Doppler Effect for Sound Waves

Steps:

• The observed frequency change indicates a Doppler shift, where relative motion between source and detector affects the detected frequency.
• For a stationary detector, if the source moves towards it, the detected frequency increases above the emitted f.
• If the source moves away, the detected frequency decreases below f.
• Here, detected frequency < f, so the source must be moving away from the detector.

Why D is correct:

• Doppler formula for source moving away from stationary observer: f' = f / (1 + v_s/c), where v_s is source speed and c is sound speed, yielding f' < f.

Why the others are wrong:

• A: Diffraction bends waves around obstacles but does not change frequency.
• B: Reflection can reverse direction but preserves frequency.
• C: Source moving towards Y would increase frequency via Doppler effect (f' = f / (1 - v_s/c) > f).

Final answer: D