A discontinuity smaller than the transducer may produce fluctuating amplitude as the search unit is moved laterally if testing is performed in the:

• A. Fraunhofer zone
• B. Near field
• C. Snell field
• D. Shadow zone

Answer: (B)

In the near field, the ultrasonic beam is complex and highly variable because multiple wavefronts interfere before the beam settles into a simple pattern. A discontinuity smaller than the transducer interacts with different parts of that intricate field as you move the probe laterally. The energy that is scattered back to the receiver can change significantly with tiny lateral shifts, so you see fluctuating amplitudes as you sweep the search unit.

As you get into the far field, the beam becomes more uniform and the energy distribution changes more gradually. A small flaw then tends to produce a steadier indication, with less sensitivity to small lateral probe movements. This is why a small discontinuity is more likely to cause amplitude fluctuations when testing in the near field.