Question

The Doppler effect results from a change in which property of sound?

• A. Amplitude of sound
• B. Frequency of sound
• C. Direction of sound
• D. None of the above

Hint:

Moving targets shift the echo pitch - it is a frequency change.

Answer 1

The Correct Option is B

Step 1: The Doppler effect is the apparent change in the frequency of a wave when there is relative motion between the source (or reflector) and the observer (the transducer).

Step 2: In medical ultrasound, the transducer emits sound at a fixed frequency. When these waves strike moving targets, usually red blood cells, the reflected echoes return at a shifted frequency. This frequency shift is the Doppler shift.

Step 3: If the target moves toward the transducer, the returning echo has a higher frequency than the transmitted sound; if it moves away, the frequency is lower. The size of the shift is used to calculate the velocity of blood flow.

Step 4: Hence the Doppler effect is a change in frequency, not amplitude or direction, making frequency the correct answer.