Ultrasonic testing is used for the detection of flaws in critical parts, such as forgings in gas turbines[3]. Benign features within the microstructure, such as grain boundaries, also reflect part of the ultrasonic pulse. These signals will generally be of a lower amplitude than flaw signals, since they represent a softer discontinuity in material properties. Figure 1 shows a simplified distribution of such ‘grain noise’ signals, and a possible distribution of signals resulting from small flaws. Two difficulties arise due to the presence of grain noise: (1) The smallest flaw signals cannot be observed since they are masked by the grain noise. This defines the limiting resolution of the scan. (2) The largest noise signals are mistaken for flaws, leading to ‘false calls’ and the rejection of sound material.