Q 11-14-128JEE MainJEE Main 2019 (12 Apr, Shift 2)Easy
A small speaker delivers $2$ W of audio output. At what distance from the speaker will one detect $120$ dB intensity sound? [Given reference intensity of sound as $10^{-12}\ \text{W/m}^2$]
Answer: (A) $40$ cm
$120 = 10\log\dfrac{I}{10^{-12}} \Rightarrow I = 1\ \text{W/m}^2$.
$$\frac{2}{4\pi r^2} = 1 \Rightarrow r = \frac{1}{\sqrt{2\pi}} \approx 0.40\ \text{m} = 40\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics