Flexural mode metal cap transducer design for specific frequency air coupled ultrasound generation

Flexural transducers are effective ultrasonic generators in fluid media, where standard piezoelectric transducers suffer a significant performance loss due to a large impedance mismatch. The flexural modes of piezoelectrically actuated metal caps are routinely used to make low frequency (typically 40 kHz) air coupled transducers for simple distance measurements. Such transducer types offer many intrinsic advantages including an integrated metal buffer for environmental shielding, good fluid coupling for generation and detection of ultrasound, and large amplitude signals for a low driving voltage. In this work, we investigate the design of arbitrary and specifically higher frequency (> 100 kHz) flexural metal cap probes. The analytical theory of vibrating plates was used to determine how the geometry of the cap affects the frequencies of its normal modes. Finite element modelling (FEM) was used to simulate a more realistic system. A first set of prototype transducers was built and investigated. The prototype behaviour is in general agreement with the theoretical and FEM models, but with shifted modal frequencies. The prototype transducers have a strong mode at 140 kHz, which can be used to generate ultrasound in air.