Noise cancellation inside a rectangular duct

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Noise cancellation inside a rectangular duct

 

Author: Leung, Shek-ping
Title: Noise cancellation inside a rectangular duct
Degree: M.Sc.
Year: 1994
Subject: Noise control
Air ducts -- Noise
Hong Kong Polytechnic -- Dissertations
Department: Multi-disciplinary Studies
Pages: 110, [52] leaves : ill. (some col.) ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1183035
URI: http://theses.lib.polyu.edu.hk/handle/200/320
Abstract: Most of the previous researches were focusing on the controllability and the stability of the active noise control systems, rarely focusing on the properties of the acoustic waves under the active cancellation condition. Moreover, their experimental results only reflected the optimum attenuation of the systems which were measured at the control point. Recently, some of the researchers found that the noise levels at the downstream of control point should be higher than that at the control point. Some of them believed that it should be resulted from the end reflection. One of the possible means to minimize/avoid the end reflection was to install the acoustic foam at the end of the duct. This research hence aimed at studying the effectiveness of the acoustic foam in minimizing/avoiding the end reflection and also the properties of the acoustic waves inside a rectangular duct. Traditionally, there were two active cancellation methods for the duct noise control. The first one was the conventional method. It positioned the secondary source(s) perpendicular to the duct, but the control point was positioned at the centre of the duct. Another method was called the "Energy Absorption Method". Its setup was same as the first one, but the control point was positioned at the surface of the secondary source. In order to reduce the errors due to the complexity of the active control system, this research employed a simple open loop monopole (one primary and one secondary source) system. The input signals were simple sine waves and the experiments were carried out under no flow condition (ideal condition) in order to eliminate the turbulence effect. Using the conventional method with the secondary source perpendicular to the duct, attenuation at the downstream of control point was proved to be lower than that at the control point. It reminded designers that the monitored noise level at the control point could not truly reflect the actual attenuation at the end of the duct. Moreover, results also revealed that noise problem at the upstream of control point was resulted due to constructive interference and standing wave development. Experimental results of the Energy Absorption Method revealed that the noise attenuation at the control point was high, but that at the downstream of control point was low. It was because only the incident wave to the surface of the secondary source was absorbed. As the surface of the secondary source was limited, the energy absorption was low. In order to increase the attenuation at the end of the duct, more secondary sources were required. However, it would eventually increase the cost and the complexity of the active control system. Other than the aforesaid cancellation methods, this research also tried to investigate the effectiveness of a new cancellation method. The setup of the new method was that the secondary source was positioned inside the duct and parallel to the primary source. Experimental results revealed that significant attenuation was achieved at the duct end and was sustainable. Moreover, significant attenuation at the upstream of control point was possible. Besides, standing wave problem at the upstream of control point was not noted. As the results were encouraging, it had the potential application in the active noise control system. This research should be considered as a starting point for further development of this method.

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