T.O. 33B-1-15-17Figure 5-17.Concave Sound Entry Surface.5.1.7.2.2.2For a convex surface, the acoustic power that reaches an internal discontinuity is reduced by refraction at the testsurface (see Figure 5-18). Signals received from a discontinuity have less amplitude than signals received from thesame size discontinuity in a test specimen with a flat sound entry surface.Figure 5-18. Convex Sound Entry Surface.5.1.7.2.2.3NOTEThe following paragraph and Figure 5-17 and Figure 5-18 are applicable whenshoes are made of plastic, as is most common. However, shoes may be fabricatedfrom the same material as the test part. If this is done, the sound will propagatestraight into the test part. Refraction does not occur, because the velocity in theshoe equals the velocity in the test part. For immersion techniques, no shoe isrequired, but refraction will be greater than illustrated in Fig. 5-18 and 5-19.
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