Nondestructive testing using air-borne ultrasound. 
Over the last two decades, more efficient transducers were developed for the generation and reception of air-borne ultrasound, thus enabling the non-contact, non-contaminating inspection of composite laminates and honeycomb structures widely used in the aerospace industry.  This paper presents the fundamentals of making air-borne ultrasonic measurement, and point out special considerations unique to propagating ultrasound in air and through solids.  Transducer beam profile characterization, thickness dependence and resonance effects in the transmission of air-coupled ultrasound through plates, and the detection and imaging of defects and damage in solid laminates and honeycomb sandwich will be discussed and illustrated with examples.  Finally, a manual scan system developed for implementing air-borne ultrasonic imaging in the field and on aircraft will be introduced.