It is an eerie experience to sit outside a cave, in the dusk of early evening, and watch the bats come out, flying swiftly and silently, like moving shadows. They have a marvelous way of avoiding obstacles; and unless they are frightened, they never collide with each other. Though we hear not even a whisper of sound, the bats are actually making a series of sharp squeaks, so high-pitched that the human ear cannot detect them. However, extremely sensitive machines have recorded the squeaks.
Scientists call such high-pitched sounds ultrasonic. Ultra means beyond, and the word sonic refers to sound. The science that studies this type of sound is called ultrasonics. The ultrasonic squeaks of the bat help it to avoid obstacles because the sounds are reflected back to the bat as echoes from anything solid with which it might collide. (See the diagram). During one experiment undertaken by naturalists some bats were masked so that they could not see at all; then turned loose in a room where metal wires were strung a foot apart. The bats flew without any difficulty. But when their ears were plugged or their mouths were gagged, they collided with the wires and even with the walls of the room. For a long time people thought that bats were guided by special senses in their wings, but when the wings were covered with nail polish the bats flew as well as usual. We know now that their system for avoiding collision depends on their ability to hear ultrasonic echoes. In short, bats are equipped with a sort of natural radar.