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Non-inertial ultra-wideband acoustic transducers

In introductory and even some advanced textbooks covering ultrasonic transducers including piezoelectric discs, the transducers used for excitation are normally introduced as electrically driven mechanical oscillators operated reversely for detection. A refined treatment based on original work from the early 60's of the last century demonstrates that even in this simple case, electromagnetic-mechanical coupling is restricted to interfaces with the volume of transducer discs operating in part as inertial mass, which can also be provided by suitable backing with improved results. Geometrical effects in combination with the oscillating masses lead to resonances of the transducers limiting the applications. Thin transducer discs or film transducers, which are in comparison to the oscillating masses in the generated or detected acoustic waves approximately mass free, can be operated such that inertial effects in the transducer are reduced respectively almost avoided. Even though such transducers are available on a commercial basis and are well introduced for the generation and detection of Lamb waves, the basic underlying principles are usually not highlighted. These effects are experimentally demonstrated and compared to expectations based on basic principles. Schemes suitable to overcome bandwidth restrictions given by geometrical effects are discussed and an application of wideband transducers for Lamb waves used for stress detection is exemplified.