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SONAR

SONAR (underwater sound navigation ranging) can be either of the passive or active type. Passive sonars were developed first and rely upon listening for noise generated by the target vessel, usually submarines (however, submarines also use sonar to detect other ships). The most difficult aspect of passive sonar use is distinguishing target noise from that of the surrounding sea (referred to as ambient noise) and particularly that of the searching platform. Active sonars are popularly characterized by the famous ping known to anybody who has ever watched a Hollywood submarine movie. The ping is a sound wave generated by the searcher that is bounced back off the objects, thus giving the sonar operator a picture of the object in the path of the sound wave.

U.S. sonar development began before World War I when the Submarine Signal Company, formed in 1901, developed steam‐operated underwater warning bells that could be heard for up to 10 nautical miles. By 1912, warning bells were used to supplement the work of lighthouses in marking hazards to navigation off the coasts of North America, South America, Europe, and Asia.

In February 1917, the U.S. Navy Consulting Board created a Subcommittee on Submarine Detection. Two passive sonar detectors developed by a staff member of the Submarine Signal Company, Professor R. A. Fessenden, were installed on navy destroyers, but their performance proved disappointing.

World War II saw active sonar systems predominate in U.S. ships and submarines, in contrast to the Germans, who concentrated on large fixed passive array systems. The American approach helped mitigate the effect of ocean noise that proved such a problem with passive sonars. Navy General Board guidelines of 1938 called for two sonars per destroyer and one unit for lesser craft. However, wartime demands for escort vessels and the low rate of sonar production prevented these guidelines from being followed. Instead, the scarce equipment was put out among destroyer escorts. U.S. submarines typically carried a passive device along with a combined ranging and sounding set.

During the Cold War, passive developments included large arrays of hydrophones mounted conformally along submarine hulls to achieve very well defined and very long range receiving beams; systems for passive range finding; PUFFS (Passive Underwater Fire Control Feasibility Study, a short range triangulation device using three passive sonars mounted along the length of a submarine); and submarine‐towed arrays. The towed array came into use to mitigate the effect of a vessel's own noise upon passive sonar systems; it consists of a string of passive hydrophones towed at some distance behind the ship. A further advantage of the towed array is that it can be made as long as necessary to detect sounds with long (very low frequency) wavelengths.

Today's most advanced U.S. submarines, the SSN‐688I and the SSN‐21, use the AN/BSY‐1 integrated sonar and fire control system that includes both active and passive sonar types. In addition to MAD (magnetic anomaly detector) sensors (a means of locating submarines by detecting changes in the earth's magnetic fields caused by large metal objects), aircraft use small sonobuoys as a means of detecting submarines. Helicopters hover above the ocean surface and dip scanning sonars that emit a ping in all directions at once.
[See also Antisubmarine Warfare Systems; Destroyers and Destroyer Escorts.]

Bibliography

Norman Friedman , U.S. Naval Weapons Systems, 1982; repr. 1985, 1988.

David E. Michlovitz

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Sonar

Sonar

Sonar, an acronym for so und n avigation a nd ra nging, is a system that uses sound waves to detect and locate objects underwater.

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 (191418), 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.

The entire operation was conducted in complete secrecy, but the first working model was not ready until after the war ended. The project operated under the code name "asdic" (which stood for Allied Submarine Detection Investigating Committee). The device kept that name until the late 1950s, when the American term "sonar" was adopted.

How it works

The principle behind sonar is simple: a pulse of ultrasonic waves is sent into the water where it bounces off a target and comes back to the source (ultrasonic waves are pitched too high for humans to detect). The distance and location can be calculated by measuring the time it takes for the sound to return. By knowing the speed of sound in water, the distance is computed by multiplying the speed by one-half of the time traveled (for a one-way trip). This is active sonar ranging (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 (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration are all forms of underwater noise. In passive sonar ranging, no pulse signal is sent. Instead, the searcher listens for the characteristic sound of another boat or submarine. By doing so, the searcher can identify the target without revealing his own location. This method is most 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 its presence. 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 recreational ships. Fishing boats use active sonar to locate schools of fish. Other applications of sonar include searching for shipwrecks, probing harbors where visibility is poor, mapping the ocean floor, and helping submerged vessels navigate under the Arctic Ocean ice sheets.

