Intensity proportional to amplitude squared

Steps:

• Intensity I of a sound wave is proportional to the square of its amplitude A: I ∝ A².
• For original amplitude x, I ∝ x².
• New amplitude is 3x, so new intensity I' ∝ (3x)² = 9x².
• Thus, I' = 9I.

Why D is correct:

• Sound wave intensity follows I ∝ A², so tripling amplitude multiplies intensity by 9.

Why the others are wrong:

• A: Assumes inverse square proportionality, which is incorrect for intensity-amplitude relation.
• B: Assumes direct proportionality to amplitude, ignoring the square dependence.
• C: Ignores amplitude's effect on intensity.

Final answer: D