On the efficiency of screens near roadside cuttings

The prediction of acoustic propagation from a monofrequency coherent line source in a cutting with impedance boundary conditions over a noise-screen onto surrounding flat grassland is presented. It is well known that over flat ground the spectra for single-noise screens have significant marked differences for propagation over absorbing ground where the screen obstructs surface wave attenuation over the absorbing ground cover. The aim here is to extend the study of this phenomenon for screens adjacent to cuttings or dips surrounded by absorbing ground. The study is based on a numerical model using boundary element techniques that enables the excess attenuation and insertion loss for various noise barriers and cuttings of complex profile and surface cover to be calculated. The model is applied to single-foundation noise barriers to which additional absorbent or rigid side-panels are added to create profiles and a double-barrier configuration is also studied. Spectra of insertion loss, change in insertion loss and excess attenuation results for a broadband traffic-source are presented. It is concluded that “multiple-edged” barriers only show a minor increase in acoustic efficiency over simple vertical screens for a noise source located in perturbed flat ground conditions such as a cutting; in addition it is shown that a screen placed close to the source shows sign of partial improvement.