Sonar Sonar ( originally an acronym for SOund Navigation And Ranging ) is a technique that uses sound propagation ( usually underwater , as in submarine navigation ) to navigate , communicate with or detect objects on or under the surface of the water , such as other vessels . Two types of technology share the name " sonar " : " passive " sonar is essentially listening for the sound made by vessels ; " active " sonar is emitting pulses of sounds and listening for echoes . Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of " targets " in the water . Acoustic location in air was used before the introduction of radar . Sonar may also be used in air for robot navigation , and SODAR ( an upward looking in - air sonar ) is used for atmospheric investigations . The term " sonar " is also used for the equipment used to generate and receive the sound . The acoustic frequencies used in sonar systems vary from very low ( infrasonic ) to extremely high ( ultrasonic ) . The study of underwater sound is known as underwater acoustics or hydroacoustics . History . Although some animals ( dolphins and bats ) have used sound for communication and object detection for millions of years , use by humans in the water is initially recorded by Leonardo da Vinci in 1490 : a tube inserted into the water was said to be used to detect vessels by placing an ear to the tube . In the 19th century an underwater bell was used as an ancillary to lighthouses to provide warning of hazards . The use of sound to ' echo locate ' underwater in the same way as bat s use sound for aerial navigation seems to have been prompted by the " Titanic " disaster of 1912 . The world ' s first patent for an underwater echo ranging device was filed at the British Patent Office by English meteorologist Lewis Richardson a month after the sinking of the Titanic , and a German physicist Alexander Behm obtained a patent for an echo sounder in 1913 . The Canadian engineer Reginald Fessenden , while working for the Submarine Signal Company in Boston , built an experimental system beginning in 1912 , a system later tested in Boston Harbor , and finally in 1914 from the U . S . Revenue ( now Coast Guard ) Cutter Miami on the Grand Banks off Newfoundland Canada . In that test , Fessenden demonstrated depth sounding , underwater communications ( Morse Code ) and echo ranging ( detecting an iceberg at two miles ( 3 km ) range ) . The so - called Fessenden oscillator , at ca . 500 Hz frequency , was unable to determine the bearing of the berg due to the 3 metre wavelength and the small dimension of the transducer ' s radiating face ( less than 1 metre in diameter ) . The ten Montreal - built British H class submarine s launched in 1915 were equipped with a Fessenden oscillator . During World War I the need to detect submarine s prompted more research into the use of sound . The British made early use of underwater hydrophones , while the French physicist Paul Langevin , working with a Russian immigrant electrical engineer , Constantin Chilowski , worked on the development of active sound devices for detecting submarines in 1915 using quartz . Although piezoelectric and magnetostrictive transducers later superseded the electrostatic transducers they used , this work influenced future designs . Lightweight sound - sensitive plastic film and fibre optics have been used for hydrophones ( acousto - electric transducers for in - water use ) , while Terfenol - D and PMN ( lead magnesium niobate ) have been developed for projectors . ASDIC . In 1916 , under the British Board of Invention and Research , Canadian physicist Robert William Boyle took on the active sound detection project with A B Wood , producing a prototype for testing in mid - 1917 . This work , for the Anti - Submarine Division of the British Naval Staff , was undertaken in utmost secrecy , and used quartz piezoelectric crystals to produce the world ' s first practical underwater active sound detection apparatus . To maintain secrecy no mention of sound experimentation or quartz was made - the word used to describe the early work ( ' supersonics ' ) was changed to ' ASD ' ics , and the quartz material to ' ASD ' ivite : hence the British acronym " ASDIC " . In 1939 , in response to a question from the Oxford English Dictionary , the Admiralty made up the story that it stood for ' Allied Submarine Detection Investigation Committee ' , and this is still widely believed , though no committee bearing this name has been found in the Admiralty archives . By 1918 , both France and Britain had built prototype active systems . The British tested their ASDIC on in 1920 , and started production in 1922 . The 6th Destroyer Flotilla had ASDIC - equipped vessels in 1923 . An anti - submarine school , HMS " Osprey " , and a training flotilla of four vessels were established on Portland in 1924 . The US Sonar QB set arrived in 1931 . By the outbreak of World War II , the Royal Navy had five sets for different surface ship classes , and others for submarines , incorporated into a complete anti - submarine attack system . The effectiveness of early ASDIC was hamstrung by the use of the depth charge as an anti - submarine weapon . This required an attacking vessel to pass over a submerged contact before dropping charges over the stern , resulting in a loss of ASDIC contact in the moments leading up to attack . The hunter was effectively firing blind , during which time a submarine commander could take evasive action . This situation was remedied by using