Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Rather than go into the details of this formulation, a few fundamental
relationships can be pointed out. The signal-to-noise ratio (S/N), and
therefore, the detectability of a defect:
1. Increases with increasing flaw size (scattering amplitude). The detectability
of a defect is directly proportional to its size.
2. Increases with a more focused beam. In other words, flaw detectability is
inversely proportional to the transducer beam width.
3. Increases with decreasing pulse width (delta-t). In other words, flaw
detectability is inversely proportional to the duration of the pulse (∆t)
produced by an ultrasonic transducer. The shorter the pulse (often higher
frequency), the better the detection of the defect. Shorter pulses
correspond to broader bandwidth frequency response. See the figure
below showing the waveform of a transducer and its corresponding
frequency spectrum.