A study of perforated panels for sound absorption

Pao Yue-kong Library Electronic Theses Database

A study of perforated panels for sound absorption

 

Author: Law, Lok-yin Peter
Title: A study of perforated panels for sound absorption
Degree: M.Phil.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Noise control
Acoustical materials
Architectural acoustics -- Measurement
Department: Dept. of Mechanical Engineering
Pages: ix, 97 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1719352
URI: http://theses.lib.polyu.edu.hk/handle/200/142
Abstract: A general analytical model for the evaluation of sound attenuation and absorption coefficient of common sound absorbers has been established. Compared with common theoretical predictions, this model is more general because it includes the sound attenuation by interference effect above the sound absorbers, and the absorption coefficient of the absorbers. The study uses various kinds of sound absorbers as testing specimens, including fibrous absorbers, micro-perforated panel absorbers, perforated panel absorbers, and mixed construction (fibrous materials covered with micro-perforated panel or perforated panel) absorbers. Outdoor sound measurement is conducted to obtain the sound attenuation by interference effect above the sound absorbers, the impedance tube and reverberation testing method is used to determine the normal and average absorption coefficient of the absorbers respectively. The first part of our study is to introduce the basic theory of predicting the acoustic impedance and sound attenuation above the sound absorption materials. Outdoor sound measurements are conducted in order to determine the surface impedance of the fibrous absorbers and to measure the sound attenuation above the absorbers. Based on the outdoor sound propagation theory, a model is proposed for predicting sound absorption coefficient in spherical wave condition. In the common theoretical model for sound absorber, they assume infinite distance between the sound source and receiver. The proposed model can be used for both cases of infinite and finite distance between the sound source and receiver, and therefore it can provide more accurate prediction. The absorption coefficient of the absorbers depend on the range of receiver and the position of source and receiver. In the second part of the thesis, the classical Maa's theory is firstly reviewed. The unproved formula of oblique acoustic impedance of micro-perforated panel absorber as proposed by Maa is validated by the author using numerical fitting method by outdoor sound propagation theory. The impedance model of fibrous materials covered with micro-perforated panel is introduced. Outdoor sound measurement is conducted to consolidate the impedance and realize the sound attenuation by interference effect above the absorbers. Furthermore, a proposed theory based on the outdoor sound propagation theory and the classical Dah-You Maa theory is presented for the sound absorption performance of the micro-perforated panel absorber. Compared to Maa's theory, the model is more general because it includes the sound attenuation and the absorption coefficient of the micro-perforated panel absorber. The theoretical prediction has good agreement with the experimental results. Moreover, the absorption coefficient of fibrous materials covered with micro-perforated panel are predictable based oh the proposed theory. In the final section of the thesis, a theory for predicting the surface impedance of perforated facings on sound absorption materials is established. Boundary element method (BEM), the existing approximate BEM and 3-dimensional approximate BEM is used to verify the impedance model of the absorbers based on the proposed model. Good agreements between the predicted results and BEM results have been obtained. Moreover, an equivalent electrical circuit approach (EECA) is briefly reviewed in order to verify the normal and average absorption coefficient in spherical wave condition of sound absorbers. This is a more general analytical model of predicting sound attenuation by interference effect and absorption coefficient of micro-perforated panel absorber as well as perforated panel absorber.

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