New type of matching layer for air-coupled ultrasonic transducers

摘要

This paper presents a new concept in the design of an impedancenmatching structure for air coupled ultrasonic transducers. A reflectivenlayer structure is inserted between the transducer and propagationnmedium with a small air space. Adjusting the air space and thenreflectivity of the inserted structure causes the transducer impedancento match with the impedance of the propagation medium. Two suchnstructures were investigated as a reflector: a polymer thin film and anthick plate with many holes. Wave impedance theory was applied to thesenreflecting structures, and the impedance of a thin film layer at thenincident surface was calculated using boundary conditions. Impedance ofnholed plate is calculated in a similar fashion. It was found that thencalculated impedance of these structures approximately matched thenimpedance of the PZT air transducer (40 KHz). The acoustic pressurenoutput was maximized by adjusting the position of the matchingnstructure. A theoretical gain of up to 10 dB in the acoustic pressurenwas predicted under ideal circumstances, and the experimentalnobservations showed a gain of 9.5 dB in the acoustic pressure for a 12nΜm polyethylene film placed at a distance of ~0.1 mm from thentransducer's surface. The increase was 9.8 dB for a polyvinylidenenfluoride (PVDF) film and 9.7 dB for a 1.5 mm printed circuit board withnmany holes at a distance of ~25 Μm from the transducer'snsurface