The results of a project that investigated the effectiveness of in situ noise barriers in Florida are presented. The prediction accuracy of the FHWA Traffic Noise Model (TNM) is compared with STAMINA 2.0 and 2.1 (Florida-specific). A total of 20 barrier sites were visited during a 3-year period that resulted in 844 discrete 20-min equivalent sound level (Leq) measurements behind the barriers. Barrier insertion loss was determined using the ANSI indirect barrier method. A methodology was developed to estimate shadow zone length created behind highway noise barriers. All of the barriers tested were effective (>5 dB:LAeq insertion loss at distances equivalent to the first row of homes, where LAeq is the A-weighted Leq) except one site because of marginal additional shielding from a berm–barrier combination. Only three sites had an insertion loss of less than 5 dB at distances representative of the second row of homes. Overall, measurements indicate that the barriers provide substantial sound level reduction for residents along the highway. TNM was the best prediction model when considering all test sites; however, the STAMINA models were more accurate at predicting source level. TNM predictions using the Average pavement input overpredicted the reference sound levels measured at these sites. TNM predictions using the OGAC (open-graded asphalt concrete) input were improved (under 2 dB:LAeq of error) over those using the Average pavement type input. This result is expected because Florida uses an open-graded asphalt friction mix.
A detailed noise prediction model was used to compare 11 highway noise barrier locations in Florida. Insertion losses, ground effects, shadow zones, and overall trends were determined or analyzed, or both. Each location was modeled using STAMINA2.0 (current FHWA regulatory model), STAMINA2.1 (Florida’s version of STAMINA2.0 with state-specific emission levels), the Traffic Noise Model (often referred to as TNM; this model will replace STAMINA2.0 in the year 2002), and the University of Central Florida Community Noise Model (CNM5.0). The modeled results were then statistically compared with the measured results. Statistical evaluation results were similar for all models for overall, absolute prediction compared with the measured value, with STAMINA2.1 being slightly better. All models provided adequate results, but ranges of error were significant. When the propagation components were explored, by comparing reference levels with those behind the barrier, the TNM was significantly better. The results also provided further insight into the benefited regions behind the barrier, a more detailed understanding of how the models perform for this complex interaction with the ground and sound wave, and how background levels change the actual size and shape of the benefited region.
A description is given of new results based on measurements of 19 noise barriers in the state of Florida and the innovative techniques used during data analysis. This work is a continuation of an ongoing study of noise barrier effectiveness in Florida. A new empirical method was developed to estimate the length of shadow zones behind highway noise barriers. This new method can lead to more effective design of future highway noise barriers. The method required an estimate of location and strength of a simulated background source to determine a more realistic edge of the shadow zone. This work also produced custom software that estimates and graphs the formation of shadow zones behind barriers, given readily available site information.
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