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 \(10^{-12}\,W/m^2\)]
Pick an option to check your answer.
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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