The range equation relates distance from the reflector to which two quantities?

• A. time-of-flight, distance
• B. frequency, wavelength
• C. time-of-flight, propagation speed
• D. propagation speed, density

Answer: (C)

Understanding how distance relates to time-of-flight and propagation speed. In ultrasound, the distance to a reflector is determined by how long the pulse takes to travel to the reflector and back (time-of-flight) and how fast sound travels in the medium (propagation speed). The range equation is d = (c × t) / 2, where c is the speed of sound in tissue and t is the total round-trip time. The speed is the key factor that converts time into distance, and the round-trip nature of the travel requires dividing by two.

This is why time-of-flight and propagation speed are the quantities involved. Frequency and wavelength describe the wave’s oscillation and spacing, not the distance to a reflector, and density does not appear in the standard range equation (even though it influences speed, the equation uses speed itself).