There is disclosed an ultrasonic piezoelectric transducer consisting of two separate transducer elements bonded together. The individual elements may be similar or different in size, thickness, and/or material. The material which bonds the two elements together may serve as one electrode, and two other...http://www.google.de/patents/US4276491?utm_source=gb-gplus-sharePatent US4276491 - Focusing piezoelectric ultrasonic medical diagnostic system
