If the long axis of a discontinuity is parallel to the sound beam, the indication from the discontinuity is best described as:

• A. Large in proportion to the length of the discontinuity
• B. Small in proportion to the length of the discontinuity
• C. Representative of the length of the discontinuity
• D. Such that complete loss of back reflection will result

Answer: (B)

In ultrasonic testing, the strength of the echo depends on how much of the flaw’s surface is facing the sound beam. When the long axis of a discontinuity runs parallel to the beam, the reflecting area presented to the beam is small. That limited area back-reflects energy, so the back wall echo is weak. Because the echo amplitude is governed by that effective reflecting area, the signal is small relative to the actual length of the discontinuity, even if the flaw is long. That’s why the best description is that the indication is small in proportion to the length of the discontinuity.