Question

Two identical sources S1 and S2 are moving towards an observer P. The two sound sources S1 and S2 produce notes of frequency 500hz. If the speed of S1 and S2 are 20ms-1 and 23ms-1 respectively, calculate the beat frequency heard by P. (Speed of sound = 340ms-1)

I don't understand the questions well. Beat frequency formula is f1-f2. But if both produce the same frequency of 500hz, then won't I get 0? And how does velocity come into this question?

Answer 1

Fo = (340+Vo)/(340-Vs1)*Fs1.
Fo = (340+0)/(340-20) * 500 = 531.25 Hz.
= The freq. heard by the observer.

Fo = (340+0)/(340-23) * 500 = 536.28 Hz.

Fb = 536.28 - 531.25 = 5.028 Hz = Beat
freq.

Note: The Freq. of S1 and S2 are equal,
but their velocities are different. Therefore, the frequencies heard by the
observer are not equal. So the beat freq. is not zero.










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