I have come to learn that the Doppler shift equation is asymmetric. That is, the Doppler shift is not the same when source is moving towards the observer or when the observer is moving towards the observer (both with same speed). I have looked at the derivation of the Doppler shift equation and it says that that's because the relative speed of the observer and the source with respect to the medium is not the same, although their relative speed to each other is. I can see how that affects the math, but I cannot imagine physically why there's a difference(in both cases the observer detects crests faster than normal). Here's a situation that conveys where I am confused: imagine a blindfolded man who's seated in a remote controlled wheelchair 50 meters from an ambulance. Suppose the ground is so smooth the man wouldn't know it if he started moving, and supposed the man also has an airtight mask on so he wouldn't feel any air brushing on his face. And let us make the man sleep through the acceleration of the chair so he wouldn't know he started moving. The man suddenly hears the siren's frequency rise. Is there any physical difference between the two cases that could enable the man to determine if he's approaching the ambulance or the ambulance is approaching him?