Doppler Effect Requires Relative Motion

Steps:

• Identify Doppler effect as frequency shift in waves due to relative motion between source and observer.
• Note the observer is stationary, so motion must come from the source for sound waves in a medium.
• Eliminate options unrelated to motion: polarization, amplitude changes, or inherent frequency variation.
• Confirm A directly states source motion, matching the effect's cause.

Why A is correct:

• Doppler formula for sound, $f' = f \frac{v}{v \pm v_s}$, derives frequency change from source speed $v_s$ relative to medium speed $v$.

Why the others are wrong:

• B: Sound waves are longitudinal, incapable of polarization like transverse electromagnetic waves.
• C: Amplitude variation modulates intensity, not frequency, producing no Doppler shift.
• D: Inherent frequency changes indicate modulation, not motion-induced Doppler effect.

Final answer: A