A point-like singing bird emits approximately constant acoustic power while flying directly towards a stationary listener. Neglect absorption of sound. If the measured sound-intensity level rises by 20 dB, what is the bird's new distance from the listener compared with its initial distance?
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Question

A point-like singing bird emits approximately constant acoustic power while flying directly towards a stationary listener. Neglect absorption of sound. If the measured sound-intensity level rises by 20 dB, what is the bird's new distance from the listener compared with its initial distance?

Options

G1 Physics · Accepted Answer

A 20 dB rise means 20 = 10log₁₀(I₂/I₁). Therefore, I₂/I₁ = 100. For a point source of constant power, I ∝ 1/r². Hence, I₂/I₁ = r₁²/r₂² = 100. Thus, r₁/r₂ = 10, so r₂ = r₁/10. Correct option: C.