Outdoor sound propagation from road tra$c is modelled by solving a boundary integral equation formulation of the wave equation using boundary element techniques in two dimensions. In the "rst model, the source representing a tra$c stream can be considered as a coherent line source of sound. The results can then be transformed to derive a pseudo-three dimensional solution to the problem. In the second model the line source is incoherent. For receivers near the ground, the second model predicted signi"cantly higher values of ground attenuation than the "rst. The "rst model generally produced better agreement with ground attenuation results obtained using the U.K. tra$c noise prediction model. For conditions when a noise barrier was present and the ground was absorbent, the incoherent line source model generally predicted signi"cantly higher values of attenuation than those from the barrier and ground attenuation calculated separately. Over a range of receiver positions and barrier heights a similar, but less marked e!ect was observed when the coherent line source model was used. On dual carriageway roads, it is possible to incorporate barriers on the central reservation as a noise control measure. These are &&median'' noise barriers. The incoherent line source model is used to assess the performance of median barriers in reducing noise when installed alone and also with associated roadside barriers. A sound absorbent median noise barrier 1 m in height produced consistent values of insertion loss of between 1 and 2 dB over the range of receiver positions and ground conditions considered. When the median barrier was used in conjunction with a roadside barrier it produced a consistent improvement in insertion loss of between 1 and 2 dB over the range of conditions considered.
2002 Elsevier Science Ltd.