Author: Lam, Tak-wah
Title: The feedback control of thermal noise from a Rijke tube
Degree: M.Sc.
Year: 1999
Subject: Noise control
Acoustical engineering
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Department of Mechanical Engineering
Pages: iv, 49, [34] leaves : ill. (some col.) ; 30 cm
Language: English
Abstract: A simple feedback control mechanism has been developed to actively control the thermal noise generated from a Rijke tube. The feedback control mechanism consists of a microphone mounted near the upper end of the Rijke tube, a loudspeaker at the lower end of the Rijke tube to produce a counter control pressure field. A phase shifter and an amplifier are made to modify the pressure picked up by the microphone and feedback to the loudspeaker. The pressure field produced by the loudspeaker changes the boundary conditions at the lower end of the Rijke tube and the Rijke tube is silenced. This experiment used a glass tube installed with a hot gauze made of nichrome wire at the lower half of a vertical glass tube. This leads to a very loud booming noise at about 380Hz, the fundamental frequency of the tube. The noise level is about 80dBA. It demonstrated standing sound waves excited by the convection currents. The effect on the booming noise caused by the following modifications are also investigated in this study. . providing a backward air current into the glass tube by a ventilation fan. . covering the downstream section of glass tube by heat insulation. . making a small hole on the tube wall. and the results confirm our understanding of the Rijke tube. The pressure field produced by the loudspeaker can be controlled by adjusting the phase shifter and amplifier. It changes the boundary conditions at the end of Rijke tube. The booming noise is cancelled when the phase angle of the controlled pressure field from 180o to 240o behind the primary field.
Rights: All rights reserved
Access: restricted access

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