T.O. 33B-1-15-13Figure 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 toinherent amplitude variations inspection within the near field is not recommended without careful calibration onreference flaws within the near field.5.1.6.2.3 FarField.The zone beyond the near field is called the far field (see Figure 5-12). In the far field (Fraunhofer zone) the intensityof the sound beam falls off exponentially as the distance from the face of the search unit increases.5.1.6.2.4 DistanceversusAmplitude.NOTEThe important thing to remember is that wide variations in amplitude fromdiscontinuities can occur when inspecting in the near field. It is always best tocompare 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.
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