Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

The absolute noise level and the absolute strength of an echo from a "small"
defect depends on a number of factors, which include:
1. The probe size and focal properties.
2. The probe frequency, bandwidth and efficiency.
3. The inspection path and distance (water and/or solid).
4. The interface (surface curvature and roughness).
5. The flaw location with respect to the incident beam.
6. The inherent noisiness of the metal microstructure.
7. The inherent reflectivity of the flaw, which is dependent on its acoustic
impedance, size, shape, and orientation.
8. Cracks and volumetric defects can reflect ultrasonic waves quite differently.
Many cracks are "invisible" from one direction and strong reflectors from
another.
9. Multifaceted flaws will tend to scatter sound away from the transducer.