Sound from single or dual element transducers is reflected off the backwall of the test piece. The return time of the echo is used to evaluate the thickness. Can be performed in echo-to-echo mode to negate the coating thickness in the case of painted surfaces.

Uses an angled wedge to impart the sound beam at a specific angle. Commonly used in manual inspection of welds.

By dividing the transducer into multiple small elements fired independently, the beam can be swept through a range of angles. Commonly used to inspect welds and perform detailed corrosion mapping. Can provide an image and an encoded record of the scan.