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If a observer is moving away from a stationary sound source with a velocity V' then observed frequency is (1-V'/v)f where v is the speed of sound and f is the frequency observed when at rest. Now if V'>v what will happen actually? What is the meaning of negative frequency?

Anindya Biswas
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This will be easier to picture with you in a boat on a lake with a wave source.

Say you travel away from the source, but slower than the waves propagate. Then the waves will travel past you and hit your boat from behind. In your formula for the doppler frequency, this will give a positive frequency.

If you speed up to exactly the speed of the waves, you will travel perfectly synchronous with the waves and no waves will hit your boat, as you are exactly travelling along with them. The doppler formula correctly gives a frequency of 0.

If you speed up even more, you will start hitting the waves with your boat from behind. The frequency you hit the waves with is again given by your formula, but now with a negative sign.

So the sign in the result tells you whether the sound waves hit you from behind, or if you are fast enough to overtake the waves and you will hit them from behind (which is equivalent to them hitting you from the front).

noah
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  • But at first he heard convex waves and in second case he heard concave wave . Don't we have to take it in our consideration??If the curvature of the wavefronts are considerable with respect to ear. – Anindya Biswas Jan 24 '20 at 16:42
  • The Doppler effect only tells you the frequency at which the wavefronts hit you (or your ear). The waveform may or may not have an effect on the sound (I doubt it but can't disprove), but this is not the point of calculating the frequency at which the wavefronts arrive. – noah Jan 24 '20 at 16:49
  • And nevertheless, you can still figure out which type of wave it is from the sign of the frequency. – noah Jan 24 '20 at 16:49
  • Generally(in 99.99999.......% cases) we hear convex sound waves . If we hear concave sound wave with the same frequency can we differentiate between them?? – Anindya Biswas Jan 25 '20 at 09:15