“…Piezoelectric ultrasonic transducer, an important electroacoustic conversion device, is used in the ultrasonic cleaning, ultrasonic welding, ultrasonic nondestructive testing, ultrasonic medical imaging, ultrasonic water treatment, underwater acoustic communication and energy collection, etc. [1][2][3][4][5][6][7][8] In the field of power ultrasonic, Langevin ultrasonic transducer, [9][10][11][12][13][14] as known as longitudinal sandwiched ultrasonic transducer, has been widely used.…”