New type of matching layer for air-coupled ultrasonic transducers.

This paper presents a new concept in the design of an impedance matching structure for air coupled ultrasonic transducers. A reflective layer structure is inserted between the transducer and propagation medium with a small air space. Adjusting the air space and the reflectivity of the inserted structure causes the transducer impedance to match with the impedance of the propagation medium. Two such structures were investigated as a reflector: a polymer thin film and a thick plate with many holes. Wave impedance theory was applied to these reflecting structures, and the impedance of a thin film layer at the incident surface was calculated using boundary conditions. Impedance of holed plate is calculated in a similar fashion. It was found that the calculated impedance of these structures approximately matched the impedance of the PZT air transducer (40 KHz). The acoustic pressure output was maximized by adjusting the position of the matching structure. A theoretical gain of up to 10 dB in the acoustic pressure was predicted under ideal circumstances, and the experimental observations showed a gain of 9.5 dB in the acoustic pressure for a 12 microm polyethylene film placed at a distance of approximately 0.1 mm from the transducer's surface. The increase was 9.8 dB for a polyvinylidene fluoride (PVDF) film and 9.7 dB for a 1.5 mm printed circuit board with many holes at a distance of approximately 25 microm from the transducer's surface.