Which NDT method uses sound waves to measure thickness?

• A. RT (Radiographic)
• B. UT (Ultrasonic)
• C. MT (Magnetic Particle Testing)
• D. PT (Penetrant Testing)

Answer: (B)

Ultrasonic testing uses sound waves to measure thickness. A high-frequency probe sends a pulse into the material, and the echo that returns from the far boundary is timed. Knowing how fast sound travels in the material, you convert the travel time into remaining thickness. This direct use of acoustic travel time makes it the method specifically suited for thickness measurements. Other methods rely on different principles—radiography images internal features with X-rays, magnetic particle testing detects surface and near-surface flaws with magnetic fields, and penetrant testing reveals surface-breaking defects with dye—so they don’t provide thickness measurements from sound waves. Keep in mind UT requires good surface contact with a couplant and material velocity calibration, but it’s the most straightforward and accurate way to gauge thickness.