In straight(normal) beam contact testing, a reduction in the back surface reflection amplitude could indicate:

• A. Inadequate coupling
• B. A near surface defect that cannot be resolved from the main bang (initial pulse)
• C. All of the above
• D. A flaw which is not normal to the beam

Answer: (C)

In straight-beam contact testing, the back surface echo depends on how much energy actually reaches the far surface and returns to the transducer. If the coupling is inadequate, energy transfer into the material is poor, so the back-wall reflection is weaker. A near-surface defect can also reduce the back-wall amplitude because energy may be reflected or scattered by that defect before it can reach the far surface, or its reflection can be buried within the initial pulse, making the back-wall signal appear smaller. Additionally, if a flaw is not normal to the beam, the energy is more likely to be scattered away from the back-wall path or undergo mode conversion, reducing the portion that returns along the original path to the transducer. Since any of these scenarios can cause a reduction in the back surface reflection, all of the above effects can lead to a weaker back-wall indication.