3. Ordering strategies

An active control system consists of a set of secondary sources and a set of sensors designed to provide the signals needed to control these sources, by means of an electronic device (the controller). Two control strategies are available: open-loop or feedforward control, and closed-loop or feedback control. Since a detailed study of these control strategies in active control is beyond the scope of this report, we refer readers wishing to explore this point in greater depth to the works by Nelson and Elliott [54] , Kuo and Morgan [55] , Hansen and Snyder [56] and Elliott [57] , which constitute the reference works in this field...