Sonar stands for Sound Navigation and Ranging. It is used to detect objects underwater using sound waves. There are two types: active sonar and passive sonar. Active sonar emits a pulse of sound, and then the operator listens for echoes. When a sound signal is sent into the water, part of it will be reflected back if it strikes an object. This allows the operator to find out how far away the object is by measuring the time between when the signal is sent out, and when the reflected sound, or echo, is received. Whales and dolphins use active sonar to identify underwater objects and to hunt for food; bats use it to navigate their way in the dark. This ability is called "echolocation."
Passive sonar is mostly used to detect submarines and surface ships. It does not emit any signal; it merely listens for sounds emitted by objects in the water, such as noise generated by motors and hull vibrations. The operator can estimate how far away an object is by measuring the curvature of the incoming sound wave. Sonar is used by the military to hunt enemy submarines, control anti-submarine weapons, direct homing torpedoes, and locate mines. But it also helps marine biologists track the animals they are trying to study.
Scientists in Denmark have determined that the shape of bats' ears help them broadcast ultrasound signals and then detect the echoes to sense their nocturnal environment. Bats' ears have very conspicuous flaps that serve as sophisticated sonar antenna for greater sensitivity. The scientists hope this knowledge will lead to better designs for man-made antennas.