Analysis of absorption and reflection mechanisms in a three-dimensional plate silencer

When a segment of a rigid duct is replaced by a plate backed by a hard-walled cavity, grazing incident sound waves induce plate vibration, hence sound reflection. Based on this mechanism, a broadband plate silencer, which works effectively from low-to-medium frequencies have been developed recently. A typical plate silencer consists of an expansion chamber with two side-branch cavities covered by light but extremely stiff plates. Such a configuration is two-dimensional in nature. In this paper, numerical study is extended to three-dimensional configurations to investigate the potential improvement in sound reflection. Finite element simulation shows that the three-dimensional configurations perform better than the corresponding two-dimensional design, especially in the relatively high frequency region. Further analysis shows that the three-dimensional design gives better plate response at higher axial modes than the simple two-dimensional design. Sound absorption mechanism is also introduced to the plate silencer by adding two dissipative chambers on the two lateral sides of a two-cavity wave reflector, hence a hybrid silencer. Numerical simulation shows that the proposed hybrid silencer is able to achieve a good moderate bandwidth with much reduced total length in comparison with pure absorption design.