Abstract: An electromagnetic ultrasonic double wave transducer, comprising a shell (1), and a permanent magnet assembly, a coil (4), a shielding layer (5), and a wire (6) which are provided in the shell (1). The permanent magnet assembly comprises a first permanent magnet (2) and a second permanent magnet (3) sleeved on the first permanent magnet (2). The magnetizing directions of the first permanent magnet (2) and the second permanent magnet (3) are perpendicular to the bottom of the shell (1), and the magnetic field directions of the first permanent magnet (2) and the second permanent magnet (3) are opposite. A non-conducting non-magnetic bushing material (9) is provided between the first permanent magnet (2) and the second permanent magnet (3), and upper end faces of the first permanent magnet (2) and the second permanent magnet (3) realize magnetic circuit closing by means of a magnetic circuit closing element (8).

Abstract: An air-coupled ultrasonic interferometric method is disclosed. An air-coupled ultrasonic transducer, as a probe, is placed directly facing the surface of a workpiece, and an ultrasonic wave is reflected back and forth between the ultrasonic transducer and the surface of the workpiece; the phase difference of the first echo reflected from the surface of the workpiece and reaching the air-coupled ultrasonic transducer is measured; based on the change of the ultrasonic frequency and wavelength, the measured distance is transformed into the rate of change of the acoustic phase with respect to the acoustic angular frequency, wherein the change in the acoustic angular frequency is a product obtained by multiplying 2? by the difference between the highest frequency F2 and the lowest frequency F1 within the bandwidth fB of the air-coupled ultrasonic transducer.