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- W1512692689 abstract "The normal modes of an enclosed sound field introduce spatial, time and frequency domainartefacts to signals reproduced in such an environment, such that undesirable colourationof these signals may be perceived. Modal density with respect to frequency is minimal atlow frequencies, and in small rooms this means that widely spaced discrete modes dominatesound reproduction up to frequencies of the order of one hundred Hertz. The removal ofunwanted colouration is not straightforward; conventional passive absorptive treatmentsoffer poor performance at low frequencies, and where their use is attempted costs (in termsof space consumption) may be prohibitive.This Thesis presents a series of investigations into the active control of low frequencyacoustic resonance, using both adaptive digital filters in feedforward and feedbackconfigurations, and fixed feedforward controllers. The adaptive filters are based around theactive control of acoustic impedance, using hardware available to the project as the productof previous work at Salford University. The application of the technique to the control ofmodes in a three dimensional environment is however novel. The fixed feedforwardcontrollers use a novel application of an analytical modal decomposition of an enclosedsoundfield as the basis for a digital IIR acoustic model. This model is utilised in order tomanipulate the locations of z-plane poles and change the behaviour of the sound field.These two techniques are applied to a number of control tasks in one- and threedimensionaltest environments, using numerical models and practical hardwareimplementations. The tasks include pressure cancellation, and more usefully the control offrequency domain Q-factor and corresponding modal decay times. It is shown that activeimpedance methods are superior in the duct; the fixed feedforward controllers suffer fromthe combined effect of the finite source impedance of practical control loudspeakers withchanging and strongly modal radiation loads. In the room, both techniques are shown to becapable of useful reductions in modal Q-factor and decay time. Fixed methods offer controlover a defined spatial volume, and adaptive techniques may be further developed by therefinement of the control hardware." @default.
- W1512692689 created "2016-06-24" @default.
- W1512692689 creator A5036357553 @default.
- W1512692689 date "2000-01-01" @default.
- W1512692689 modified "2023-09-27" @default.
- W1512692689 title "The active control of low frequency room modes" @default.
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