A localized acoustic field formed in the vicinity of a bending-vibrator end and its application to piezoelectric sensors

Evanescent acoustic fields that decay exponentially from the plate surface can be created in the vicinity of a piezoelectric bending vibrator. When an object is brought into this evanescent field, the electric admittance of the vibrator undergoes some change depending on the vibrator-to-object distance d. The authors' group has found so far that as an object comes into close proximity with the plate end, an unexpected phenomenon occurs such that the Q-factor, which once decreased, starts to increase in the range where d becomes small. In this study, this unique phenomenon is examined in detail by experiments and some new findings are shown on the variation of bending-vibrator characteristics. Also, theoretical explanations are given to conjecture the mechanism of this phenomenon by using a simple model.