A parametric investigation of the performance of multiple edge highway noise barriers and proposals for design guidance

Abstract

The Boundary Element Method has been employed for a parametric investigation of the performance afforded by highway noise barriers with multi-edge tops and different acoustic treatment. Configurations investigated included single additional edges located on either the source or receiver sides of the barrier and two additional edges located symmetrically on each side of the barrier. The effect of treating the internal faces with a sound absorbing material was also investigated. The parameters investigated included the source to barrier distance, the receiver to barrier distance, the barrier height, the length of the additional edge and the gap between the additional edges and the face of the barrier. The values of each parameter were selected to be those appropriate to a practical installation. The performance of each edge variation was investigated for both reflective and absorptive faces. The relative insertion loss afforded by a given multi-edge configuration was found to be a function of the location of the source, the barrier and the receiver and also the height of the barrier. However, the sensitivity of the relative insertion loss to variations in most parameters was not great. For both reflective and absorptive treatments, the relative insertion loss of most additional edges was found to be only slightly greater on increasing the length of the edge above 0.5m. Although there was always an increase with increasing gap width over the range of widths investigated gap but the indications were that benefit to be obtained for gap widths in excess of 0.4m may not be an economic proposition. For source locations close to the barrier and receiver locations in the far field, as would be appropriate for a highway noise barrier, although the relative insertion loss afforded by a reflective additional edge located on the source side of the barrier is generally low, significant attenuation can be obtained when an absorptive treatment with a high coefficient of absorption at frequencies around 1000Hz is applied to the device. However, a reflective edge located on the source side of a barrier at very short source to barrier distances and/or high barriers was found to result in a negative value of relative insertion loss and an explanation based upon resonances in the gap between the additional edge and the barrier was postulated. The use of two additional edges located symmetrically either side of a barrier can be a very effective means of improving the performance of a highway noise barrier giving relative insertion loss values that are high and consistent over a large range of parameters for both reflective and absorptive surfaces. However, in the former case the performance can be severely reduced for combinations of short source to barrier distances and/or high barriers.