Sound and Hearing?

The sound waves travel out and hit the insect and then echo back. The echo is different depending on what is in the path of a bat. Insects give a distinct echo.

Two factors are involved. The most obvious is the time interval between when the sound was made and when it was heard. The other, less obvious factor is that the reflected sound will be Doppler shifted. The bat will hear the sound at a lower or higher frequency, depending on whether or not it is traveling away from or towards the insect. I don’t know if you want to get into the mathematics, but essentially, if the bat can determine how long it took for the sound to be reflected as well as how much the frequency has changed due to the Doppler effect, it can determine it’s distance from the insect.

@Ivan, great answer. But I would add that the bat doesn’t know all the theory behind the phenomenon. The bat just knows, through instinct and experience, that if the sound comes back and sounds this way it means one thing, and if it sounds _that_way, it means something else.