# Detecting phase shift

Points A,B,C lie equidistant on a straight line. There is a medium streaming from A to C. Thus, if a continuous sound is emitted at B, it will arrive at A with a slight phase shift relative to the reception at C.

If you substract the signal from A and C, a signal of the same frequency and phase as the one sent at B will be created. Depending on the velocity of the medium, and thus the resulting phase shift between A and C, the amplitude of the resulting signal will change. Measuring that amplitude will now give me a handle on the velocity of the medium.

This breaks down though, if i do not know that the amplitudes from A and C are the same (because one microphone is not as good as the other) - is there another way, measuring only amplitude and frequency at A, B, C , or some sum or substraction of any of these, to determine the speed of the medium? There are no particles in the medium that would produce an echo.

I labeled this 'doppler' in hopes that there is some smart interaction that will give me a velocity-dependent frequency shift...