# Doppler shift and Galilean relativity

Doppler shift has different forms if the observer or the source are in motion. I consider Doppler shift in the case of sound.

I tried to find an answer on the non symmetry of Doppler effect, especially on its apparent contrast with Galilean relativity.

What I found is that, in the case of Doppler shift for sound, there is a privileged frame of reference, which is the one where the sound wave travels, air for istance, and this is the reason why the simmetry is broken and it is not possible to talk about Galilean relativity.

I think I did not get this completely, in particular how is the existance of the privileged frame translated in a different physical phenomenon in the cases in which the observer or the source move?

I got the reason why the two phenomenon occur but it is not clear what this has to do with the presence of a privileged frame (the frame of air).

Imagine a case where one of the two is at rest with respect to the medium and the other is moving away at $c$. If the emitter is moving, the receiver still gets the (Doppler shifted) signal. If the receiver is moving away, no signal is ever received.