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Active Control of Flow-excited Acoustic Resonance

其他書名	Higher Order Acoustic Models
出版	CSIRO. Division of Building, Construction and Engineering, 1991
URL	http://books.google.com.hk/books?id=vavxZwEACAAJ&hl=&source=gbs_api

註釋The active control and attenuation of an acoustic resonance excited by the flow around a plate in a duct is described. The acoustic mode is an evanescent first-cross mode of the duct upstream and downstream of the plate and is known as a Parker-beta mode. It is shown that the minimum gain setting on the power amplifier which generates maximum attenuation is obtained when the sound pressure is fed back into the duct 180 deg. out of phase with the flow-excited sound pressure field. It is also shown that the resonant sound is attenuated at four distinctly different rates as the gain on the power amplifier increases. Initially there is a low rate, followed by a high rate and then a low rate again before the system becomes unstable and rapidly amplifies the sound. It is hypothesised that the higher rate of attenuation is due to the vortex shedding becoming uncorrelated along the trailing edge of the plate. The feedback system initially reduces the amplitude of the acoustic field approximately linearly with gain setting on the power amplifier. When the acoustic particle velocity is less than the threshold necessary to ensure that the vortices are shed in phase from the trailing edge of the plate, the vortex street no longer acts as a strong acoustic source and the resonant sound is rapidly attenuated. This feature of active control systems influencing the source of the sound is distinctly different from the active noise attenuators investigated during the past decade. Using a simple non-adaptive active feedback control system, the resonant acoustic field at the fundamental frequency is attenuated 24 dB, while the first and second harmonics are attenuated by 38 and 12 dB respectively.