Sonar

Sonar is a medium-range method of proximity sensing. The acronym derives from the words sonic navigation and ranging. The basic principle is simple: Bounce acoustic waves off of objects, and measure the time it takes for the echoes to return.
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At a block diagram of medium-range sonar system
An elementary sonar system consists of an ac pulse generator, an acoustic emitter, an acoustic pickup, a receiver, a delay timer, and an indicating device such as a numeric display, CRT, LCD, or pen recorder. The transmitter sends out acoustic waves through the medium, usually water or air. These waves are reflected by objects, and the echoes are picked up by the receiver. The distance to an object is determined on the basis of the echo delay, provided the speed of the acoustic waves in the medium is known.
 
A simple sonar system is diagrammed in above figure A. A computer map can be generated on the basis of sounds returned from various directions in two or three dimensions. This can help a mobile robot or vessel navigate in its environment. However, the system can be fooled if the echo delay is equal to or longer than the time interval between pulses, as shown at B. To overcome this, a computer can instruct the generator to send pulses of various frequencies in a defined sequence. The computer keeps track of which echo corresponds to which pulse.
 
Acoustic waves travel faster in water than in air. The amount of salt in water makes a difference in the propagation speed when sonar is used on boats, for example, in depth finding. The density of water can vary because of temperature differences as well. If the true speed of the acoustic waves is not accurately known, false readings will result. In freshwater, the speed of sound is about 1400 meters per second (m/s), or 4600 feet per second (ft/s). In saltwater, it is about 1500 m/s (4900 ft/s). In air, sound travels at approximately 335 m/s (1100 ft/s).
 
In the atmosphere, sonar can make use of audible sound waves, but ultrasound is often used instead. Ultrasound has a frequency too high to hear, ranging from about 20 kHz to more than 100 kHz. One advantage of ultrasound is that the signals will not be heard by people working around machines equipped with sonar. Another advantage is the fact that it is less likely to be fooled by people talking, machinery operating, and other noises. At frequencies higher than the range of human hearing, acoustical disturbances do not normally occur as often, or with as much intensity, as they do within the hearing range.
 
In its most advanced forms, sonar can rival vision systems (also called machine vision) as a means of mapping the environment for a mobile robot or vessel. Sonar has one significant limitation: all acoustic waves, including sound and ultrasound, require a gaseous or liquid medium in order to propagate. Therefore, sonar is useless in outer space, which is practically a vacuum.