A New Type of Acoustic Modes of Vibration of Thin Piezoelectric Plates: Quasi-Longitudinal Normal Modes

摘要

Using ST-cut quartz crystal plates as an example, a new type of normal modes of acoustic vibrations is described. The modes propagate along the x axis with a velocity close or equal to that of longitudinal bulk waves propagating in the same direction and have a longitudinal component of elastic displacement no less than two orders of magnitude greater than the two other components (the shear-horizontal and shear-vertical ones) throughout the whole plate thickness. The domain of existence of the quasi-longitudinal modes consists of a set of limited zones that contain the "allowed" values of the plate thickness H/λ (H is the plate thickness and λ is the wavelength) and are separated by "forbidden" zones corresponding to common Lamb modes. The closeness (or coincidence) of the velocities of a quasi-longitudinal mode in the plate and a longitudinal bulk wave in an unbounded crystal is a necessary but not sufficient condition for the existence of the aforementioned type of modes in ST,x quartz.