To see in the dark, nocturnal world that they spend their woken hours in bats 'see' using ultrasonic calls. After making these calls in a series of melodic swoops the bat can create a picture of the surrounding environment by sensing and analysing the ultrasonic sound reflections. Unbelievably, this unique sense is so refined that a bat can chase it's prey through the air using sound alone.
The sensory-spacial sounds made by bats range from around 20-200kHz which is beyond the limit of human hearing and the designed frequency responses of most standard microphones. Aside from their state of the art sonar systems however, bats make audible (to the human ear) squeaky calls and, as the clip below also demonstrates, can make quite a rumpus if they decide to nest in the walls of a building. Using contact microphones stuck to the walls of a wooden chalet I could hear a colony of bats squeaking and scrabbling as they wake up to hunt when darkness falls.