Figure 5-12. Schematic Presentation of Sound Beam.
The smaller the transducer element diameter or the lower the frequency, the shorter the near field will be. Due to
inherent amplitude variations inspection within the near field is not recommended without careful calibration on
reference flaws within the near field.
The zone beyond the near field is called the far field (see Figure 5-12). In the far field (Fraunhofer zone) the intensity
of the sound beam falls off exponentially as the distance from the face of the search unit increases.
Distance versus Amplitude.
The important thing to remember is that wide variations in amplitude from
discontinuities can occur when inspecting in the near field. It is always best to
compare discontinuity signals with signals from reference standards, such as flat-
bottom holes, having the same metal travel distance as the discontinuity.
Figure 5-13 is a typical curve of amplitude response versus distance from the transducer face.
Figure 5-13. Amplitude Response Curve of Typical Search Unit.