Development of air-coupled ultrasound transducers for nondestructive evaluation

We report the development of air-coupled ultrasound transducers for noncontact nondestructive evaluation applications. Air-coupled ultrasound is an attractive inspection technique for materials or structures that are not suitable for contact or immersion ultrasound inspections. However, due to large acoustic impedance mismatch between air and materials in the system, tremendous acoustic power losses exist. To overcome this problem, the efficiency of the ultrasound transducers needs to be improved. We have developed large-gap capacitive micromachined ultrasound transducers (CMUTs) that have high transduction efficiency in air. The CMUT has a patterned dielectric insulation layer that supports large acoustic output but eliminates dielectric breakdown or charging failures. Acoustic tests showed that the CMUTs had significantly higher transduction efficiency than the state-of-the-art commercial transducers. This indicates improvement of air-coupled CMUTs is achievable and may lead to wide use of CMUTs for air inspections.