[See also Ultrasonics ]

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sonar

sonar (sō´när), device used underwater for locating submerged objects and for submarine communication by means of sound waves. The term sonar is an acronym for sound navigation ranging. The main component of sonar equipment is an electroacoustic transducer that is in direct contact with the water. It is suspended from the hull of a ship or on a cable from a low-flying helicopter. The transducer converts electric energy into acoustic energy (thus acting as a projector), much as does a loudspeaker, and converts acoustic energy into electric energy (serving as a hydrophone), as does a microphone. A pulse of electric energy vibrates the diaphragm of the projector, sending sound waves through the water. These waves are concentrated into a sound beam, which scans the water when the projector is rotated. After the sound wave is emitted, the projector is converted into a hydrophone and listens for an echo. The cycle is repeated periodically. A returning echo is converted into an electric current by the transducer and may be interpreted (for range, bearing, and the nature of the target) aurally or by a cathode-ray tube, as is done with radar signals. The various types of sonar in use can be put into three classes: direct listening, communications, and echo ranging. In direct listening, also known as passive sonar, the object under observation generates the sounds that are received. In communications and echo ranging the sonar must generate its own signals. Sonar operates in the 10- to 50-kilocycle acoustical frequency range. It is used for communication between submerged submarines or between a submarine and a surface vessel, for locating mines and underwater hazards to navigation, and also as a fathometer, or depth finder. Sonar is widely used by commercial fishermen for locating shoals of fish. Research has indicated that sonar used for echo ranging can affect some dolphins and porpoises and especially beaked whales. In some instances it may startle them and cause them to surface too rapidly, producing a disorder similar to decompression sickness (in which nitrogen bubbles form in body tissues); this may be linked to strandings of those species.

See J. W. Horton, Fundamentals of Sonar (1957); D. G. Tucker, Underwater Observation Using Sonar (1966).

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sonar

so·nar / ˈsōˌnär/ • n. a system for the detection of objects under water and for measuring the water’s depth by emitting sound pulses and detecting or measuring their return after being reflected. ∎  an apparatus used in this system. ∎  the method of echolocation used in air or water by animals such as whales and bats.

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sonar

sonar (Acronym for sound navigation and ranging) Underwater detection and navigation system. The system emits high-frequency sound that is reflected by underwater objects and detected on its return.

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sonar

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sonar

sonar •dinar • seminar • sonar • Fragonard

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sonar

sonar (ˈsəʊnɑː) sound navigation and ranging

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SONAR

SONAR

K. LEE LERNER

SONAR, an acronym for Sound Navigation and Ranging, is a technique based on echolocation used for the detection of objects underwater.

Historical development of SONAR. Ancient drawings depict the use of long tubes as non-mechanical underwater listening devices to detect and transmit sound in water. In the late nineteeth century, scientists began to explore the physical properties associated with sound transmission in water. In 1882, a Swiss physicist, Daviel Colladen, attempted to calculate the speed of sound in the known depths of Lake Geneva. Based upon the physics of sound transmission articulated by English physicist Lord Rayleigh (18421914), and the piezoelectric effect discovered by French scientist Pierre Curie (15091906), in 1915, French physicist Paul Langevin (18721946) invented the first system designed to utilize sound waves and acoustical echoes in an underwater detection device.

In the wake of the Titanic disaster, Langevin and his colleague Constantin Chilowsky, a Russian engineer then living in Switzerland, developed what they termed a "hydrophone" as a mechanism for ships to more readily detect icebergs (the vast majority of any iceberg remains below the ocean surface). Similar systems were put to immediate use as an aid to underwater navigation by submarines.

Improved electronics and technology allowed the production of greatly improved listening and recording devices. Because passive SONAR is essentially nothing more than an elaborate recording and sound amplification device, these systems suffered because they were dependent upon the strength of the sound signal coming from the target. The signals or waves received could be typed (i.e. related to specific targets) for identifying characteristics. Although skilled and experienced operators could provide reasonably accurate estimates of range, bearing, and relative motion of targets, these estimates were far less precise and accurate than results obtained from active systems unless the targets were very closeor were very noisy.

The threat of submarine warfare during World War I made urgent the development of SONAR and other means of echo detection. The development of the acoustic transducer that converted electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Although active SONAR was developed too late to be widely used during World War I the push for its development produced enormous technological dividends. Early into World War II, the British Anti-Submarine Detection and Investigation Committee (its acronym, ASDIC, became a name commonly applied to British SONAR systems) made efforts to outfit every ship in the British fleet with advanced detection devices. The use of ASDIC proved pivotal in the British effort to repel damaging attacks by German submarines.

