The relative strengths of observed defect signals and grain noise depend on many factors including the UT instrument being used, the frequency and focusing properties of the transducer, the surface curvature, and the metal microstructure itself. In defect detection work, one endeavors to choose inspection parameters that maximize the signal-to-noise (S/N) ratio, ie. the response from the defect divided by the average response from the competing grain noise. For material characterization work, on the other hand, it is often useful to choose inspection parameters that maximize the backscattered grain noise. Below are two ultrasonic C-scans of a metal specimen containing a set of nine reference reflectors, namely flat-bottomed holes (FBH’s) each having a diameter of 1/64 inch. For the inspection on the right the ultrasonic responses from the holes stand out more clearly from the grain noise background. We will revisit this same figure later when we discuss the concept of ultrasonic pulse volume and its relationship to S/N ratios.
Below is a C-scan from the inspection of a cylindrical Nickel-alloy billet with the gain (signal amplification) boosted to highlight the backscattered grain noise. There one sees obvious variations in backscattered grain noise with position. Upon further study, these were traced to gradual, systematic changes in the average grain diameter from region to region, with regions having larger grains producing more backscatter.
