An active control of noises in a duct is presented. To avoid the instability caused by acoustic feedback from speakers into microphones, Swinbanks' method is applied by using unidirectional speakers and unidirectional microphones.
The maximum reduction of noises is simulated by considering the effects of flow and decay modes from the speakers to the microphones. The simulation shows that the effect of flow is meager and that of decay modes is also ignorable if the distances between the microphones and the speakers are apart more then twice the cross-sectional dimension of the duct.
Experiments have been carried out with flow and without flow. For low frequency noises between 100 to 300 Hz, the experimental results show good agreements with those of our simulation for noise reductions.