SONAR and RADAR. Although they rely on two fundamentally different types of wave transmission, SONAR and Radio Detection And Ranging (RADAR) and both are remote sensing systems. While active SONAR transmits acoustic (i.e., sound) waves, RADAR sends out and measures the return of electromagnetic waves.

In both systems these waves return echoes from certain features or targets that allow the determination of important properties or attributes of the target (e.g., shape, size, speed, distance to target, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are mostly used for ground or atmospheric observations. Because SONAR signals easily penetrate water, they are ideal for navigation and measurement under water. Within the ocean, the speed of sound varies with changes in temperature and pressure, and these conditions can also be determined by alterations in SONAR signals.

SONAR usually operates at frequencies in the 10,00050,000 Hz range. Higher higher frequencies provide more accurate location data, but propagation losses (i.e. loss of signal strength over distance) also increase with frequency.

FURTHER READING:

BOOKS:

Van Trees, Harry L. Radar-Sonar Signal Processing and Gaussian Signals in Noise. Indianapolis, IN: John Wiley & Sons, 2001.

Waite, A. D. Sonar for Practising Engineers. Indianapolis, IN: John Wiley & Sons, 2001.

ELECTRONIC:

Canadian Center for Remote Sensing, "History of Remote Sensing." 2001. <http://www.ccrs.nrcan.gc.ca/ccrs/org/history/morleye.htm> (February 1, 2003).

SEE ALSO

P-3 Orion Anti-Submarine Maritime Reconnaissance Aircraft
Remote Sensing
SOSUS (Sound Surveillance System)
Undersea Espionage: Nuclear vs. Fast Attack Subs

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SONAR

SONAR

Historical development of SONAR

SONAR and RADAR

SONAR technology

Resources

SONAR (Sound Navigation And Ranging) is a technique based on echolocation used for the detection of objects underwater.

Historical development of SONAR

Ancient peoples have long used underwater tubes as devices to detect and transmit sound in water. In the later nineteeth century, scientists began to explore the properties of sound transmission in water. In 1882, a Swiss physicist Daviel Colladen attempted to calculate the speed of sound in the known depths of Lake Geneva. Based upon the physics of sound transmission articulated by English physicist Lord Rayleigh, (18421914) and the piezoelectric effect discovered by French scientist Pierre Curie (15091906), in 1915, French physicist Paul Langevin (18721946) invented the first system designed to utilize sound waves and acoustical echoes in an underwater detection device.

In the wake of the Titanic disaster, Langevin and his colleague Constantin Chilowsky, a Russian engineer then living in Switzerland, developed what they termed a hydrophone, a mechanism for ships to more readily detect icebergs. Similar systems were put to immediate use as an aid to underwater navigation by submarines.

Improved electronics and technology allowed the production of greatly improved listening and recording devices. Because passive SONAR is essentially nothing more than an elaborate recording and sound amplification device, these systems suffered because they were dependent upon the strength of the sound signal coming from the target. The signals or waves received could be typed (i.e. related to specific targets) for identifying characteristics. Although skilled and experienced operators could provide reasonably accurate estimates of range, bearing, and relative motion of targets, these estimates were far less precise and accurate than results obtained from active systems unless the targets were very closeor were very noisy.

The threat of submarine warfare during World War I made urgent the development of SONAR. and other means of echo detection. The development of the acoustic transducer that converted converting electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Although active SONAR was developed too late to be widely used during WWI, the push for its development reaped enormous technological dividends. Not all of the advances, however, were restricted to military use. After the war, echosounding devices were placed aboard many large French ocean-liners.

Early into World War II, the British AntiSubmarine Detection and Investigation Committee (its acronym, ASDIC, became a name commonly applied to British SONAR systems) made efforts to outfit every ship in the British fleet with advanced detection devices. The use of ASDIC proved pivotal in the British effort to repel damaging attacks by German submarines.

