released a new tool for highway traffic noise prediction and noise barrier design, the Traffic Noise Model (TNM). In order to assess the accuracy and make recommendations on the use of TNM for the FHWA, the Volpe Center Acoustics Facility performed numerous roadside measurements, obtaining over 100 hours of traffic noise data from highways around the country. A majority of the measurement sites included noise barriers protecting homes, schools, or recreational parks in the area. These barriers were either berms or walls constructed of various materials with varying heights and configurations. For each site, acoustical, meteorological, and traffic data were collected simultaneously throughout the measurement period. Spectrum analyzers were used to collect 1/3-octave band A-weighted equivalent sound levels, and the microphones were deployed at distances from 50 to 300 feet behind the barrier and at two heights off the ground, the number of microphones used being site dependent; a reference microphone was also deployed. Preliminary results indicate that these barriers are providing substantial noise reduction to the protected area, with attenuation over distance varying depending on the noise barrier configuration. Results also indicate that TNM is adequately modeling these typical sites; the calculated sound levels are generally within 2 dB of the measured levels. Also, the effects of wind are seen to influence the accuracy of the model; since TNM does not account for wind, the model is, in general, over- and under-predicting during different wind conditions.\n