Piezoelectric vibrator and method of adjusting vibrating frequency thereof

The disclosure is directed to a piezoelectric vibrator or resonator which includes a vibrating plate employing expansion mode vibration, and a groove or grooves formed at a center and/or in the vicinity of the center of the vibrating plate in a direction intersecting sides of the vibrating plate contributing to the vibration so as to make vibration propagating speed slower than that in the absence of the groove or grooves, and also to a method of adjusting vibrating frequency of such a piezoelectric vibrator.