For true non-destructive material characterization, a contact-less (air coupled) method of ultrasound is highly desirable.  However, exorbitant acoustic impedance mis-match between the coupling air and the test media presents a natural impediment to Non-Contact Ultrasound (NCU).  This impedance mis-match can be as high as seven orders of magnitude for some materials.  Therefore, high frequency NCU propagation in materials has been regarded as an impossibility.  On the other hand, low frequency ultrasound – 25kHz to 100kHz – can be propagated through materials in non-contact mode.  However, these frequencies are practically useless for the interrogation of most materials where the industry demands high resolution and high detectability.  Recent advances in high transduction piezoelectric transducers (100kHz to 5 MHz) and an exceptionally high dynamic range nanosecond accuracy ultrasonic system have advanced the NCU practice to levels that rival conventional contact method.  In this paper we provide an introduction to NCU and present its applications for analyzing a variety of materials.

Final Frontier of Ultrasound: Realization of High Frequency Non-Contact Analysis