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Question

The sound level at a point 5.0 m away from a point source is 40 dB. What will be the level at a point 50 m away from the source?


A

40 dB

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B

60 dB

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C

20 dB

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D

10 dB

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Solution

The correct option is D

20 dB


Assume the power of the point source is P

Intensity I1 at a point 5 m away from it will be I1 = P4π(5)2

Given = sound level here = 40 dB

 40 = 10 log (I1I0)

At point 50 m away intensity

I2 = P4π(50)2 = P4π(5)2 × (10)2 = I1(10)2

β2 = 10 log (I2I0)

= 10 log (I1I0 × 102)

β2 = 10 log (I1I0)  20 log 10

β2 = 40  20

=20 dB


Physics

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