Hybrid passive/active duct noise control by using micro-perforated plates

Abstract

Mitigation of duct noise is an important and challenging research topic which finds wide applications. Our previous work led to a so-called plate silencer, consisting of an expansion chamber with two side-branch cavities covered by plates. Working in a purely reactive manner, the device can produce a wide stopband from low to medium frequencies provided the plate is extremely light and stiff, to ensure a strong reflection of acoustic sound wave upstream in the duct. However, a plate with a slightly weak bending stiffness will result in non-uniform transmission loss (TL) spectrum with narrowed stopband, which is detrimental for broadband noise control. In this study, a hybrid passive and active silencer is proposed by following two steps. First, micro-perforation is introduced into the plate silencer. Drilled with sub-millimetre holes, the micro-perforated plate aims to elicit the sound absorption in order to compensate for the deficiency in the passband caused by the insufficient sound reflection in a certain frequency range due to weaker plate stiffness. To this end, a theoretical model, capable of dealing with the strong coupling between the vibrating micro-perforated plate and the sound fields inside the cavity and the duct, is developed. Through proper balancing between sound absorption and reflection, the plate silencer with micro-perforation provides a more flattened and uniform TL and a widened stopband by more than 20% while relaxing the harsh requirement on the bending stiffness of the plate by about 20%. Theoretical predictions are validated by experimental data, with phenomena explained through numerical analyses. Furthermore, piezoelectric actuator is introduced to the system of the plate silencer with micro-perforations, leading to a so-called hybrid passive and active noise control system. The aim for adding the active part is to further broaden the effective range of the plate silencer with micro-perforation to even lower frequencies. To realize the goal, an additional plate attached with piezoelectric actuators is inserted to the duct wall adjacent to the original micro-perforated plate and backed by an independent rigid cavity. A theoretical model, capable of characterizing vibro-acousto-electromechanical coupling of the entire system, is established. The hybrid system is investigated under passive and active operation modes. Numerical analyses reveal the influences of major parameters, providing guidance for the design. It is shown that the lower limit of the working range of the plate silencer with micro-perforation can be further extended to the low frequency with a reasonable control voltage under relatively simple control strategy.