Adaptive active control of low-frequency noise in a duct

A method for realizing noise control by a technique of adaptive signal processing is suggested on the basis of a physical model of a progressive wave in a duct. A mathematical model is constructed and a transfer function of the adaptive system for noise control is also worked out. Adaptive filtering is realized with the ADSP2101. We also analyzed and compared the effects of algorithms such as RLS, LMS and LSL on noise control. It was shown, without the feedback of sound, that a 27.5 dB mean-noise-reduce-volume (MNRV) for broadband noise from 0 to 500 Hz in frequency could be achieved with the method. However, when acoustic feedback takes place and an air-stream loudspeaker is used, the MNRV was only 4.86 dB. But if the loudspeaker had a plain frequency feature, MNRV would have been improved by 10.2 dB. The technique is applied to reduce the noise from engines and their exhaust gas pipes. It is, in principle, used for noise canceling in a closed three-dimensional space