Active Global Control of Time-Harmonic Sound Waves

This thesis deals with the active global control of time-harmonic sound waves. The theoretical foundations of the modelling of the problem in the frequency domain, the underlying inverse problem and the minimum sound power of isotropic source pairs are described. Transform domain adaptive filters, which are used in practice for the real-time solution of the inverse problem, are then derived from the gradient descent method. A brief literature review of active noise control systems with the objective of controlling the wave fields in infinite domains is given. This work was carried out in the context of noise pollution by ferries at berth.
A direct boundary element ansatz for the simulation of active noise control near-field array systems based on speaker models was formulated and its limitations were investigated in an experimental setting. The choice of boundary conditions for the underlying Neumann problem was validated via laser Doppler measurements of the speaker's membranes. This is followed by a comparison of the experimentally determined and simulated radiation characteristics. The technical implementation of a near-field multichannel active noise control system is then described. The hardware and iterative transform domain algorithms for the identification of the frequency response functions in the system and controlling the acoustic wave field in real-time, as implemented on a field programmable gate array, are presented.