The application of system modelling to digital electronic systems for active control of acoustic noise

Abstract

Active noise control (ANC) is concerned with achieving some degree of cancellation between unwanted acoustic noise and the output from one or more actively controlled secondary sources. The minimum configuration for an A.N.C. system is a detecting microphone to detect the unwanted acoustic noise and a single loudspeaker to act as a secondary source. The basic problem is to specify the electronic system needed between the detecting microphone and the loudspeaker to achieve cancellation. The system has an inherent feedback pathbetween the loudspeaker and the detecting microphone that may cause instability. The system described in this thesis uses the principle of parallel feedback to ensure system stability. The basic idea of this is to use a second electronic system in a feedback path parallel to the electroacoustic feedback path to, in effect, remove this feedback. Incorporating both this feedback modelling and the appropriate forward transfer function between thedetecting microphone and the loudspeaker should produce a practical stable working system. This thesis considers implementation of these two electronic systems as sampled data systems employing finite impulse response (FIR) filters. Measurements on an experimental duct are used as input data for the study of system performance, this data being analysed using aleast squares approach to find best fit. FIR filters. The system is assessed in terms of the lengths of FIR filters required in the feedforward and feedback paths to achieve satisfactory operation, i.e. that the system produce a significant level of attenuation and be stable. The predicted level of performance of the ANC system indicates that the use of two independent FIR filters to model the feedforward and feedback paths produces a stable working system, capable of producing a significant level of attenuation over a broad frequency range. However, the length of FIR filter needed in each path has to be long in order to achieve a satisfactory level of performance.