several ships cooperating and by the adoption of " ahead throwing weapons " , such as Hedgehog and later Squid , which projected warheads at a target ahead of the attacker and thus still in ASDIC contact . Developments during the war resulted in British ASDIC sets which used several different shapes of beam , continuously covering blind spots . Later , acoustic torpedoes were used . At the start of World War II , British ASDIC technology was transferred for free to the United States . Research on ASDIC and underwater sound was expanded in the UK and in the US . Many new types of military sound detection were developed . These included sonobuoy s , first developed by the British in 1944 under the codename " High Tea " , dipping / dunking sonar and mine detection sonar . This work formed the basis for post war developments related to countering the nuclear submarine . Work on sonar had also been carried out in the Axis countries , notably in Germany , which included countermeasures . At the end of World War II this German work was assimilated by Britain and the US . Sonars have continued to be developed by many countries , including Russia , for both military and civil uses . In recent years the major military development has been the increasing interest in low frequency active systems . SONAR . During the 1930s American engineers developed their own underwater sound detection technology and important discoveries were made , such as thermocline s , that would help future development . After technical information was exchanged between the two countries during the Second World War , Americans began to use the term " SONAR " for their systems , coined as the equivalent of RADAR . Performance factors . The detection , classification and localisation performance of a sonar depends on the environment and the receiving equipment , as well as the transmitting equipment in an active sonar or the target radiated noise in a passive sonar . Sound propagation . Sonar operation is affected by variations in sound speed , particularly in the vertical plane . Sound travels more slowly in fresh water than in sea water , though the difference is small . The speed is determined by the water ' s bulk modulus and mass density . The bulk modulus is affected by temperature , dissolved impurities ( usually salinity ) , and pressure . The density effect is small . The speed of sound ( in feet per second ) is approximately : This empirical ly derived approximation equation is reasonably accurate for normal temperatures , concentrations of salinity and the range of most ocean depths . Ocean temperature varies with depth , but at between 30 and 100 meters there is often a marked change , called the thermocline , dividing the warmer surface water from the cold , still waters that make up the rest of the ocean . This can frustrate sonar , because a sound originating on one side of the thermocline tends to be bent , or refracted , through the thermocline . The thermocline may be present in shallower coastal waters . However , wave action will often mix the water column and eliminate the thermocline . Water pressure also affects sound propagation : higher pressure increases the sound speed , which causes the sound waves to refract away from the area of higher sound speed . The mathematical model of refraction is called Snell ' s law . If the sound source is deep and the conditions are right , propagation may occur in the ' deep sound channel ' . This provides extremely low propagation loss to a receiver in the channel . This is because of sound trapping in the channel with no losses at the boundaries . Similar propagation can occur in the ' surface duct ' under suitable conditions . However in this case there are reflection losses at the surface . In shallow water propagation is generally by repeated reflection at the surface and bottom , where considerable losses can occur . Sound propagation is affected by absorption in the water itself as well as at the surface and bottom . This absorption depends upon frequency , with several different mechanisms in sea water . Long - range sonar uses low frequencies to minimise absorption effects . The sea contains many sources of noise that interfere with the desired target echo or signature . The main noise sources are waves and shipping . The motion of the receiver through the water can also cause speed - dependent low frequency noise . Scattering . When active sonar is used , scattering occurs from small objects in the sea as well as from the bottom and surface . This can be a major source of interference . This acoustic scattering is analogous to the scattering of the light from a car ' s headlights in fog : a high - intensity pencil beam will penetrate the fog to some extent , but broader - beam headlights emit much light in unwanted directions , much of which is scattered back to the observer , overwhelming that reflected from the target ( " white - out " ) . For analogous reasons active sonar needs to transmit in a narrow beam to minimise scattering . Target characteristics . The sound " reflection " characteristics of the target of an active sonar , such as a submarine , are known as its target strength . A complication is that echoes are also obtained from other objects in the sea such as whales , wakes , schools of fish and rocks . Passive sonar detects the target ' s " radiated " noise characteristics . The radiated spectrum comprises a continuous spectrum of noise with peaks at certain frequencies which can be