Active noise cancellation for the radiated noise out of ventilation exhaust systems in buildings

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Active noise cancellation for the radiated noise out of ventilation exhaust systems in buildings


Author: Cheng, Shuk-fong Justine
Title: Active noise cancellation for the radiated noise out of ventilation exhaust systems in buildings
Degree: M.Sc.
Year: 1998
Subject: Noise control
Ventilation -- Noise
Acoustical engineering
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Structural Engineering
Pages: iii, 80, [78] leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: In this research report, the radiated noise problems of ventilation exhaust systems in buildings are identified by survey work. In order to select the appropriate noise reduction system, the performance of different noise attenuators will be assessed by survey and experiment work. This research will focus on the feasibility of active noise cancellation systems for the practical ventilation systems. The active noise silencer has the advantages of low power consumption and low pressure drop. Compared with the dissipative and reactive noise silencer, the active noise silencer requires smaller plant room space for the low frequency noise attenuation. The operation frequency band of active noise silencer is wider than that of reactive noise silencer. From the literature survey, the approach of active noise cancellation has been applied in the commercial and industrial markets in the foreign countries including the U.S. and Japan. A survey is carried out to study the noise problem generated by the air exhaust systems and the noise control methods used for the air exhaust system. Two buildings are also selected for detailed discussion. From the survey results, passive dissipative noise reduction systems such as splitter silencers, duct linings and acoustic enclosures are commonly used to reduce the low frequency noise generated in air exhaust systems. It is found that the designers of noise control systems rarely consider the use of reactive or active noise silencers which can attenuate low frequency noise more effectively. From the survey findings, appropriate silencers are urgently required to solve the low frequency radiated noise problem to meet the latest Noise Control Ordinance. In this research, experimental work is carried out to simulate the noise attenuation and study the performance for the dissipative, reactive and active silencers. Different configurations of active noise cancellation are also studied. The experiment results show that the dissipative silencer can effectively reduce the noise above 500 Hz and reactive silencer can effectively attenuate low frequency tonal noise. The active noise cancellation can achieve the attenuation above 10dB in the frequency range 63 to 250 Hz. A feasibility study of active noise cancellation in the ventilation exhaust system is also carried out. The performance of the ventilation system and noise attenuation system in the selected two buildings are examined. Both the literature survey and the feasibility study show that it will be more economical if the active silencer is adopted to attenuate low frequency noise whereas the dissipative silencer is adopted to attenuate middle and high frequency noise. The combination of these silencers can provide more overall noise reduction than just any one of these silencers alone. The physical size of noise silencers are reduced. Furthermore, the energy consumption and electricity cost of ventilation systems can be reduced because the pressure drop of noise attenuators is decreased. When the energy efficiency of ventilation systems is required in future, the active noise cancellation system will be a potential noise attenuator.

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