The active control of low frequency room modes
The normal modes of an enclosed sound field introduce spatial, time and frequency domain artefacts to signals reproduced in such an environment, such that undesirable colouration of these signals may be perceived. Modal density with respect to frequency is minimal at low frequencies, and in small rooms this means that widely spaced discrete modes dominate sound reproduction up to frequencies of the order of one hundred Hertz. The removal of unwanted colouration is not straightforward; conventional passive absorptive treatments offer poor performance at low frequencies, and where their use is attempted costs (in terms of space consumption) may be prohibitive. This Thesis presents a series of investigations into the active control of low frequency acoustic resonance, using both adaptive digital filters in feedforward and feedback configurations, and fixed feedforward controllers. The adaptive filters are based around the active control of acoustic impedance, using hardware available to the project as the product of previous work at Salford University. The application of the technique to the control of modes in a three dimensional environment is however novel. The fixed feedforward controllers use a novel application of an analytical modal decomposition of an enclosed soundfield as the basis for a digital IIR acoustic model. This model is utilised in order to manipulate 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 threedimensional test environments, using numerical models and practical hardware implementations. The tasks include pressure cancellation, and more usefully the control of frequency domain Q-factor and corresponding modal decay times. It is shown that active impedance methods are superior in the duct; the fixed feedforward controllers suffer from the combined effect of the finite source impedance of practical control loudspeakers with changing and strongly modal radiation loads. In the room, both techniques are shown to be capable of useful reductions in modal Q-factor and decay time. Fixed methods offer control over a defined spatial volume, and adaptive techniques may be further developed by the refinement of the control hardware.