Interference from multiple sound paths causes periodic loudness changes

Steps:

• Varying loudness while walking indicates sound waves combining in patterns.
• Repeated rises and falls suggest alternating constructive and destructive wave interactions.
• This pattern arises from waves taking direct and indirect (reflected) paths to the listener.
• Interference explains the periodic variation as the student moves between points.

Why C is correct:

• Interference occurs when two or more waves superpose, producing constructive interference (increased amplitude, louder sound) or destructive interference (decreased amplitude, quieter sound) per the principle of superposition.

Why the others are wrong:

• A. Diffraction spreads waves around barriers but does not produce repeated loudness variations.
• B. Doppler shift alters frequency due to relative motion, affecting pitch, not periodic loudness.
• D. Reflection redirects waves but requires interference with other waves to cause varying loudness.

Final answer: C