FIELD EVALUATION OF ACOUSTICAL PERFORMANCE OF PARALLEL HIGHWAY NOISE BARRIERS IN CALIFORNIA

The effect of multiple reflections between two parallel highway noise barriers on the acoustical performance of each of the barriers has been a subject of considerable controversy. Mathematical and scale modeling predict possible large reductions (degradations) in the effectiveness of noise barriers with smooth, hard surfaces (such as masonry or concrete) due to parallel configurations. However, noise measurements under conditions of actual highway traffic, typical barrier heights, and separations have yet to confirm the large degradations. The methods and results of a parallel noise barrier research project performed by the California Department of Transportation are presented. Field measurements of noise, traffic, and meteorology were made in three stages: before barrier construction, after construction of the near barrier, and after construction of the barrier on the opposite side of a highway. The selected site was typical of many parallel barrier configurations in California. More than 100 simultaneous, A-weighted, 15-min energy-averaged noise levels were measured at 11 microphones from 5 to 23 ft high and 15 to 200 ft behind the near barrier. The noise data were matched by crosswind vector wind velocities and normalized for differences in traffic. Analysis results showed degradations of 0 to 1.9 dBA, independent of wind. Vector wind velocities of -3 to +11 mph caused variations in noise levels of up to 9 dBA at 200 ft behind the near barrier.