Nearly everyone is familiar with the way in which the whistle of a train moving toward them lowers in pitch as the train passes. The change in pitch is caused by the change in the movement of the train relative to the listener—first it is moving toward the listener, but after it passes it is moving away. This change in pitch is called Doppler shifting. Now let's apply this principal to the bat's echolocation system.

REF.: Bats alter frequency of squeaks to aid hunt for prey. The San Diego Union, Mon., Oct. 23, 1989. p. D1. Image: Echolocation of Bats by Shung_CCA BY SA 3.0. Explanation of chart: A – Chiroptera B – prey d – distance E – emitted wave of bat R – reflected wave of prey