used for classification . Countermeasures . " Active " ( powered ) countermeasures may be launched by a submarine under attack to raise the noise level , provide a large false target , and obscure the signature of the submarine itself . " Passive " ( i . e . , non - powered ) countermeasures include : Active sonar . Active sonar uses a sound transmitter and a receiver . When the two are in the same place it is monostatic operation . When the transmitter and receiver are separated it is bistatic operation . When more transmitters ( or more receivers ) are used , again spatially separated , it is multistatic operation . Most sonars are used monostatically with the same array often being used for transmission and reception . Active sonobuoy fields may be operated multistatically . Active sonar creates a pulse of sound , often called a " ping " , and then listens for reflection s ( echo ) of the pulse . This pulse of sound is generally created electronically using a sonar projector consisting of a signal generator , power amplifier and electro - acoustic transducer / array . A beamformer is usually employed to concentrate the acoustic power into a beam , which may be swept to cover the required search angles . Generally , the electro - acoustic transducers are of the Tonpilz type and their design may be optimised to achieve maximum efficiency over the widest bandwidth , in order to optimise performance of the overall system . Occasionally , the acoustic pulse may be created by other means , e . g . ( 1 ) chemically using explosives , or ( 2 ) airguns or ( 3 ) plasma sound sources . To measure the distance to an object , the time from transmission of a pulse to reception is measured and converted into a range by knowing the speed of sound . To measure the bearing , several hydrophone s are used , and the set measures the relative arrival time to each , or with an array of hydrophones , by measuring the relative amplitude in beams formed through a process called beamforming . Use of an array reduces the spatial response so that to provide wide cover multibeam systems are used . The target signal ( if present ) together with noise is then passed through various forms of signal processing , which for simple sonars may be just energy measurement . It is then presented to some form of decision device that calls the output either the required signal or noise . This decision device may be an operator with headphones or a display , or in more sophisticated sonars this function may be carried out by software . Further processes may be carried out to classify the target and localise it , as well as measuring its velocity . The pulse may be at constant frequency or a chirp of changing frequency ( to allow pulse compression on reception ) . Simple sonars generally use the former with a filter wide enough to cover possible Doppler changes due to target movement , while more complex ones generally include the latter technique . Since digital processing became available pulse compression has usually been implemented using digital correlation techniques . Military sonars often have multiple beams to provide all - round cover while simple ones only cover a narrow arc , although the beam may be rotated , relatively slowly , by mechanical scanning . Particularly when single frequency transmissions are used , the Doppler effect can be used to measure the radial speed of a target . The difference in frequency between the transmitted and received signal is measured and converted into a velocity . Since Doppler shifts can be introduced by either receiver or target motion , allowance has to be made for the radial speed of the searching platform . One useful small sonar is similar in appearance to a waterproof flashlight . The head is pointed into the water , a button is pressed , and the device displays the distance to the target . Another variant is a " fishfinder " that shows a small display with shoal s of fish . Some civilian sonars ( which are not designed for stealth ) approach active military sonars in capability , with quite exotic three - dimensional displays of the area near the boat . When active sonar is used to measure the distance from the transducer to the bottom , it is known as echo sounding . Similar methods may be used looking upward for wave measurement . Active sonar is also used to measure distance through water between two sonar transducers or a combination of a hydrophone ( underwater acoustic microphone ) and projector ( underwater acoustic speaker ) . A transducer is a device that can transmit and receive acoustic signals ( " pings " ) . When a hydrophone / transducer receives a specific interrogation signal it responds by transmitting a specific reply signal . To measure distance , one transducer / projector transmits an interrogation signal and measures the time between this transmission and the receipt of the other transducer / hydrophone reply . The time difference , scaled by the speed of sound through water and divided by two , is the distance between the two platforms . This technique , when used with multiple transducers / hydrophones / projectors , can calculate the relative positions of static and moving objects in water . In combat situations , an active pulse can be detected by an opponent and will reveal a submarine ' s position . A very directional , but low - efficiency , type of sonar ( used by fisheries , military , and for port security ) makes use of a complex nonlinear feature of water known as non - linear sonar , the virtual transducer being known as a " parametric array " . Project ARTEMIS . Project ARTEMIS was a one - of - a - kind low - frequency sonar for surveillance that was deployed off Bermuda for several years in the early 1960s . The active portion was deployed from a World War II tanker , and the receiving array was a built into a fixed position on an offshore bank . Transponder . This is an active sonar device that receives a stimulus and immediately ( or with a delay ) retransmits the received signal or a predetermined one . Performance prediction . A sonar target is small relative to the sphere , centred around the emitter , on which it is located . Therefore , the power of the reflected signal is very low , several orders of magnitude less than the original signal . Even if the reflected signal was of the same power , the following example ( using hypothetical values ) shows the problem : Suppose a sonar system is capable of emitting a 10 , 000 W / m² signal at 1 m , and detecting a 0 . 001 W / m² signal . At 100 m the signal will be 1 W / m² ( due to the inverse - square law ) . If the entire signal is reflected from a 10 m² target , it will be at 0 . 001 W / m² when it reaches the emitter , i . e . just detectable . However , the original signal will remain above 0 . 001 W / m² until 300 m . Any 10 m² target between 100 and 300 m using a similar or better system would be able to detect the pulse but would not be detected by the emitter . The detectors must be very sensitive to pick up the echoes . Since the original signal is much more powerful , it can be detected many times further than twice the range of the sonar ( as in the example ) . In active sonar there are two performance limitations , due to noise and reverberation . In general one or other of these will dominate so that the two effects can be initially considered separately . In noise limited conditions at initial detection : where SL is the source level , TL is the transmission loss ( or propagation loss ) , TS is the target strength , NL is the noise level , DI is the directivity index of the array ( an approximation to the array gain ) and DT is the detection threshold . In reverberation limited conditions at initial detection ( neglecting array gain ) : where RL is the reverberation level and the other factors are as before . Passive sonar . Passive sonar listens without transmitting . It is often employed in military settings , although it is also used in science applications , " e . g . " , detecting fish for presence / absence studies in various aquatic environments - see also passive acoustics and passive radar . In the very broadest usage , this term can encompass virtually any analytical technique involving remotely generated sound , though it is usually restricted to techniques applied in an aquatic environment . Identifying sound sources . Passive sonar has a wide variety of techniques for identifying the source of a detected sound . For example , U . S . vessels usually operate 60 Hz alternating current power systems . If transformer s or generators are mounted without proper vibration insulation from the hull or become flooded , the 60 Hz sound from the windings can be emitted from the submarine or ship . This can help to identify its nationality , as most European submarines have 50 Hz power systems . Intermittent sound sources ( such as a wrench being dropped ) may also be detectable to passive sonar . Until fairly recently , an experienced , trained operator identified signals , but now computers may do this . Passive sonar systems may have large sonic databases , but the sonar operator usually finally classifies the signals manually . A computer system frequently uses these databases to identify classes of ships , actions ( i . e . the speed of a ship , or the type of weapon released ) , and even particular ships . Publications for classification of sounds are provided by and continually updated by the US Office of Naval Intelligence . Noise limitations . Passive sonar on vehicles is usually severely limited because of noise generated by the vehicle . For this reason , many submarines operate nuclear reactors that can be cooled without pumps , using silent convection , or fuel cell s or batteries , which can also run silently . Vehicles ' propellers are also designed and precisely machined to emit minimal noise . High - speed propellers often create tiny bubbles in the water , and this cavitation has a distinct sound . The sonar hydrophone s may be towed behind the ship or submarine in order to reduce the effect of noise generated by the watercraft itself . Towed units also combat the thermocline , as the unit may be towed above or below the thermocline . The display of most passive sonars used to be a two - dimensional waterfall display . The horizontal direction of the display is bearing . The vertical is frequency , or sometimes time . Another display technique is to color - code frequency - time information for bearing . More recent displays are generated by the computers , and mimic radar - type plan position indicator displays . Performance prediction . Unlike active sonar , only one way propagation is involved . Because of the different signal processing used , the minimum detectable signal to noise ratio will be different . The equation for determining the performance of a passive sonar is : where SL is the source level , TL is the transmission loss , NL is the noise level , DI is the directivity index of the array ( an approximation to