SONAR and RADAR

Although they rely on two fundamentally different types of wave transmission, SONAR and Radio Detection And Ranging (RADAR) and both are remote sensing systems. Whilst active SONAR transmits acoustic (i.e., sound) waves, RADAR sends out and measure the return of electromagnetic waves,

Both systems these waves return echoes from certain features or targets that allow the determination of important properties or attributes of the target (e.g., shape, size, speed, distance to target, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are mostly used for ground or atmospheric observations. Because SONAR signals easily penetrate water, they are ideal for naviagtion and measurement under water.

SONAR technology

SONAR equipment is used on most ships for measuring the depth of the water. This is accomplished by sending an acoustic pulse and measuring the time for the echo, or return from the bottom. By knowing the speed of sound in the water, the depth is computed by multiplying the speed by one half of the time traveled (for a one-way trip). SONAR is also used to detect large underwater objects and to search for large fish concentrations. More sophisticated SONAR systems for detection and tracking are found aboard naval vessels and submarines. In nature, bats are well known for making use of echo-location, as are porpoises and some species of whales. SONAR should not be confused with ultrasound, which is simply sound at frequencies higher than the threshold of human hearing - greater than 15,00020,000 cycles per second or hertz (Hz). High-power ultrasound is used on a small scale to break up material and for cleaning purposes. Low-power ultrasound in the megahertz range is used for internal imaging of the body and therapeutically, for treatment of muscle and tissue injuries.

SONAR is directional, so the signals are sent in narrow beams in various directions to search the water. SONAR usually operates at frequencies in the 10,00050,000 Hz range. Though higher frequencies provide more accurate location data, propagation losses also increase with frequency. Lower frequencies are therefore used for longer range detection (up to 10 mi [17,600 yd]) at the cost of location accuracy.

Acoustic waves are detected using hydrophones that are essentially underwater microphones. Hydrophones are often deployed in large groups, called arrays, forming a SONAR net. SONAR arrays also give valuable directional information on moving sources. Electronics and signal processing play a large role in hydrophone and general SONAR system performance.

The propagation of sound in water is quite complex and depends very much on the temperature, pressure, and depth of the water. Salinity, the quantity of salt in water, also changes sound propagation speed. In general, the temperature of the ocean is warmest at the surface and decreases with depth. Water pressure, however, increases with depth, due to the water mass. Temperature and pressure, therefore, change what is called the refractive index of the water. Just as light is refracted, or bent by a prism, acoustic waves are continuously refracted up or down and reflected off the surface or the bottom. A SONAR beam propagating along the water in this way resembles a car traveling along regularly spaced hills and valleys. As it is possible for an object to be hidden between these hills, the water conditions must be known in order to properly assess SONAR performance.

In location and tracking operations, two types of SONAR modes exist, active and passive. Echolocation is an active technique in which a pulse is sent and then detected after it bounces off an object. Passive SONAR is a more sensitive, listening-only SONAR that sends no

KEY TERMS

Acoustics The study of the creation and propagation of mechanical vibration causing sound.

Active SONAR Mode of echo location by sending a signal and detecting the returning echo.

Array A large group of hydrophones, usually regularly spaced, forming a SONAR net.

Hertz A unit of measurement for frequency, abbreviated Hz. One hertz is one cycle per second.

Hydrophone An underwater microphone sensitive to acoustic disturbances.

Passive SONAR A sensitive listening-only mode to detect presence of objects making noise. PropagationTraveling or penetration of waves through a medium.

Radar A method of detecting distant objects based on the reflection of radio waves from their surfaces.

Refractive index Degree to which a wave is refracted, or bent.

Sonar SOund Navigation And Ranging. A device utilizing sound to determine the range and direction to an underwater object.

Ultrasound Acoustic vibrations with frequencies higher than the human threshold of hearing.

Wave A motion, in which energy and momentum is carried away from some source, which repeats itself in space and time with little or no change.

pulses. Most moving objects underwater make some kind of noise. This means that they can be detected just by listening for the noise. Examples of underwater noise are marine life, cavitation (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration. Some military passive SONARs are so sensitive they can detect people talking inside another submarine. Another advantage of passive SONAR is that it can also be used to detect an acoustic signature. Each type of submarine emits certain acoustic frequencies and every vessels composite acoustic pattern is different, just like a fingerprint or signature. Passive SONAR is predominantly a military tool used for submarine hunting. An important element of hunting is not to divulge ones own position. However, if the passive SONAR hears nothing, one is obliged to turn to active mode but in doing so, risks alerting the other of his presence. The use of SONAR in this case has become a sophisticated tactical exercise.

