Numerical and experimental analysis of focused air-coupled ultrasonic transducer based on cross-linked polypropylene film

Air-coupled ultrasonic transducers based on novel piezoelectrets with extremely small acoustic impedance have some distinctive advantages including no coupling agent, flexibility, and eco-friendliness, compared to conventional ultrasonic transducers. Such advantages make them unique in the applications of nondestructive testing. In this study, a focused air-coupled ultrasonic transducer featuring a sphere-shaped transduction surface is designed by introducing flexible irradiated cross-linked polypropylene (IXPP) films. The characteristic of focusing is evaluated by numerical and experimental analysis. The results show that, by bending the IXPP film into a sphere, the ultrasonic energy can be concentrated to a certain small area to achieve better sensitivity and higher resolution. Furthermore, the IXPP focusing transducer has an obvious focusing effect when the opening radius D of the transducer is increased. Meanwhile, with the reducing radius R of the spherical surface and increasing frequency f of excitation, the value of the deviation coefficient α can be made smaller so that the transducer designed in this way can obtain a better focusing effect.