the array gain ) and DT is the detection threshold . The figure of merit of a passive sonar is : Military applications . Modern naval warfare makes extensive use of both passive and active sonar from water - borne vessels , aircraft and fixed installations . Although active sonar was used by surface craft in World War II , submarines avoided the use of active sonar due to the potential for revealing their presence and position to enemy forces . However , the advent of modern signal - processing enabled the use of passive sonar as a primary means for search and detection operations . In 1987 a division of Japan ese company Toshiba reportedly sold machinery to the Soviet Union that allowed their submarine propeller blades to be milled so that they became radically quieter , making the newer generation of submarines more difficult to detect . The use of active sonar by a submarine to determine bearing is extremely rare and will not necessarily give high quality bearing or range information to the submarines fire control team ; however , use of active sonar on surface ships is very common . Active sonar is used by submarines when if the tactical situation dictates it is more important to determine the position of a hostile submarine than conceal their own position . With surface ships it might be assumed that the threat is already tracking the ship with satellite data . Any vessel around the emitting sonar will detect the emission . Having heard the signal , it is easy to identify the sonar equipment used ( usually with its frequency ) and its position ( with the sound wave ' s energy ) . Active sonar is similar to radar in that , while it allows detection of targets at a certain range , it also enables the emitter to be detected at a far greater range , which is undesirable . Since active sonar reveals the presence and position of the operator , and does not allow exact classification of targets , it is used by fast ( planes , helicopters ) and by noisy platforms ( most surface ships ) but rarely by submarines . When active sonar is used by surface ships or submarines , it is typically activated very briefly at intermittent periods to minimize the risk of detection . Consequently active sonar is normally considered a backup to passive sonar . In aircraft , active sonar is used in the form of disposable sonobuoy s that are dropped in the aircraft ' s patrol area or in the vicinity of possible enemy sonar contacts . Passive sonar has several advantages . Most importantly , it is silent . If the target radiated noise level is high enough , it can have a greater range than active sonar , and allows the target to be identified . Since any motorized object makes some noise , it may in principle be detected , depending on the level of noise emitted and the ambient noise level in the area , as well as the technology used . To simplify , passive sonar " sees " around the ship using it . On a submarine , nose - mounted passive sonar detects in directions of about 270° , centered on the ship ' s alignment , the hull - mounted array of about 160° on each side , and the towed array of a full 360° . The invisible areas are due to the ship ' s own interference . Once a signal is detected in a certain direction ( which means that something makes sound in that direction , this is called broadband detection ) it is possible to zoom in and analyze the signal received ( narrowband analysis ) . This is generally done using a Fourier transform to show the different frequencies making up the sound . Since every engine makes a specific sound , it is straightforward to identify the object . Databases of unique engine sounds are part of what is known as " acoustic intelligence " or ACINT . Another use of passive sonar is to determine the target ' s trajectory . This process is called Target Motion Analysis ( TMA ) , and the resultant " solution " is the target ' s range , course , and speed . TMA is done by marking from which direction the sound comes at different times , and comparing the motion with that of the operator ' s own ship . Changes in relative motion are analyzed using standard geometrical techniques along with some assumptions about limiting cases . Passive sonar is stealthy and very useful . However , it requires high - tech electronic components and is costly . It is generally deployed on expensive ships in the form of arrays to enhance detection . Surface ships use it to good effect ; it is even better used by submarines , and it is also used by airplanes and helicopters , mostly to a " surprise effect " , since submarines can hide under thermal layers . If a submarine ' s commander believes he is alone , he may bring his boat closer to the surface and be easier to detect , or go deeper and faster , and thus make more sound . Examples of sonar applications in military use are given below . Many of the civil uses given in the following section may also be applicable to naval use . Anti - submarine warfare . Until recently , ship sonars were usually with hull mounted arrays , either amidships or at the bow . It was soon found after their initial use that a means of reducing flow noise was required . The first were made of canvas on a framework , then steel ones were used . Now domes are usually made of reinforced plastic or pressurized rubber . Such sonars are primarily active in operation . An example of a conventional hull mounted sonar is the SQS - 56 . Because of the problems of ship noise , towed sonars are also used . These also have the