Apparent frequency when the source is moving away from the observer the relation between the frequencies is: $$f' = \frac{v}{v+ v_{s} } f$$

Apparent frequency when the observer is moving away from the source the relation between the frequencies is: $$f' = \frac{v- v_{o} }{v} f$$

According to Galilean relativity the form of physical laws must be the same except the sign of velocity. But here you see the form of the equation is not conserved. Is there any solution to this problem?


1 Answer 1


Unlike light, sound can only travel through a medium - in most situations, air.

The velocities in your equations are relative to a fixed reference frame - that of the body of air in which the sound is travelling (which in a typical physics problem is the same as the ground's reference frame).

So there really is a tangible difference between the case where the source is moving (relative to the air), and the case where the observer is.

This difference accounts for the asymmetry in the equations.

  • $\begingroup$ What if we use Lorentz transformations to find the Doppler effect, will the difference between the two formulas remain ? $\endgroup$
    – Kashmiri
    Feb 28, 2021 at 15:58

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