Cancellation of acoustic noise in a pipe using digital adaptive filters

This paper describes a series of experiments on the control of acoustic noise by antiphase electroacoustic feedback, utilising a 4-chip digital adaptive filter developed by British Telecom Research Laboratories. An active control system must provide accurate modelling of an acoustic path (for 20dB attenuation the error in magnitude of the modelling must not exceed 10% and the error in phase must not exceed 6°), but also a should be made inherently stable despite the closed loop necessarily formed by the (detector)/(filter system)/(secondary source)/(detector). The system described here overcomes the problem in two parts. There is a digital adaptive filter arranged to act in the same way as a conventional echo canceller to kill the feedback loop, and there is a second such filter arranged to provide the required feedforward transfer function for cancellation to be achieved. The characteristics of these filters are described and also illustrated by experimental results.