The ultrasonic wave generator of the present invention always drives a transducer with the resonant frequency f.sub.o of the transducer. Although the resonant frequency f.sub.o changes due to the temperature change etc., the driving frequency automatically follows the change of the resonant frequency...http://www.google.es/patents/US3967143?utm_source=gb-gplus-sharePatente US3967143 - Ultrasonic wave generator
