If one wants to control the noise from a duct, then one must have a sufficient number of sensors and actuators so that the control system is observable and controllable. The number of sensors and actuators for controlling radiating noise from a duct has to be incorporated with the number of modes that one wants to control. These considerations motivated the present study which considers a control system that has fewer sensors and actuators than the number of propagating modes in a duct. The control performance and robust reliability of such a kind of control system are investigated, theoretically and numerically. The likelihood of success in reducing the radiation power of a duct is verified by theoretical analysis and numerical simulations. In addition, the control performance of the control system is verified by experiments. (C) 1999 Academic Press.