Sound minimisation for local active control

This thesis is concerned with sound minimisation in space and frequency for local studies have been performed to develop methods of active control. designing larger zones of quiet than are obtained with conventional systems, while a comprehensive study of a practical active headrest system was performed with the aim of improving its performance. A general formulation for 2-norm and oo-norm sound minimisation over a space is described, which in contrast to conventional approaches, which use pressure or pressure and particle velocity cancellation at one point, has no cancellation points within the minimisation region. This method therefore provides the best overall attenuation of sound within the space due to the optimal design. The performance of the sound minimisation method for local active control has been evaluated in this work using computer simulations. Larger zones of quiet were achieved compared with conventional methods for two or three secondary sources. These new design methods could be applied to a practical application, such as a headrest system. The methods described above are then extended to active control of broadband noise. This involves the acoustic pressure minimisation over both space and frequency. The perfoiTnance was investigated through computer simulations, and the results showed that good attenuation could be achieved at desired regions over both space and frequency. These methods could be realised in practice by using virtual microphones to estimate the pressure fields away from the position of the microphones. Hz and H2/H0. methods for designing feedback controllers have also been presented, and implemented in an active headrest system which used single channel feedback controllers to attenuate the noise at a virtual microphone. This is the Orst time that real-time virtual microphone feedback controllers have been implemented in an active headrest system. The theoretical and practical stability and performance issues of the active headrest system have also been investigated in this work. The results showed that good performance could only be achieved with unstable open-loop controllers due to the reduced magnitude of the virtual plant response. This novel analysis could provide guidelines to designing better performing active headrest systems. The performance of an active headrest system has also been evaluated in a realistic environment to predict performance in a passenger aircraft.