Occupational Hygienic Department, Scholl of Public Health, Tehran University of Medical Sciences, Tehran,Iran
The paper presents the results of an investigation on the acoustic performance of vertical profile parallel barriers with quadratic residue diffuser tops and faces. A 2D boundary element method (BEM) is used to predict the barrier insertion loss. The results of rigid and with absorptive coverage are also calculated for comparisons. Using QRD on the top surface and faces of all vertical profile parallel barrier models presented here is found to improve the efficiency of barriers compared with fully absorptive equivalent parallel barrier at the examined receiver positions. It is found that reducing the design frequency of QRD shifts the performance improvement towards lower frequency, and therefore the most efficient model for vertical profile parallel traffic noise barrier is a setup treated with QRDs tuned to around 400 Hz. The overall performance improvement by the above diffusive barrier is predicted to be 5.8 dB (A) compared to its rigid equivalent barrier. It is also found that if increase in absorption coefficient of QRD by well reduction destroys the effect of wells in resonance; it will also have negative effect on the performance of parallel QRD barrier and will reduce the overall A-weighted insertion loss of the reactive barriers.