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Question

A small speaker delivers 2Wof audio output. At what distance from the speaker will one detect 120dB intensity sound? (Given reference intensity of sound is 10-12W/m2)


A

40cm

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B

20cm

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C

10cm

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D

30cm

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Solution

The correct option is A

40cm


Step 1. Given data:

Power of output audio, P=2W

Intensity of sound, B=120dB with reference intensity I0=10-12W/m2.

Step 2. Finding distance from speaker:

B=10log10II0

here,I is the intensity

120=10log10I10-12
12=log10I10-12

1012=I×1012

I=1

Intensity equals power per unit area

I=P4πr2=24πr2

r=P4πI=24×3.14×1

r=0.39m40cm

Hence, option A is correct.


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