A loudspeaker at a rock concert generates $10^{- 2} W / m^{2}$ at $20 m$ at a frequency of $1 k H z$ . Assume that the speaker spreads its energy uniformly in three dimensions.
What is the total acoustic power output of the speaker?

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Solution

$P = 4 π r^{2} I = 4 \times 3.14 \times 400 \times 10^{- 2} = 50.3 W$

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