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Reflection of Sound Waves

A tuning fork...

Question

A tuning fork of frequency 328 Hz is moved towards a wall at a speed of 2 ms $^{- 1}$ . An observer standing on the same side as the fork hears two sounds, one directly from the fork and the other reflected from the wall. No. of beats per second is:
(Velocity of sound in air 330ms $^{- 1}$ ).

A

4

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B

5

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C

6

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D

7

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Solution

The correct option is A 4
Frequency of fork, $f_{1} = (\frac{v}{v + v_{s}}) f$ $= (\frac{330}{332}) 328$
Frequency of echo, $f_{2} = (\frac{330}{328}) 328$
No. of beats $= f_{1} - f_{2}$
$= 330 (328) (\frac{4}{330 (328)})$
$= 4$

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