Externally applied electric field in piezoelectric material causes mechanical stress in addition to electric charge displacement. This effect is bidirectional in that mechanical strain can in turn cause electric charge displacement. The bidirectionality suggests that a piezoelectric element can be used simultaneously as both a sensor and an actuator for bending vibration control of a thin plate. To do this, the key problem that needs to be solved is to separate the two kinds of electric charge displacement generated in the piezoeoectric element due to the mechanical strain and the electric control signal. By introducing a dummy piezo-electric element to provide a reference on the effect due to the electric control signal only, successful separation of the two kinds of charge displacement signals has been accomplished. The effectiveness of the simultaneous use for the control system is demonstrated in substantial reductions of the sound pressure transmitted and the bending vibration in the plate.

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