During the past fifteen years there has been a lot of research work on active control of sound radiation by smart structures with both embedded structural actuators and sensors. The main principles of this new approach have been investigated and are now well established. Both feed–forward and feedback control schemes have been studied. Feed–forward control is particularly suited to the control of tonal disturbances for which a reference signal is available. In this case the structural actuators are driven in such a way as to rearrange the vibration of the panel so that the total sound radiation detected via the structural actuators is minimised. Normally single channel systems are used with distributed sensor–actuator pairs designed to control the most efficient radiating mode of the structure. This approach is known as Active Acoustic Structural Control (ASAC). Feedback control is instead advantageous for the control of steady state broad band disturbances. Its implementation relies heavily on the collocation and duality of the sensor–actuator transducers in which case the feedback control system is bound to be unconditionally stable. In principle, the most advantageous approach is to implement single channel feedback ASAC systems, although some fundamental problems related to stability have been highlighted when distributed sensor–actuator pairs are used. However recent work has shown that excellent control results can also be obtained with arrays of decentralised feedback control systems using point actuators and sensors. In this case the control system is driven to implement Active Vibration Control (AVC) only which however can be set to minimise both the vibration and sound radiation by the structure. Although the main principles of both ASAC and AVC approaches for the control of sound radiation by smart structures have been pinned down, there is still a lot of research work in progress on the sensor–actuator transducers. In this paper the main features of both strain and inertial sensor and actuator transducers are discussed with reference to both ASAC and AVC control systems. Particular emphasis is given to the physics of control and to the stability issues involved in both adaptive feed–forward and feedback control systems. Finally some new directions for research are proposed where the control transducers are also made adaptive in order to provide better collocation and duality propertie
