A study of Helmholtz resonators on broadband noise control and their potential application on ventilation

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A study of Helmholtz resonators on broadband noise control and their potential application on ventilation

 

Author: Wang, Xu
Title: A study of Helmholtz resonators on broadband noise control and their potential application on ventilation
Degree: Ph.D.
Year: 2012
Subject: Ventilation -- Noise.
Noise control.
Resonators.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xvii, 125 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2551288
URI: http://theses.lib.polyu.edu.hk/handle/200/6799
Abstract: Noise abatement and ventilation control are two important aspects in improving the indoor environmental comfort in buildings. The aims of this thesis were to achieve a better understanding of the broadband noise control and ventilation effects of Helmholtz resonators. Helmholtz resonator (or simply called resonator), a noise control device that has a resonance peak, is useful when noise centralized in a narrow frequency band. In this thesis, both the lumped- and distributed-parameter models of the Helmholtz resonator were considered. The latter model considering the multi-dimensional wave propagation inside its neck and cavity gave a better prediction of its resonance frequency. To obtain a boarder noise attenuation band, combing several resonators is a possible way. This thesis reported a theoretical study of sound propagation in a one-dimensional duct with identical side-branch resonators mounted periodically. Bloch wave theory and the transfer matrix method were used to investigate wave propagation in these spatially periodic resonators. Three types of stop-bands were discussed, and their bandwidths were predicted. The results predicted by the theory fit well with the computer simulation using a three-dimensional finite element method and the experimental results. This study indicated that the wave coupling in this periodic system resulted in the distribution of the frequency band into the stop- and the pass-bands. The long-term significance is that periodic resonators may more effectively control noise in ducts by broadening the bandwidth they attenuate. This thesis also considered the disorder in the periodic duct-resonator system. Two cases were investigated: the disorder in periodic distance and the disorder in the geometries of Helmholtz resonators. The latter case was then compared to the traditional case of an array of differently tuned resonators without periodic mount. It was found that the analysis of the disorder in the geometries of resonators with periodic distance being kept unchanged provides a useful way for the design of such a system to achieve a relatively wide noise attenuation band and to track some narrow noise peaks within it. Apart from their extensive application on noise control, Helmholtz resonators were found to have the effect on ventilation. This thesis presented the results of a theoretical investigation of the airflow through the neck into the cavity of a side-branched Helmholtz resonator. It was found that the motion of the airflow in the opening may result in an air jet formed in the resonator that provided the fresh air from the ventilation duct. This air jet was composed of the "escaped" air masses, and the "escaped" air masses resulted from the behavior of the airflow in the region of opening-enclosure interface that can be regarded a sink-jet model. This ventilation method can be considered as "AC" ventilation with the electrical analogy, which is far less understood than the traditional ventilation method. It is hoped that the present study can provide a stepping stone for investigation of both the acoustic and ventilation performance of Helmholtz resonators, and seeking their potential application in improving the indoor environmental comfort in buildings.

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