Doppler Shifts always occur if the sound source and receiver are in motion.

• A. True
• B. False
• C. Only when the reflector is moving
• D. Only when the velocity is above a threshold

Answer: (B)

Doppler shifts come from relative motion along the path the sound travels between source and observer. If the source and the receiver move but stay at the same distance from each other along that path (no component of velocity toward or away from the other), there’s effectively no change in the frequency that the receiver detects. Conversely, even a small velocity component toward or away from the other along the beam changes the observed frequency: moving toward increases the frequency, moving away decreases it. There isn’t a required speed threshold—the amount of shift scales with the relative velocity along the line of sight.

In ultrasound terms, you’ll see a shift when the scatterers (like blood cells) have a velocity component toward or away from the transducer, or when the transducer or medium moves in a way that changes that line-of-sight distance. If motion is perpendicular to the beam, the shift is minimal or zero.