The behaviour of multiple channel adaptive systems for the control of periodic sound

Boucher, Christopher Charles (1992) The behaviour of multiple channel adaptive systems for the control of periodic sound. University of Southampton, Doctoral Thesis.

Abstract

This thesis investigates the behaviour and performance of multiple channel adaptive systems for use in the active control of periodic sound. The convergence of one particular control algorithm, the multiple error LMS algorithm, is considered in detail in terms of the eigenvalues of the relevant Hessian matrix, which determine the principal axis of the control error surface. By transforming the primary field into this coordinate system it is shown how the convergence is composed of the sum of a number of independent control modes. The use of an effort weighting term in the system cost function is shown to reduce the effects of badly conditioned modes and so reduce excessive secondary source power demands. The multiple error LMS algorithm requires models of the acoustic transfer functions between all of the secondary sources and all of the error microphones. The effect of measurement errors in these models on the stability and steady state solutions of the algorithm are investigated. It is shown how the addition of modest levels of effort weighting can significantly improve the likelihood of a given system being stable, even when subject to large errors. The use of multiple independent control systems, acting on the same acoustic space, offers considerable benefits in terms of the required processing capability. These control systems have been modelled as a single equivalent controller, allowing the earlier stability analysis to be directly applied in this case. A generalized extension to the theory for the LMS algorithm is presented which shows how this algorithm is one extreme of a family of algorithms, the Newton's algorithm being the other extreme. These algorithms are compared and used to generate new hybrid algorithms. Practical methods for implementing these algorithms, which help to reduce the computational overhead, are also discussed. The results from a series of in-flight experiments are presented. These experiments were conducted in a B.Ae. 748 turbo-propaircraft and attempted to control the fundamental blade passage frequency of the aircraft and its first two harmonics. Twenty six loudspeaker and microphone combinations were tested, split into two main configurations. The results from these tests are compared with predicted optimum solutions and the effects of an observed phase jitter is analysed. The use of synchrophasing in combination with active control is also assessed.

This record has no associated files available for download.

Contributors

Download statistics