Ultrasonic Waves: Humans can normally hear sound frequencies between 20 and 20,000 Hz (20kHz). When a sound wave's frequency lies above 20 kHz, it is called an ultrasonic wave. While we cannot hear ultrasonic waves, we apply them in various technologies such as sonar systems, sonograms, surgical tools, and cleaning sytems. Some animals also use ultrasonic waves in a specialized technique called echolocation that alows them to pinpoint objects and other animals, even in the dark. Sonar: Sonar stands for SOund NAvigation Ranging. Sonar is used in navigation, forecasting weather, and for tracking aircraft, ships, submarines, and missiles. Sonar devices work by bouncing sound waves off objects to determine their location. A sonar unit consists of an ultrasonic transmitter and a receiver. On boats, the receiver is mounted on the bottom of the ship. To measure water depth, for instance, the transmitter sends out a short pulse of sound, and later, the receiver picks up the reflected sound. The water depth is determined from the time elapsed between the emission of the ultrasonic sound and the reception of its reflection off the sea-floor. In the diagram below, a ship sends out ultrasonic waves (green) in order to detect schools of fish swimming beneath. The waves reflect off the fish (white), and return to the ship where they are detected and the depth of the fish is determined. Echolocation: In 1944, Donald R. Griffin coined the term echolocation. Echolocation is the use of echoes of sound produced by certain animals to detect obstacles and food. Animals that live where lighting is unpredictable use echolocation. Some of these animals are bats, porpoises, some kinds of whales, several species of birds, and some shrews. The first step in echolocation is emitting a sound. High-frequency sounds provide better resolution of targets than lower-frequency sounds. Not every animal uses ultrasonic sounds in echolocation, but they are more effective. Still, sounds used in echolocation can be produced in the voice box, the mouth, or some other part of the head. Then, a highly refined auditory system detects the returning echoes (the sounds that bounced of the object). In order for echolocation to work, the outgoing pulses of sound need to register in the organism's brain, so it can be compared to its echo. Using echolocation, some animals can effectively catch prey and "see" in the dark.