USE OF SCHROEDER-PHASED WAVE-FORM TO INVESTIGATE CONVERGENCE AND TRACKING OF THE LMS ALGORITHM IN ACTIVE SOUND CONTROL

A Schroeder-phased test waveform is used to investigate convergence an d tracking of the LMS system identification process in the filtered-X LMS active sound cancellation scheme. The coherence between the test w aveform and the error microphone is calculated off-line. The power in the test waveform is increased until coherence between the two signals is achieved. The active sound cancellation algorithm is activated and the LMS system identification is adaptively modified on-line using di fferent power in the test waveform. An error between the actual model, as determined by a Bode plot, and the LMS FIR model is calculated. Wh en the power in the test waveform becomes large enough that coherence between the test signal and the error microphone is achieved, the mode l error decreases to a minimum. Further increase in test waveform powe r shows no further reduction in model error. It is also shown that whe n the part of the control signal which cancels sound is used in the on -line system identification algorithm, the model at the cancellation f requency degrades. This problem does not occur when only the test wave form is used in the system identification algorithm. Finally, it is sh own, both analytically and experimentally, that the zeros in the FIR m odel emulate dynamic poles by spacing themselves uniformly around the unit circle in the z-plane. (C) 1997 Elsevier Science Ltd.