[1] Therefore, most conventional ultrasound methods require the use of some type of acoustic coupling medium in order to efficiently transmit the energy from the sensor to the test material.
Couplant materials can range from gels or jets of water to direct solder bonds.
An electromagnetic acoustic transducer (EMAT), is a type of non-contact ultrasound that generates an ultrasonic pulse which reflects off the sample and induces an electric current in the receiver.
The sensitivity of these devices continues to improve; a newly developed piezoelectric transducer can produce frequencies in the MHz that can easily propagate through even high acoustic impedance materials such as steel and dense ceramics.
In general non-contact ultrasound would facilitate testing of materials or components that are continuously rolled on a production line, in extremely hot environments, coated, oxidized, or otherwise difficult to physically contact.