A study of the effect of periodic structure on the attenuation performance of the mufflers

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A study of the effect of periodic structure on the attenuation performance of the mufflers

 

Author: Shi, Xiaofeng
Title: A study of the effect of periodic structure on the attenuation performance of the mufflers
Degree: Ph.D.
Year: 2016
Subject: Air ducts -- Acoustic properties.
Ventilation -- Noise.
Noise control.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xix, 129 pages : illustrations
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2859545
URI: http://theses.lib.polyu.edu.hk/handle/200/8429
Abstract: The use of multiple mufflers is often a way used to improve the sound attenuation performance of the mufflers. When the mufflers are periodically mounted on the duct, the transmission loss of the periodic mufflers is determined by the characteristics of both the muffler itself and the periodic structure. This thesis therefore provides a systematic investigation of the effect of the periodic arrangement on the transmission loss of the mufflers including the simple expansion chamber muffler and the micro-perforated muffler. The study on the wave propagation in such periodic structures provides how the periodic structure influences the performance of the mufflers which can contribute to the design of the periodic mufflers. The theory of various mufflers is investigated. For resonator mufflers, the resonance frequency is mainly determined by its physical parameters. In order to adapt to the changes, a semi-active resonator via the control of the termination impedance of the resonator is used. A theoretical study is conducted to investigate the effect of flow on the semi active Helmholtz resonator in a low Mach number flow duct. To improve the attenuation of the Helmholtz resonator at lower frequencies, a Helmholtz resonator with a spiral neck is proposed and the theoretical results show that the resonance frequency can be effectively lowered by incorporating the spiral neck which have potential application of tonal noise control within a limited space.
The expansion chamber muffler is an effective device for noise reduction in duct systems. The transmission loss of the single expansion muffler has a periodic character that is often used for the periodic noise control. The Bloch wave theory and the transfer matrix method are used to study the wave propagation in periodic expansion chamber mufflers and the dispersion characteristics are examined. The theory is validated against finite element method simulation. Compared to a single expansion chamber muffler, the stopbands of the finite periodic structure is mainly due to its dispersion characteristics. With different configuration, the results indicate that the periodic structure can enhance the transmission loss within a narrower frequency range or change effective noise control frequency ranges with different distance between mufflers. Because of the high acoustic resistance and low mass reactance due to the sub-millimeter perforation, the micro-perforated muffler can provide considerable sound attenuation of duct noise. The wave propagation in periodic micro-perforated mufflers is studied theoretically, numerically and experimentally. This study indicates that the combination of the Bragg reflection due to the periodic structure and the resonance of the micro-perforated muffler can result in different transmission loss. The proposed periodic placement of micro-perforated mufflers can provide lower frequency noise control within a broader frequency range or enhance transmission loss around the resonant frequency.

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