Although most vibration and noise control systems are open loop stable, the natural damping in these systems can be quite small. Thus, the first objective of a feedback control system is to ensure closed loop stability. To improve on open loop performance, asymptotic stabilization requires controllers that dissipate energy and drive the system response to zero. Under forcing, undamped systems may resonate, resulting in unbounded response. Natural or feedback damping can eliminate resonances and guarantee bounded response for bounded forcing. This chapter applies the mathematical tools introduced in Chapter 3 to the development of passive control systems for distributed vibration and noise. Passive controllers are defined as control laws that are not explicitly based on the system model.
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