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This paper is concerned with the erection of median barriers as an imaginary noise source when a pair of roadside noise barriers is located at highways. A simple rigid median barrier causes a significant degradation introducing extra multiple reflections and can decrease the performance of parallel noise barriers. Different median barrier models were designed including rigid profiled, sloped, absorptive and diffusive shapes to reduce the mentioned negative effect. The acoustical performance of designed screens is predicted by a verified two dimensional boundary element method. Comparison between the results of A-weighted road traffic noise spectrum with rigid barriers has shown that only sloped barriers can make an improvement in which barrier model KAP with lower diffraction edges indicates the highest efficiency. Although employing grass on the top surface of median barriers could make some improvements relative to their equivalent rigid barrier, the average improvement in insertion loss was significant when the thickness of absorptive material was increased. Application of Quadratic Residue Diffusers (QRD) and Primitive Root Diffusers (PRD) on the top surface of the median barriers have shown lower performance than the median barriers with QRD and PRD stem faced toward the source point. Among the designed median barriers, the highest efficiency was seen in a median barrier with 1 meter thickness and PRD stem surface by 2.1 dB (A).

Multiple Reflection Reduction of Security Screens Installed among Parallel Noise Barriers Introducing New Shapes