Noise reduction systems by structural-acoustic coupling using membrane are proposed in order to overcome the geometric restriction of the reactive muffler such as an expansion chamber. The expansion chamber has a disadvantage taking up excessive space for installation to get higher noise reduction performance. In this study, methods of leading impedance mismatching by structural motion are proposed. The suggested systems are compared with an expansion chamber. Theoretical analysis is conducted using modal expansion approach, which enables predictions of transmission loss.
First, noise reduction system using membrane and a cavity is suggested. Membrane is installed as a part of the duct wall and an air cavity is covered outside the membrane-duct system to prevent breakout noise. In this case membrane is coupled with the air inside the cavity and duct, which causes rapid impedance mismatching and noise reduction. Theoretical prediction and experimental results show good noise reduction performance which is valid for narrow band low frequency.
Second, membrane is installed inside the duct vertical to the direction of plane wave propagation. This system is suggested in order to give no volume change to the rigid duct. The frequency at which the maximum transmission loss occurs is found to be near the natural frequency of the installed membrane. Analytical method shows good agreements with the experimental results in predicting the frequency at which the maximum transmission loss occurs.
From the two cases, we can infer that noise reduction using membrane is applicable to the system where the narrow band noise control with lower geometric restriction than the expansion chamber is needed.