If a reflector moves twice as far from the transducer, how does that affect the time-of-flight of the sound pulse?

• A. quadruple it
• B. double it
• C. no change
• D. halve it

Answer: (B)

Time-of-flight equals the time the pulse spends traveling to the reflector and back, which is determined by the round-trip distance divided by the speed of sound in tissue. If the reflector is at distance D, TOF = 2D / c. Move the reflector to twice that distance (2D); the round-trip distance becomes 4D, so TOF = 4D / c. That is twice the original TOF (2D / c). So the time-of-flight doubles when the reflector moves twice as far. This holds as long as the speed of sound in the medium stays essentially constant.