A) 10 cm
B) 30 cm
C) 40 cm
D) 20 cm
Correct Answer: C
Solution :
Loudness of sound is given by \[dB=10\log \frac{I}{{{I}_{0}}}\]\[\left( \begin{align} & \text{I is intensity of sound} \\ & {{\text{I}}_{0}}\text{ is reference intensity of sound} \\ \end{align} \right)\] \[\therefore \]\[120=10\log \left( \frac{I}{{{I}_{0}}} \right)\]\[\Rightarrow I=1W/{{m}^{2}}\] Also\[I=\frac{P}{4\pi {{r}^{2}}}=\frac{2}{4\pi {{r}^{2}}}\] \[\therefore \]\[r=\sqrt{\frac{2}{4\pi }}=\sqrt{\frac{1}{2\pi }}m=0.399m\approx 40cm\]
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