How do you calculate thickness from time-of-flight measurements?

• A. Thickness = time-of-flight × velocity.
• B. Thickness is independent of time-of-flight.
• C. Thickness = (velocity × time-of-flight) / 2, using the correct material velocity.
• D. Thickness = velocity ÷ time-of-flight.

Answer: (C)

In ultrasonics, the time-of-flight is the total time for a pulse to travel to the far surface and back to the transducer. To get the actual thickness, you use the material’s sound velocity for the one-way path and account for the round trip: thickness equals velocity times the one-way time, which is half the measured time-of-flight. So the correct relationship is thickness = (velocity × time-of-flight) / 2, using the correct velocity for the material (often obtained from calibration or material data). This is why you divide by 2: the measurement covers both going to the far surface and returning. The other options miss this two-way travel or mix up the relationship, and would give incorrect values or units.