Wave Motion 4 Question 28

26. A whistle emitting a sound of frequency 440Hz is tied to a string of 1.5m length and rotated with an angular velocity of 20rad/s in the horizontal plane. Calculate the range of frequencies heard by an observer stationed at a large distance from the whistle. (Speed of sound =330m/s ).

(1996, 3M)

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Answer:

Correct Answer: 26. (b, c)

Solution:

  1. vs= Speed of source (whistle) =Rω=(1.5)(20)m/s

vs=30m/s

Maximum frequency will be heard by the observer in position P and minimum in position Q. Now,

fmax=f(vvvs)

where, v= speed of sound in air =330m/s

=(440)(33033030)Hz

fmax=484Hz

and

fmin=f(vv+vs)=(440)(330330+30)fmin=403.33Hz

Therefore, range of frequencies heard by observer is from 484Hz to 403.33Hz.



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