Hybrid noise control in duct by using micro-perforated plate

A plate silencer consists of an expansion chamber with two side-branch cavities covered by light but extremely stiff plates. It works effectively with wide stopband from low-tomedium frequencies only if the plate is extremely stiff, to ensure a strong reflection of acoustic wave to the upstream in the duct. However, the plate with slightly weak bending stiffness will results in non-uniform transmission loss (TL) amplitude in the spectra and the stopband becomes narrower. In this study, a hybrid silencer is proposed by introducing micro-perforations into a very light and moderately stiff plate. The purpose of the microperforations is to add the effect of sound absorption to compensate for the deficiency in the passband caused by the insufficient sound reflection in certain frequency range due to plate with weaker stiffness. A theoretical model is developed and modal analysis is conducted to investigate the vibro-acoustic coupling of the system. It is found that the stopband could be widened by 30%. Meanwhile, the use of micro-perforation allows relaxing the harsh requirement on the bending stiffness of plate. The balance between the compromised sound reflection and enhanced sound absorption is the key to achieve a flattened and widened stopband in the transmission loss.