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Title: Noise attenuation by vibroacoustic coupling
Authors: Chiu, Yu-him
Degree: M.Phil.
Issue Date: 2004
Abstract: A series of investigation done by Huang and Choy [1-5] demonstrated that the performance of membrane type silencer could provide a satisfactory performance in duct noise control. This silencer consists of a cavity-backed flexible membrane. Its compact size, environmental friendly properties, zero pressure drop, high transmission loss and broad stopband make it surpass the conventional reactive silencers such as expansion chamber and Helmholtz resonator. However, this membrane type silencer still cannot provide a good performance in very low frequency range (e.g. <200Hz) unless a larger cavity is employed. It is because the membrane type silencer needs a cavity mounted behind the membrane to prevent the breakout noise from the duct. This cavity not only increases the size of the silencer but also prohibits its low-frequency performance. As the air inside the cavity constrains the motion of the membrane, the response of the membrane at low frequencies is compromised. This phenomenon is well-known in vibroacoustics and called "cavity stiffness". To solve this problem, Huang [6] proposed to use magnetic forces to overcome the cavity stiffness. Taking the advantage of magnetic force, a magnetic-induced stiffness (negative stiffness) is expected to cancel the positive cavity stiffness as well as the structural stiffness. However, Huang's analysis totally bypasses the issue of practical implementation and assumes a piston behaviour for the membrane in duct noise control. Such practical issues are explained in this thesis. Moreover, this thesis will also show that, in some regions of the parametric space, the magnetic forces can also increase the structural stiffness of the silencer. Such an increase in structural stiffness may be larger than the increase of magnetic-induced stiffness and hence the total stiffness of the silencer will increase rather than decrease after using magnetic forces. Therefore, the objective of this research is twofold. One is to understand the effect of magnetic force on the vibration of a magnetic/ferromagnetic membrane, and the other one is to see whether such magnetic force can be utilized for neutralizing the cavity stiffness of a cavity backed membrane. The first part of our study addresses the 2-dimensional theoretical analysis on the noise attenuation mechanism by a membrane type silencer. This study shows that the performance of the membrane type silencer at low frequencies is determined by how the resonance of the cavity-backed membrane is achieved at low frequencies. The second part of our study focuses on the feasibility of using the magnetic force to reduce cavity stiffness so that it will resonate at low frequencies. Although the mathematical function of the magnetic force is a simplified one, it is still possible to demonstrate that the existence of magnetic forces can render a magnetic-induced stiffness to reduce the cavity stiffness. Thirdly, Finite Element Analysis (FEA) is employed to study the performance of the proposed silencer. The Finite Element Models presented in this thesis fully couples the membrane vibration, acoustics pressure and magnetic field. Crucial numerical results are validated by experiments. The test rig consists of a versatile tensile gear to adjust the membrane tension, and an efficient LabView(R) code which controls data acquisition via a DAQ card. A four-microphone, two-load method is used for the transmission loss measurement. Detailed analysis is carried out to understand the sound energy loss in the system. Results validate the predictions despite some uncertainties in the test rigs, and show that the magnetic force can indeed provide an alternative engineering solution for reducing the cavity stiffness of a cavity-backed membrane if parameters are carefully chosen.
Subjects: Hong Kong Polytechnic University -- Dissertations
Noise control
Air ducts -- Noise control
Pages: xii, 110, viii leaves : ill. ; 30 cm
Appears in Collections:Thesis

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