Nondestructive Evaluation techniques such as eddy current or ultrasonics do not give a direct visual indication of the key characteristics of a flaw. Some form of signal interpretation is required in order to determine flaw type, size, and shape. One tool that is frequently employed in the field is the “reference defect”; in the case of ultrasonic testing, reference defects typically consist of notches and flat-bottom or side-drilled holes in a calibration block. Not only are these artificial sound reflectors used to aid in the interpretation of flaw signals, but they may also be used to check that the NDE equipment is working properly each time it is used.