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The idea of using sound to determine the depth of a lake or ocean was first proposed in the early nineteenth century. Interest in this technique, called underwater ranging, was renewed in 1912 when the luxury sailing vessel Titanic collided with an iceberg and sank. Two years later, during World War I (1914–18), a single German submarine sank three British cruisers carrying more than 1,200 men. In response, the British government funded a massive effort to create an underwater detection system.
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  The idea of using sound to determine the depth of a lake or ocean was first proposed in the earlynineteenth century. Interest in this technique, called underwater ranging, was renewed in 1912when the luxury sailing vessel Titanic collided with an iceberg and sank. Two years later, duringWorld War I (1914  –  18), a single German submarine sank three British cruisers carrying morethan 1,200 men. In response, the British government funded a massive effort to create anunderwater detection system.The entire operation was conducted in complete secrecy, but the first working model was notready until after the war ended. The project operated under the code name asdic (which stoodfor Allied Submarine Detection Investigating Committee). The device kept that name until thelate 1950s, when the American term sonar was adoptedw  How it works The principle behind sonar is simple: a pulse of ultrasonic waves is sent into the water where itbounces off a target and comes back to the source (ultrasonic waves are pitched too high forhumans to detect). The distance and location can be calculated by measuring the time it takes forthe sound to return. By knowing the speed of sound in water, the distance is computed bymultiplying the speed by one-half of the time traveled (for a one-way trip). This is active sonarranging (echolocation). Words to Know Active Sonar : Mode of echo location by sending a signal and detecting the returning echo. Passive sonar: Sensitive listening-only mode to detect the presence of objects making noise. Ultrasound: Acoustic vibrations with frequencies higher than the human threshold of hearing.Most moving objects underwater make some kind of noise. Marine life, cavitation (smallcollapsing air pockets caused by propellers), hull popping of submarines changing depth, andengine vibration are all forms of underwater noise. In passive sonar ranging, no pulse signal issent. Instead, the searcher listens for the characteristic sound of another boat or submarine. Bydoing so, the searcher can identify the target without revealing his own location. This method ismost often used during wartime.  However, since a submarine is usually completely submerged, it must use active sonar at times,generally to navigate past obstacles. In doing so, the submarine risks alerting others of itspresence. In such cases, the use of sonar has become a sophisticated military tactical exercise.Sonar devices have become standard equipment for most commercial and many recreationalships. Fishing boats use active sonar to locate schools of fish. Other applications of sonar includesearching for shipwrecks, probing harbors where visibility is poor, mapping the ocean floor, andhelping submerged vessels navigate under the Arctic Ocean ice sheets.Sonar is a system that uses transmitted and reflected underwater sound waves to detect andlocate submerged objects or measure the distances underwater. It has been used for submarineand mine detection, depth detection, commercial fishing, diving safety and communication atsea. The Sonar device will send out a subsurface sound wave and then listens for returningechoes, the sound data is relayed to the human operators by a loudspeaker or by being displayedon a monitor.As early as 1822, Daniel Colloden used an underwater bell to calculate the speed of soundunderwater in Lake Geneva, Switzerland. This early research led to the invention of dedicatedsonar devices by other inventors. Lewis Nixon Lewis Nixon invented the very first Sonar type listening device in 1906, as a way of detectingicebergs. Interest in Sonar was increased during World War I when there was a need to be able todetect submarines.  Paul Langévin In 1915, Paul Langévin invented the first sonar type device for detecting submarines called an echo location to detect submarines using the piezoelectric properties of the quartz. He was toolate to help very much with the war effort, however, Langévin's work heavily influenced futuresonar designs.The first Sonar devices were passive listening devices - no signals were sent out. By 1918, bothBritain and the U.S had built active systems, in active Sonar signals are both sent out and thenreceived back. Acoustic communication systems are Sonar devices where there is both a soundwave projector and receiver on both sides of the signal path. The invention of the acoustictransducer and efficient acoustic projectors made more advanced forms of Sonar possible. Sonar - SOund, NAvigation and Ranging  The word Sonar is an American term first used in World War II, it is an acronym for SOund,NAvigation and Ranging. The British also call Sonar, ASDICS, which stands for Anti-Submarine Detection Investigation Committee. Later developments of Sonar included the echosounder, or depth detector, rapid-scanning Sonar, side-scan Sonar, and WPESS (within-pulseectronic-sector-scanning) Sonar. There are two major kinds of sonar, active and passive. Active sonar creates a pulse of sound, often called a ping , and then listens for reflections of thepulse. The pulse may be at constant frequency or a chirp of changing frequency. If a chirp, thereceiver correlates the frequency of the reflections to the known chirp. The resultant processinggain allows the receiver to derive the same information as if a much shorter pulse of the sametotal power were emitted. In general, long-distance active sonars use lower frequencies. Thelowest have a bass BAH-WONG sound. To measure the distance to an object, one measuresthe time from emission of a pulse to reception.Passive sonars listen without transmitting. They are usually military (although a few arescientific). Passive sonar systems usually have large sonic databases. A computer systemfrequently uses these databases to identify classes of ships, actions (i.e. the speed of a ship, or thetype of weapon released), and even particular ships. 1.1 The sonar system An active sonar system is an apparatus used for obtaining information about underwater objectsand events by transmitting sound waves and observing the return echoes 1 The sound wavesproduced by sonar systems used for fish detection and biomass estimation are of the same natureas those produced by musical instruments, moving vehicles, machinery, human speech organs,etc. However, the human ear has a restricted range of perception of sound limited approximatelyto frequencies between 50 and 12000 Hertz (cycles per second, abbreviated Hz). Sonar systemsused in fisheries utilize ultra sounds, i.e., the sounds of frequency 12000-500000 Hz, (i.e., 12-500 kHz) which are not detectable by the human ear. 1 An apparatus used only for receiving the sounds generated by underwater objects is called apassive sonar system, which can be utilized in marine biology for detecting sounds generated byfish and other aquatic animals.The users of the sonar system, i.e., sailors, fishermen, and marine researchers have adopted thefollowing terms:- a sonar system that transmits vertically is called an “echo - sounder” (Fig. 1a).  - a sonar system that transmits horizontally is called a “sonar” (Fig. 1b).    Figure 1 Detection and location of fish by (a) echosounder (b) sonar  The functioning of both kinds of appartus is the same. Therefore when discussing the basictheory of acoustics and the functioning of acoustic equipment in general we will use the term “sonar system”, and when discussing the practical use of a particular kind of equipment we will   use one of the customary names, i.e., “echo - sounder” or “sonar”.  
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