ACTIVE CONTROL OF STRUCTURE-BORNE NOISE

The successful practical application of active noise control requires an understanding of both its acoustic limitations and the limitations of the electrical control strategy used. This paper is concerned with the active control of sound in enclosures. First, a review is presente d of the fundamental physical limitations of using loudspeakers to ach ieve either global or local control. Both approaches are seen to have a high frequency limit, due to either the acoustic modal overlap, or t he spatial correlation function of the pressure field. These physical performance limits could, in principle, be achieved with either a feed back or a feedforward control strategy. These strategies are reviewed and the use of adaptive digital filters is discussed for both approach es. The application of adaptive feedforward control in the control of engine and road noise in cars is described. Finally, an indirect appro ach to the active control of sound is discussed, in which the vibratio n is suppressed in the structural paths connecting the source of vibra tion to the enclosure. Two specific examples of this strategy are desc ribed, using an active automotive engine mount and the incorporation o f actuators into helicopter struts to control gear-meshing tones. In b oth cases good passive design can minimize the complexity of the activ e controller.