Ultrasound has been used to detect undersea naval vessels, to measure the depth of the ocean floor, and to locate schools of fish. When used in these ways, ultrasound is usually referred to as sonar, an acronym for SOund NAvigation and Ranging. The frequencies used in most sonar systems range from 5 to 50 kHz. Ultrasonic detectors also measure chemical fluid levels automatically in tanks and containers, as well as in the fuel tanks of aircraft. In addition, ultrasonic devices are used in burglar alarms. When an intruder trips an ultrasonic alarm, a signal can be relayed to the local police station, and the burglar apprehended.
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