Other, non-military, applications of SONAR, apart from finding fish, include searching for shipwrecks, probing harbors where visibility is poor, oceanography studies, searching for underwater geological faults and mapping the ocean floor.

Resources

BOOKS

Anderton, Craig. The Sonar Insider. New York: Schirmer Trade Books, 2004.

Jeanette, A., et al. (eds.). Echolocation in Bats and Dolphins. Chicago: U. of Chicago Press, 2004.

Waite, A.D. Sonar for Practicing Engineers. John Wiley & Sons, 2001.

David Lunney

K. Lee Lerner

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Sonar

SONAR

SONAR, an acronym for Sound Navigation And Ranging, is a technique based on echolocation used for the detection of objects underwater.


Historical development of SONAR

Ancient peoples have long used tubes as non-mechanical underwater listening devices to detect and transmit sound in water . In the later nineteeth century, scientists began to explore the physical properties associated with sound transmission in water. In 1882, a Swiss physicist Daviel Colladen attempted to calculate the speed of sound in the known depths of Lake Geneva. Based upon the physics of sound transmission articulated by English physicist Lord Rayleigh, (1842–1914) and the piezoelectric effect discovered by French scientist Pierre Curie (1509–1906), in 1915, French physicist Paul Langevin (1872–1946) invented the first system designed to utilize sound waves and acoustical echoes in an underwater detection device.

In the wake of the Titanic disaster, Langevin and his colleague Constantin Chilowsky, a Russian engineer then living in Switzerland, developed what they termed a "hydrophone" as a mechanism for ships to more readily detect icebergs (the vast majority of any iceberg remains below the ocean surface). Similar systems were put to immediate use as an aid to underwater navigation by submarines.

Improved electronics and technology allowed the production of greatly improved listening and recording devices. Because passive SONAR is essentially nothing more than an elaborate recording and sound amplification device, these systems suffered because they were dependent upon the strength of the sound signal coming from the target. The signals or waves received could be typed (i.e. related to specific targets) for identifying characteristics. Although skilled and experienced operators could provide reasonably accurate estimates of range, bearing, and relative motion of targets, these estimates were far less precise and accurate than results obtained from active systems unless the targets were very close—or were very noisy.

The threat of submarine warfare during World War I made urgent the development of SONAR. and other means of echo detection. The development of the acoustic transducer that converted converting electrical energy to sound waves enabled the rapid advances in SONAR design and technology during the last years of the war. Although active SONAR was developed too late to be widely used during WWI, the push for its development reaped enormous technological dividends. Not all of the advances, however, were restricted to military use. After the war, echosounding devices were placed aboard many large French ocean-liners.

Early into World War II, the British Anti-Submarine Detection and Investigation Committee (its acronym, ASDIC, became a name commonly applied to British SONAR systems) made efforts to outfit every ship in the British fleet with advanced detection devices. The use of ASDIC proved pivotal in the British effort to repel damaging attacks by German submarines.


SONAR and RADAR

Although they rely on two fundamentally different types of wave transmission, SONAR and Radio Detection And Ranging (RADAR ) and both are remote sensing systems. Whilst active SONAR transmits acoustic (i.e., sound) waves, RADAR sends out and measure the return of electromagnetic waves,

Both systems these waves return echoes from certain features or targets that allow the determination of important properties or attributes of the target (e.g., shape, size, speed, distance to target, etc.). Because electromagnetic waves are strongly attenuated (diminished) in water, RADAR signals are mostly used for ground or atmospheric observations. Because SONAR signals easily penetrate water, they are ideal for naviagtion and measurement under water.


SONAR technology

SONAR equipment is used on most ships for measuring the depth of the water. This is accomplished by sending an acoustic pulse and measuring the time for the echo, or return from the bottom. By knowing the speed of sound in the water, the depth is computed by multiplying the speed by one half of the time traveled (for a oneway trip). SONAR is also used to detect large underwater objects and to search for large fish concentrations. More sophisticated SONAR systems for detection and tracking are found aboard naval vessels and submarines. In nature, bats are well known for making use of echolocation, as are porpoises and some species of whales. SONAR should not be confused with ultrasound, which is simply sound at frequencies higher than the threshold of human hearing - greater than 15,000-20,000 cycles per second, or hertz (Hz). Ultrasound is used on a very small scale, at high power, to break up material and for cleaning purposes. Lower power ultrasound is used therapeutically, for treatment of muscle and tissue injuries.

