Abstract: A plurality of small bubbles are continuously generated by transmitting to washing water in a vessel, a high frequency ultrasonic wave from an ultrasonic vibrator which generates a high frequency output from a high frequency oscillator and intermittently transmitted low frequency ultrasonic waves from ultrasonic vibrators wherein the intermittent low frequency ultrasonic waves are produced from low frequency outputs from low frequency oscillators for short periods of time. The plurality of small bubbles are destroyed by the intermittent ultrasonic waves and the pressure from the sound of the ultrasonic waves in the washing water is thereby increased and washing performance is improved.

Abstract: A drive circuit is used in a Langevin type ultrasonic bolt-tightening motor in which a motor drive voltage having a given frequency is applied to a piezo-electric element in a stator section, the resulting longitudinal and torsional vibrations being effective to rotate a motor section. The drive circuit has a longitudinal vibration sensor for detecting the longitudinal vibration in the stator section, a torsional vibration sensor for detecting vibration in the stator section and a frequency controller for controlling the frequency of the motor drive voltage such that the phase difference between the detection signals of the longitudinal and torsional vibration sensors becomes 90 degrees so as to provide maximum efficiency. Thus, the frequency of the motor drive voltage can be feedback controlled to maintain an optimum drive frequency despite the varying of the optimum drive frequency due to changes in various factors.