sonar, or sound navigational ranging, is the use of sound to explore the oceans, and for military and salvage purposes. And as the swim-bladders of fish that are gas filled are good reflectors, sonar (fish-finders) is used by commercial fisheries to detect fish shoals.

Sound, particularly low-frequency sound, travels well through water—the singing of humpback whales allows them to communicate over hundreds of kilometres underwater. Dolphins have their own sonar systems, which they use to locate their prey. The sound pulses they produce are reflected back by any objects in the water that have a different acoustic density from the water. They may be either hard objects or gas bubbles that will reflect the sound so long as they are larger than the wavelength of the sound signal. High-frequency sounds have short wavelengths and so ‘see’ smaller objects, but do not travel through water as well as low-frequency sounds with long wavelengths.

Pulses of sound are used by echo sounders; a series of pings of sound is transmitted from the vessel on the surface and the shortest time for the echo to return gives a measure of the depth. Sound travels at about 1,500 metres per second, so if the echo returns in two seconds, the depth is 1,500 metres (5,000 ft), that is, one second for the signal to reach the bottom and another second for it to return. But the speed of sound through water varies with the temperature of the water, so over very deep water an accurate sounding can only be gained if the temperature profile of the water column is known.

But sonar does not just work vertically downwards. Upward-looking sonars are mounted on autonomous underwater vehicles that have been programmed to go far under pack ice and ice shelves in polar regions. These sonars not only enable the vehicle to stay below the ice but have also provided direct measurements of the rate at which the ice is thinning as a result of climate change. Side-scan sonar is the transmission of sound beams at an angle to insonify a swath of the seabed. The returning echoes generate a radar-like record of the seabed, picking out variations in its topography and make-up. Strong echoes are returned from gravels and rocks, especially if the rock face is facing the transmitter, whereas muddy bottoms reflect weak echoes. GLORIA was a long-range oblique sonar system developed to survey large swaths of the deep ocean bed 12 kilometres (7.5 mls.) to either side of the ship's track. It was mounted in a large body that was towed on a faired cable below the thermocline, so the reception of the faint acoustic echoes would be optimized. It revealed many new seabed features, such as immense debris flows fanning out from the volcanic islands of the Canary Islands and Hawaii. Sea Beams are hull-mounted devices that are simpler to operate, but achieve similar, if less extensive, seabed surveys.

Side-scan sonar not only reveals the characteristics of the bottom topography but in shallow water can be used to locate pipelines or, in marine archaeology, to detect shipwrecks on the seabed. The full technological development of side-scan is swath bathymetry in which multiple frequencies are used; the lower frequencies penetrating the sediments to provide data on the internal structure of the seabed. Sound signals can also be coded to switch devices such as acoustic releases to recover current meter moorings in the deep ocean, or even transmit in situ data from devices being towed.

Marine archaeologists use forward-sector scanning sonar, targeting a site where there may be a shipwreck by making several runs from different directions so that a sonar ‘shadow’ picture can be built up of the feature on the seabed.

Other scientific sonars include acoustic doppler current profilers (ADCPs) in which an array of high-frequency sound (70–100 kHz) is transmitted into the water, and the frequencies of the return echoes are analysed by computer. At such high frequencies the echoes come from particles, including plankton, suspended in the water. If these are moving relative to the device, the tones (frequencies) of the echoes are shifted (just as the sound of a train changes as it passes; the so-called Doppler shift). If the particles are drifting passively in the currents, this gives a direct measure of variations in the currents with depth.

For military purposes, passive listening systems are used initially to detect the approach of enemy devices. Once something is heard, active sonars transmitting sounds are turned on to locate the potentially offensive devices (submarines, torpedoes, or mines). However, the active sonar will not only locate any offensive devices, but also reveal the presence and position of the vessel transmitting it, so it is only used in the final stages of an engagement.

Recently (2004) the use of high-powered military sonars has been blamed for mass strandings of whales, the strong sound signals allegedly fatally damaging the whales' sonar systems. See also deep scattering layers; salvage.

www.arl.psu.edu/capabilities/uss_acou_sonars.htmlnews.bbc.co.uk/1/hi/world/americas/2131524-.stm

M. V. Angel

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

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 (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.

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 ]

sonar , device used underwater for locating submerged objects and for submarine communication by means of sound waves. The term sonar is an acronym for so und na vigation r anging. 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, 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 suggested that sonar used for echo ranging can cause a disorder similar to decompression sickness (in which nitrogen bubbles form in body tissues) in some beaked whales and dolphins and that this may be linked to strandings of those species.

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

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.

sonar ˈsōˌnär n.
1. 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.

2. an apparatus used in this system.

SONAR, acronym derived from Sound-Navigation, Ranging. It was a US device which, like ASDIC, was developed between the wars to detect submerged submarines. In 1943 the Royal Navy adopted SONAR as the name for its ASDIC equipment.

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.

sonar See SIDE-SCAN SONAR; SONIC LOG; and ECHO-SOUNDING.

sonar •dinar • seminar • sonar • Fragonard

sonar (ˈsəʊnɑː) sound navigation and ranging