Wave Motion 4 Question 22

21. A stationary source emits sound of frequency f0=492Hz. The sound is reflected by a large car approaching the source with a speed of 2ms1. The reflected signal is received by the source and superposed with the original. What will be the beat frequency of the resulting signal in Hz? (Given that the speed of sound in air is 330ms1 and the car reflects the sound at the frequency it has received).

(2017 Adv.)

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Answer:

Correct Answer: 21. (a)

Solution:

Frequency observed at car

f1=f0(v+vCv)(v= speed of sound )

Frequency of reflected sound as observed at the source

f2=f1(vvvC)=f0(v+vCvvC)

Beat frequency =f2f0

=f0[v+vCvvC1]=f0[2vCvvC]=492×2×2328=6Hz



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