If the distance to a target is doubled, the time-of-flight for the pulse round-trip is:

• A. 4 times
• B. 8 times
• C. the same
• D. 2 times

Answer: (D)

Time-of-flight grows directly with how far the pulse must travel, and ultrasound travels to the target and back, so the round-trip distance is twice the one-way distance. With a constant speed of sound in the medium, time equals distance divided by speed. The original round-trip time is t = (2d)/c. If the distance to the target doubles to 2d, the round-trip distance becomes 4d, giving t' = (4d)/c, which is twice the original time. So doubling the distance makes the time-of-flight twice as long.