The ultrasonic inspection of metals for internal defects is similar in many respects to searching for fish using an ultrasonic fish finder. In each case a sonic transducer sends out a short-duration sound pulse and then “listens” for returning echoes. The defect is analogous to the fish, and the metal grains are analogous to weeds, rocks and other scatterers that can mask the presence of the fish.
One common method of displaying “raw” ultrasonic data is termed an “A-scan”. There the transducer is fixed in position and one looks at the strength of the returning echoes as a function of time, or equivalently of penetration depth. In our fishing analogy, the boat is moored in one spot.
Another common data display type is an ultrasonic C-scan. There the transducer is scanned in two dimensions above the specimen being inspected. The display typically shows the peak response within a time or depth interval of interest as a function of transducer position. In our fishing analogy, the boat moves across the lake in raster search pattern.