Novel methods of transduction for active control of harmonic sound radiated by vibrating surfaces
Date
1996
Authors
Burgemeister, Kym A.
Editors
Advisors
Hansen, Colin Henry
Snyder, Scott D.
Snyder, Scott D.
Journal Title
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Thesis
Citation
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Abstract
Large electric transformers such as those used in high voltage substations radiate an annoying
low frequency hum into nearby communities. Attempts have been made to actively control the
noise by placing a large number of loudspeakers as control sources around noisy transformers
to cancel the hum. These cancellation systems require a large number of loudspeakers to be
successful due to the imposing size of the transformer structures. Thus such systems are very
expensive if global noise reduction is to be achieved.
The aim of this thesis is to investigate theoretically and experimentally the use of thin perforated
panels closely placed to a heavy structure to reduce the radiation of unwanted
harmonic noise. These panels can themselves be vibrated to form a control source radiating over
a large surface surrounding the primary source. The problem of the equipment overheating inside
the enclosure is alleviated because the holes in the panels still allow natural cooling.
An initial study is carried out to determine the resonance frequencies of perforated panels. The
use of previously determined effective elastic properties of the panels and Finite Element
Analysis to theoretically calculate their resonance frequencies is examined.
Secondly the attenuation provided by active noise control using perforated panels as control
sources is explored by use of a coupled analysis, where the primary source is assumed to
influence the radiation of the perforated control panel. This analysis was found to predict poorly
the amount of attenuation that could be achieved, so an uncoupled analysis is undertaken, where
both the primary and control sources are assumed to radiate independently of each other. Not
only does this greatly simplify the theoretical analysis but it also enables prediction of attenuation
levels which are comparable to those determined experimentally. The theoretical model is
reformulated to enable comparison of the sound power attenuation provided by perforated panel
control sources with that of traditional acoustic and structural control sources.
Finally, the use of modal filtering of traditional acoustic error sensor signals to give transformed
mode (or power mode) sensors is examined. The independently radiating acoustic transformed
modes of the panel are determined by an eigenanalysis and a theoretical analysis is presented for
a farfield acoustic power sensor system to provide a direct measurement of the total radiated
acoustic power. The frequency dependence of the sensor system, and the amount of global sound
power attenuation that can be achieved is examined. Experimental measurements are made to
verify the theoretical model and show that a sound power sensor implemented with acoustic
sensors can be used in a practical active noise control system to increase the amount of
attenuation that can be achieved. Alternatively the sound power sensor can be used to reduce the
number of error channels required by a control system to obtain a given level of attenuation when
compared to traditional error criteria. The power mode sensor analysis is then applied to the
perforated panel control system, with similar results.
School/Discipline
Engineering (Department of Mechanical Engineering)
Dissertation Note
Thesis (Ph.D.)--Engineering (Department of Mechanical Engineering), 1996.
Provenance
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