*Article* **Fabrication and Modeling of Matching System for Air-Coupled Transducer**

**Jinjie Zhou, Jiaqi Bai and Yao Liu \***

School of Mechanical Engineering, North University of China, Taiyuan 030051, China; zhoujinjiechina@126.com (J.Z.); baihuahua6@outlook.com (J.B.) **\*** Correspondence: liuyao@nuc.edu.cn; Tel.: +86-19834530812

**Abstract:** The tremendous acoustic impedance difference between the piezoelectric composite and air prevents the ultrasonic transition, resulting in low amplitude for the received signal for the composite defect detection using an air-coupled transducer. The matching system, which includes the matching layers and bonding layers attached to the piezoelectric composite, can reduce the acoustic impedance difference and benefit the acoustic transition. In this paper, the fabrication method and modeling for the matching layers are proposed to optimize the transducer performance. The effects of bonding layer material on the transducer performance are also discussed. Experiments were conducted for modeling validation. The proposed model can predict the matching layer acoustic properties with an error of less than 11%. The bonding layer using the same material as the first matching layer can help to increase the sensitivity by about 33% compared to the traditional epoxy bonding. The optimized air-coupled ultrasonic transducer, based on the results of this study, has a 1283 mV amplitude in the air, which is 56% higher than commercially available transducers, and can identify the defects in two typical non-metallic composite materials easily.

**Keywords:** air-coupled acoustic transducer; matching layer; acoustic impedance; bonding layer
