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JEE Mains · Physics · STD 11 - Waves and sound
  • JEE Mains 2019
  • Easy
  • Numerical

 A small speaker delivers \(2\, W\) of audio output. At what distance from the speaker will one detect \(120\, dB\) intensity sound ... \(cm\) ? [Given reference intensity of sound as \(10^{-12}\,W/m^2\)]

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Answer & Solution

Correct Answer

(C) \(40\)

Step-by-step Solution

Detailed explanation

Loudness of sound is given by

\(\mathrm{dB}=10 \log \frac{I}{\mathrm{I}_{0}}\) (\(I\) is intensity of sound, \(I_0\) is reference intensity of sound)

\(120=10 \log \left(\frac{I}{I_{0}}\right)\)

\(\Rightarrow I=1 \mathrm{W} / \mathrm{m}^{2}\)

Also, \(\mathrm{I}=\frac{\mathrm{P}}{4 \pi \mathrm{r}^{2}}=\frac{2}{4 \pi \mathrm{r}^{2}}\)

\(\therefore r=\sqrt{\frac{2}{4 \pi}}=\sqrt{\frac{I}{2 \pi}} \mathrm{m}=0.399 \mathrm{m}\)

\(\approx 40 \mathrm{cm}\)

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