Behaviour of an adaptive algorithm with a moving primary source

Adaptive algorithms, such as LMS are often used in active noise control systems to update the strength of the secondary sources. Generally, the assumption of 'time invariant system' is introduced and the convergence behaviour of the algorithm is examined. If we try to apply the active scheme to traffic noise control, however, the time invariant assumption is apparently invalid due to the drastic changes in the primary paths. This paper describes preliminary investigations of such cases, i.e., the tracking ability of the adaptive algorithm with moving primary source. The feedforward structure is assumed in the control system, and the well known multichannel filtered-x LMS is used as an adaptive algorithm. The convergence properties are examined in both frequency and time domains. The results of the simulations in the frequency domain indicate that the attenuation varies irregularly compared with the case of a stationary primary source. In the time domain calculation, approximately 2 to 3 dB stable attenuation was obtained, however, an adverse effect was also observed especially when the primary source passed in front of the system.