“Bats come out at night and can be difficult to spot—but with this bat detector, you will be able to track them down!
While some bats do make audible clicks, most produce inaudible clicks that are between 20kHz and 200kHz. Since vision is severely restricted at night and in caves, bats rely on echolocation, which works in a nearly identical fashion to submarine SONAR.
Essentially, the bat generates a high-frequency click that bounces off surrounding objects. The reflected click is detected by the bat and this is enough information for the bat to determine the size and type of object nearby, as well as the distance from it.
Luckily for us, these sound waves diffract and spread in mostly all directions—so if we can create a circuit that can detect these clicks then we should be able to track bats!” - Link