Silencing of the duct noise by using stacked micro-perforated plate with air gaps

Silencing of the mid- to high- frequency noise within a duct can be done by the sound sound reduction effect. A large amount of sound reduction can be achieved by using the resonance effect of the micro-perforated plate (MPP) with backing cavity. In this work, multiple MPP layers with air gaps in the transverse direction are adopted to widen the effective frequency range. Acoustic impedances of MPP layers is arranged to change gradually to minimize the impedance mismatch. To test the idea, a rectangular duct of 30×30 mm2 is taken as the main conduit, of which one of the walls is covered with MPP stacks of 9.2 mm in thickness for 0.45 m in length. Transmission loss is estimated by the transfer matrix method. Above 1.7 kHz, TL larger than 5 dB is achieved. The present system is compared with an expansion chamber having same volume and filled with rock wool. It is seen that the proposed system is more effective for 1.1-3.8 kHz range. As a result, a compact silencer for the mid- to high- frequency noise in a duct is designed.