Horseshoe bats use the Doppler effect to determine their location. A horseshoe bat flies toward a wall at a speed of 19 m/s while emitting a sound of frequency 37 kHz. What is the beat frequency between the emission frequency and the echo?
The bat emits a sound which is reflected off the wall. This then interferes with the incoming signal to produce beats.
The incoming signal is "Doppler Shifted" twice because it has been reflected off a wall to which the bat is moving.
To get the beat frequency you find the difference between the two frequencies.
I will assume that at 20 degC the speed of sound
The beat frequency between a transmitted signal and a reflected signal off a moving object is:
In relative terms we can consider the wall to be moving towards the bat.
Police radar guns measure vehicle speeds like this.