Study of a longitudinal-flexural mode conversion transducer based on a ring-excited flat plate structure

Air-coupled transducer equipped with a flexural vibration radiator find application in an extensive range of non-destructive material testing, material monitoring, and long-range acoustic energy transmission. In this paper, a class of air-coupled longitudinal-flexural mode conversion transducers with a ring-excited thin circular plate (RTCP) radiator is studied. Based on thin plate vibration theory, the analytical solutions for the radiation directivity of the proposed transducer based on different vibration mode were derived. The finite element models of the RTCPs were established to analyse and discuss the effect of ring excitation on the vibration modes and their radiation directivity. The computational results demonstrate that the directivity of a flat plate radiator could be enhanced by the addition of a ring excitation at the valley of the normal displacement curve for all orders of axisymmetric flexural modes. The RTCP is distinguished by the absence of nodal circles, a lower frequency and a significantly larger ANVD when compared to those observed in the point-excited thin circular plate (PTCP). Furthermore, RTCP exhibits directivity comparable to that of a piston source with equivalent dimensions, yet possesses a larger ANVD. A comparison was made between the RTCP and two other radiators, an existing stepped plate radiator (SP) and a ring excitation fixed boundary radiator (RFP). The results of this comparison demonstrate that the RTCP has a larger ANVD, a narrower beamwidth (BW) of the main lobe, a smaller amplitude (AM) of the side lobe, and the capacity to radiate over a longer distance with a narrower beamwidth. Finally, two transducer prototypes were fabricated, after which the resonant frequencies, vibration modes, radiation directivity and radiated sound field were measured. The findings of this study confirm that the vibration modes and radiation directivity patterns obtained by the three methods (theoretical, finite element, and experimental) are consistent. The designed transducers have been shown to possess exceptional radiation directivity.