# Dopler Shift for Reflected Sound Waves

If a car was going at near mach-1 speed heading towards a wall and honked its horn. What frequency would the driver measure the reflected sound wave? I trying to think about this reasonably, but the answer I get is somewhat not what I initially expected.

Here is my current thinking, correct anything you might deem wrong. First let's pretend there is no reflection but instead the wave comes from another source moving head on to the observer with equal and opposite velocity. So then we have the receiver moving towards the source, and the source moving towards the receiver.

Using the Doppler shift equation, the observed frequency is upshifted by a factor of (c+v)/(c-v), where c is the speed of the sound wave, and v the speed of the car. I suppose the interesting thing is that the source and receiver are actually the same, but there is still a shift due to the reflection. Or, maybe I made a mistake in my logic.

• what if the wall is moving'
– user324939
Commented Jul 12 at 8:44