Pulse Echo Technique:

Ultrasonic inspections are largely performed by the pulse echo technique in which a single probe is used to both transmit and receive ultrasound. In addition to the fact that access is required from one surface only, further advantages of this technique are that it gives an indication of the type of defect, its size and its exact location within the item being tested. The major disadvantage is that pulse echo inspection is reliant upon the defects having the correct orientation relative to the beam in order to generate a returning signal to the probe and is not therefore considered fail safe. If the sound pulse hits the flaw at an angle other than 90o much of the energy will be reflected away and not return to the probe with the result that the flaw will not show up on the screen.

Through-transmission:

Through-transmission was used in the early days of UT and is still used in plate and bar production. A probe one side of a component transmits an ultrasonic pulse to a receptor probe on the other side. The absence of a pulse coming to the receiver indicates a defect.