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NDE applications of air-coupled ultrasonic transducers

In recent years, several new NDE applications of air-coupled, as opposed to liquid-coupled, ultrasonics have been seriously pursued. The applications range from through transmission testing of aerospace laminates and various honeycomb or foam filled materials, and solar cells, to pulse-echo flaw detection in plastic electronic packaging materials and in-line inspection of large-diameter gas pipelines. Most of the new applications have been made possible by major advances in component technologies: transducers, preamplifiers, pulsers, signal-conditioning, and impedance matching and noise-matching circuits. In this paper, we describe the transducer types and circuit topologies that are especially suitable for applications in the 20 kHz to 20 MHz frequency region. We also address the important key differences between air-coupled and liquid-coupled inspection configurations. In particular, we focus on the practical advantages and limitations that arise from the fact that gases and liquids exhibit substantially different specific densities and sound speeds.