Duct noise suppression with partition walls

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Duct noise suppression with partition walls

 

Author: Shao, Jing
Title: Duct noise suppression with partition walls
Degree: M.Sc.
Year: 2014
Subject: Noise control.
Acoustical engineering.
Hong Kong Polytechnic University -- Dissertations
Department: Faculty of Engineering
Pages: vi, 93 leaves : ill. (chiefly col.) ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2752254
URI: http://theses.lib.polyu.edu.hk/handle/200/7510
Abstract: Silencer is a device for reducing noise. Noise control can be easily tackled from medium to high frequency range. However low frequency noise control still remains a technical challenge for acoustical researchers. Since the extensive application of a perforated liner on noise control, researchers have made great efforts to pursue methods for extending the effective absorption bandwidth of this type of passive sound-absorbing structure. For various types of duct mufflers or silencers with perforated liners, it has long been a challenging problem of how to control a sound source whose frequency varies due to the change of engine operation condition or environmental conditions. A similar broadband requirement exists when an acoustic lining is used to suppress the combustion noise. In commercial aviation, the use of the high bypass ratio engine leads to a significant reduction of the tonal noise but marked increases in the broadband component. This has given the need for a new impetus for broadband absorbers. It is well known that the conventional perforated liners have narrow effective absorption bandwidth. The so-called locally reacting liner has a back cavity divided into cells of small cross sections, so that sound only propagates in the transverse direction inside the liner. This is the reason why the absorption peaks of this type of liners are sharp and narrow around a limited number of discrete resonance frequencies. Researchers try to extend the effective absorption bandwidth of a locally reacting liner by the use of a double- or a triple-layer structure, but this inevitably brings the penalty of increasing the size and weight of the acoustic treatment. One way for improving the performance of an acoustic liner makes use of the basic principle that wave reflection and scattering happen at impedance discontinuities. For some time the idea of a nonlocally reacting acoustic liner has received great attention. When the partition plates are removed from the cavity of an acoustic liner, sound propagation occurs not only in the transverse direction normal to the liner facing, but also in the longitudinal direction within the back cavity. In this situation, resonances are produced in relation to two lengths that are the depth and the length of the cavity, thereby resulting in more peaks in the absorption coefficient of the acoustic liner. In this paper, experiments is made to study how the partition walls effects the duct noise suppression with different silencers such as expansion chamber, plate silencer and MPP silencer.

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