Active Control in Vehicles and in the Inner Ear: a Review

Abstract

The low–frequency sound inside a number of aircraft and cars is now attenuated using commercial active sound– control systems. These operate either using loudspeakers to directly drive the sound field, or with shakers acting on the structure to modify its vibration and, hence, reduce excitation of the sound field. As the structure becomes larger, the number of actuators and sensors required for effective control rises significantly. Conventional, fully coupled control systems then become costly in terms of weight and sensitivity to individual failures. An alternative strategy of distributing the control over multiple local controllers will be discussed, which has been shown to be effective in a number of cases. The workings of the inner ear also provide a remarkable natural example of decentralised active vibration control, which aims to enhance the motion within the cochlea. A simple model for this cochlear amplifier, in which each of the outer hair cells act as local control loops, will be described and its use illustrated in predicting the otoacoustic emissions generated by the ear as a result of this mechanism. These emissions are used clinically to screen the hearing of young children, so it is important to understand how they are generated within the cochlea.