The active control of acoustic noise in a small enclosure

Abstract

This thesis describes the theory and implementation of a system for the active control of acoustic noise in a small enclosure ("small" inferring that only a small number of acoustic modes dominate the reverberant field in the en c1osure). The theory for a multichannel active noise control system is developed. A system could consist of a number of detectors and cancellation sources controlling the field at a number of monitor positions. The simplest system comprises a single detector and a single source and is capable of controlling the field at one position. The controller for this system could consist of a pair of electronic filters, one between the detector and the source in parallel with another cancelling the feedback from the source to the detector. It is shown that for any configuration of transducers the required controllers can be realised by repeatedly using the same filter pair as described above. A particular study is made of the active control of reverberant fields. A system was successfully implemented to partially control the reverberant field in a small enclosure (0-5 x 0-6 x 0.7m). This consisted of a single detector microphone and a single control loudspeaker controlling the field at a single monitor microphone. The system was controlled with two finite impulse response filters realised using a Texas Instruments TMS32020 microprocessor accessed via a Ferranti PC360XT personal computer. The first two modes of the reverberant field were successfully attenuated. In summary, the theory for a multichannel controller has been developed and the simplist case tested. The importance of this is that the basic unit of a multichannel controller has been successfully implemented and this unit could be replicated as the basic building block for more complex controllers. This facilitates the implementation of controllers to attenuate the sound field at a number of points in a practical enclosure.