Sound trapping and dredging barriers.

When sound barriers are installed on both sides of a noise source, degradation in performance is observed. Barriers having negative-phase-gradient surfaces successfully eliminate this drawback by trapping sound energy in between the barriers. In contrast, barriers can also be designed to "dredge" the energy flux out. An extended model considering higher-order diffractions, which resulted from the interplay of the induced surface wave and barrier surface periodicity, is presented. It is found that the sound dredging barriers provide a remarkable enhancement over the trapping ones, and hence have the potential to be widely used in noise control engineering.