Flow-through silencer design to control low-frequency noise

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Flow-through silencer design to control low-frequency noise

 

Author: Hu, Qi
Title: Flow-through silencer design to control low-frequency noise
Degree: M.Sc.
Year: 2012
Subject: Noise control.
Industrial noise.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Mechanical Engineering
Pages: vii, ii, 91 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2527601
URI: http://theses.lib.polyu.edu.hk/handle/200/6666
Abstract: To devise a silencer works for a broad frequency range especially covering relatively low frequencies is a challenging task to tackle passively. Though this objective can be achieved by traditional passive noise control methods through elaborately design, the drawbacks remain are significant. A research group led by Huang come up with an idea that is totally reactive and works effectively for low-frequency noise control, which is a broadband passive silencer so-called drumlike silencer or plate silencer. The drum-like silencer consists of an expansion chamber with two side-branch cavities covered by light membranes under high tension, which in the plate silencer the membrane is replaced by a simply supported plate as the high tension applied on the membranes is difficult to maintain. The operation mechanism of the drum-like silencer and the plate silencer is identical to each other essentially that it acts as a side-branch wave reflector, which the grazing incident sound waves induce membranes or plates to vibrate and the fluid-structure interaction causes remarkable wave reflection. The present study focuses on the upgraded design based on the great idea of the side-branch wave reflector. The first consideration for improvement is to attach some absorption properties to the initially totally reactive silencer to improve overall performance by using the micro-perforated aluminum foils instead of the original membranes. Moreover, the plate silencer introduced in the previous study requires very light but extremely stiff plates to guarantee excellent performance, which is greatly difficult to achieve this characteristic of material in practice. Hence, another consideration to improve the previous design is to moderate the strict requirements for the plate used.
The acoustic performance of the usage of micro-perforated panel is studied experimentally. The transmission loss is determined as the performance indicator, which is measured by the four-microphone, two-load method. The newly designed two-plate silencer to reduce the strict requirement of the plate applied which should be very light but extremely stiff, is studied theoretically based on numerical simulation and calculations accomplished by the commercial software package Femlab® and MATLAB® for postprocessing. The stopband specified as the logarithmic bandwidth which is the frequency ratio of the lower and higher limit of the frequency-band that the transmission loss of the silencer is above arbitrary determined criterion value, is chosen as a performance indicator for design purpose. The experimental study shows that general changes caused by replacing the original membrane without orifice by the micro-perforated aluminum foil are not greatly pronounced, and moreover, the characteristics of the MPP chosen is very similar to the original aluminum foil without orifice, which means the perforated panel does not vary the fluid-structure interaction very much. However, the overall performance is indeed improved to some degree because the spectra is more smooth-going and the spectral peak is lifted up a little bit for the usage of MPP. In addition, the mean flow in the duct will slightly cripple the operation performance of the silencer, especially for the absorption and the transmission loss, whereas the reflection will be enhanced a trifle. For the upgraded design of two-plate silencer, the theoretical research suggests that the global optimal value of the stopband is achieved when the upstream plate length is 3.5 dimensionally. It is also found that the performance remains the same when the two plates are swapped which means each plate length has its own optimal parameter in this design unrelated to its position with others. There is a major benefit for the two-plate silencer in contract with the plate silencer introduced and studied before. The requirement for the plate used in the silencer of high bending stiffness is reduced remarkably. In addition to what mentioned above, the near field effect of the gap between two plates is studied numerically, the studies show that it can be neglected in the relatively very low frequency range, and though the spectral peaks are basically unchanged, the troughs between them vary pronouncedly which the dip of troughs affects the performance of the two-plate silencer significantly as the length of the gap increasing.

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