SONAR is very directional, so the signals are sent in narrow beams in various directions to search the water. SONAR usually operates at frequencies in the 10,000-50,000 Hz range. Though higher frequencies provide more accurate location data, propagation losses also increase with frequency . Lower frequencies are therefore used for longer range detection (up to 10 mi [17,600 yd]) at the cost of location accuracy.

Acoustic waves are detected using hydrophones that are essentially underwater microphones. Hydrophones are often deployed in large groups, called arrays, forming a SONAR net. SONAR arrays also give valuable directional information on moving sources. Electronics and signal processing play a large role in hydrophone and general SONAR system performance.

The propagation of sound in water is quite complex and depends very much on the temperature , pressure , and depth of the water. Salinity, the quantity of salt in water, also changes sound propagation speed. In general, the temperature of the ocean is warmest at the surface and decreases with depth. Water pressure, however, increases with depth, due to the water mass . Temperature and pressure, therefore, change what is called the refractive index of the water. Just as light is refracted, or bent by a prism , acoustic waves are continuously refracted up or down and reflected off the surface or the bottom. A SONAR beam propagating along the water in this way resembles a car traveling along regularly spaced hills and valleys. As it is possible for an object to be hidden between these hills, the water conditions must be known in order to properly assess SONAR performance.

In location and tracking operations, two types of SONAR modes exist, active and passive. Echolocation is an active technique in which a pulse is sent and then detected after it bounces off an object. Passive SONAR is a more sensitive, listening-only SONAR that sends no pulses. Most moving objects underwater make some kind of noise. This means that they can be detected just by listening for the noise. Examples of underwater noise are marine life, cavitation (small collapsing air pockets caused by propellers), hull popping of submarines changing depth, and engine vibration. Some military passive SONARs are so sensitive they can detect people talking inside another submarine. Another advantage of passive SONAR is that it can also be used to detect an acoustic signature. Each type of submarine emits certain acoustic frequencies and every vessel's composite acoustic pattern is different, just like a fingerprint or signature. Passive SONAR is predominantly a military tool used for submarine hunting. An important element of hunting is not to divulge one's own position. However, if the passive SONAR hears nothing, one is obliged to turn to active mode but in doing so, risks alerting the other of his presence. The use of SONAR in this case has become a sophisticated tactical exercise .

Other, non-military, applications of SONAR, apart from fish finding, include searching for shipwrecks, probing harbors where visibility is poor, oceanography studies, searching for underwater geological faults and mapping the ocean floor.


Resources

books

Waite, A.D. Sonar for Practicing Engineers. John Wiley & Sons, 2001.

other

Canadian Center for Remote Sensing, "History of Remote Sensing." 2001 [cited February 1, 2003]. <http://www.ccrs.nrcan.gc.ca/ccrs/org/history/morleye.htm>.

David Lunney
K. Lee Lerner

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acoustics

—The study of the creation and propagation of mechanical vibration causing sound.

Active SONAR

—Mode of echo location by sending a signal and detecting the returning echo.

Array

—A large group of hydrophones, usually regularly spaced, forming a SONAR net.

Hertz

—A unit of measurement for frequency, abbreviated Hz. One hertz is one cycle per second.

Hydrophone

—An underwater microphone sensitive to acoustic disturbances.

Passive SONAR

—A sensitive listening-only mode to detect presence of objects making noise.

Propagation

—Traveling or penetration of waves through a medium.

Radar

—A method of detecting distant objects based on the reflection of radio waves from their surfaces.

Refractive index

—(characteristic of a medium) Degree to which a wave is refracted, or bent.

Sonar

—SOund Navigation And Ranging. A device utilizing sound to determine the range and direction to an underwater object.

Ultrasound

—Acoustic vibrations with frequencies higher than the human threshold of hearing.

Wave

—A motion, in which energy and momentum is carried away from some source, which repeats itself in space and time with little or no change.

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"Sonar." The Gale Encyclopedia of Science. . Encyclopedia.com. 17 Sep. 2018 <http://www.encyclopedia.com>.

"Sonar." The Gale Encyclopedia of Science. . Encyclopedia.com. (September 17, 2018). http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/sonar-0

"Sonar." The Gale Encyclopedia of Science. . Retrieved September 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/sonar-0

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Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.