Study on the acoustic radiation of an area-excited flat plate structure transducer

Air-coupled transducer with a flat plate radiator has a variety of applications in ultrasonic ranging, material monitoring and directional intense acoustic radiation. In this paper, a longitudinal-flexural composite mode ultrasonic transducer based on an area-excited thin circular plate (ATCP) is studied. The analytical solutions for the vibration displacement and radiation directivity of the ATCP are deduced based on the vibration theory of thin circular plates. The finite element model of an ATCP has been established, and the influence of the excitation area on the vibration frequency, vibration mode and radiation directivity has been analyzed and discussed. The findings indicate that ATCP possesses the capacity to vibrate in a composite mode, encompassing both longitudinal vibration of the inner circular plate and flexural vibration of the outer circular plate. In comparison to the point-excited thin circular plate (PTCP), ATCP has a smaller number of nodal circles. For the first-order vibration of the ATCP, as the excitation radius increases, the vibration frequency of the circular plate increases, the average normal vibration displacement (ANVD) and the beamwidth of the directivity main lobe decrease, but the number of side lobes increases. Furthermore, the ANVD of ATCP is observed to be larger, while its main lobe beamwidth is narrower, in comparison to a piston source and a stepped plate radiator of equivalent dimensions. Finally, a prototype transducer was fabricated and the resonant frequency, vibration mode, radiation directivity and axial sound pressure level were measured experimentally. The results show that the directivity of the transducers obtained by the three methods is consistent, and acoustic waves are mainly radiated in the axial direction of the transducer.