If sound waves of 3 MHz, 5 MHz, and 10 MHz are transmitted through the same tissue, reflections from which frequency would reach the transducer first?

• A. 3 MHz
• B. 5 MHz
• C. 10 MHz
• D. Reflections from all three frequencies would have nearly identical transmit times.

Answer: (D)

The key idea is that time of flight depends on how far the echo travels and how fast the sound moves, not on the frequency of the transmitted wave. In soft tissue, the speed of sound is about 1540 m/s and is essentially the same for 3, 5, and 10 MHz signals. A reflector at depth d sends back an echo after roughly 2d/c, so the round-trip time is the same regardless of frequency. The difference you might see is in amplitude due to attenuation (higher frequencies attenuate more), not in arrival time. So echoes from all three frequencies reach the transducer at nearly the same time.