The active control of low frequency room modes

Avis, MR

Abstract

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.

Citation

Avis, M. The active control of low frequency room modes. (Thesis). University of Salford

• Thesis Type: Thesis
• Deposit Date: Aug 17, 2011
• Publicly Available Date: Aug 17, 2011
• Award Date: Jan 1, 2000