Figure 5-17. Concave Sound Entry Surface.
For a convex surface, the acoustic power that reaches an internal discontinuity is reduced by refraction at the test
surface (see Figure 5-18). Signals received from a discontinuity have less amplitude than signals received from the
same size discontinuity in a test specimen with a flat sound entry surface.
Figure 5-18. Convex Sound Entry Surface.
The following paragraph and Figure 5-17 and Figure 5-18 are applicable when
shoes are made of plastic, as is most common. However, shoes may be fabricated
from the same material as the test part. If this is done, the sound will propagate
straight into the test part. Refraction does not occur, because the velocity in the
shoe equals the velocity in the test part. For immersion techniques, no shoe is
required, but refraction will be greater than illustrated in Fig. 5-18 and 5-19.