FIGS. 11A to 11D show the relationship between the vibration element in FIG. 10 and standing waves, in which FIG. 11A is a side view of the vibration element, FIG. 11B is a chart showing the waveform of a one-pulse standing wave and the positions of piezoelectric elements for detecting vibrations, FIG. 11C is a chart showing the waveform of a two-pulse standing wave and the positions of the piezoe