advantage of being able to be placed deeper in the water . However , there are limitations on their use in shallow water . These are called towed arrays ( linear ) or variable depth sonars ( VDS ) with 2 / 3D arrays . A problem is that the winches required to deploy / recover these are large and expensive . VDS sets are primarily active in operation while towed arrays are passive . An example of a modern active / passive ship towed sonar is Sonar 2087 made by Thales Underwater Systems . Torpedoes . Modern torpedoes are generally fitted with an active / passive sonar . This may be used to home directly on the target , but wake following torpedoes are also used . An early example of an acoustic homer was the Mark 37 torpedo . Torpedo countermeasures can be towed or free . An early example was the German " Sieglinde " device while the " Bold " was a chemical device . A widely used US device was the towed AN / SLQ - 25 Nixie while Mobile submarine simulator ( MOSS ) was a free device . A modern alternative to the Nixie system is the UK Royal Navy S2170 Surface Ship Torpedo Defence system . Mines . Mines may be fitted with a sonar to detect , localize and recognize the required target . Further information is given in acoustic mine and an example is the CAPTOR mine . Mine countermeasures . Mine Countermeasure ( MCM ) Sonar , sometimes called " Mine and Obstacle Avoidance Sonar ( MOAS ) " , is a specialized type of sonar used for detecting small objects . Most MCM sonars are hull mounted but a few types are VDS design . An example of a hull mounted MCM sonar is the Type 2193 while the SQQ - 32 Mine - hunting sonar and Type 2093 systems are VDS designs . See also Minesweeper ( ship ) Submarine navigation . Submarines rely on sonar to a greater extent than surface ships as they cannot use radar at depth . The sonar arrays may be hull mounted or towed . Information fitted on typical fits is given in Oyashio class submarine and Swiftsure class submarine . Aircraft . Helicopters can be used for antisubmarine warfare by deploying fields of active / passive sonobuoys or can operate dipping sonar , such as the AQS - 13 . Fixed wing aircraft can also deploy sonobuoys and have greater endurance and capacity to deploy them . Processing from the sonobuoys or dipping sonar can be on the aircraft or on ship . Helicopters have also been used for mine countermeasure missions using towed sonars such as the AQS - 20A . Underwater communications . Dedicated sonars can be fitted to ships and submarines for underwater communication . See also the section on the underwater acoustics page . Ocean surveillance . For many years , the United States operated a large set of passive sonar arrays at various points in the world ' s oceans , collectively called Sound Surveillance System ( SOSUS ) and later Integrated Undersea Surveillance System ( IUSS ) . A similar system is believed to have been operated by the Soviet Union . As permanently mounted arrays in the deep ocean were utilised , they were in very quiet conditions so long ranges could be achieved . Signal processing was carried out using powerful computers ashore . With the ending of the Cold War a SOSUS array has been turned over to scientific use . In the United States Navy , a special badge known as the Integrated Undersea Surveillance System Badge is awarded to those who have been trained and qualified in its operation . Underwater security . Sonar can be used to detect frogmen and other scuba diver s . This can be applicable around ships or at entrances to ports . Active sonar can also be used as a deterrent and / or disablement mechanism . One such device is the Cerberus system . See Underwater Port Security System and Anti - frogman techniques # Ultrasound detection . Hand - held sonar . Limpet Mine Imaging Sonar ( LIMIS ) is a hand - held or ROV - mounted imaging sonar designed for patrol divers ( combat frogmen or clearance diver s ) to look for limpet mine s in low visibility water . The LUIS is another imaging sonar for use by a diver . Integrated Navigation Sonar System ( INSS ) is a small flashlight - shaped handheld sonar for divers that displays range . Intercept sonar . This is a sonar designed to detect and locate the transmissions from hostile active sonars . An example of this is the Type 2082 fitted on the British Vanguard class submarine s . Civilian applications . Fisheries . Fishing is an important industry that is seeing growing demand , but world catch tonnage is falling as a result of serious resource problems . The industry faces a future of continuing worldwide consolidation until a point of sustainability can be reached . However , the consolidation of the fishing fleets are driving increased demands for sophisticated fish finding electronics such as sensors , sounders and sonars . Historically , fishermen have used many different techniques to find and harvest fish . However , acoustic technology has been one of the most important driving forces behind the development of the modern commercial fisheries . Sound waves travel differently through fish than through water because a fish ' s air - filled swim bladder has a different density than seawater . This density difference allows the detection of schools of fish by using reflected sound . Acoustic technology is especially well suited for underwater applications since sound travels farther and faster underwater than in air . Today , commercial fishing vessels rely almost completely on acoustic sonar and sounders to detect fish . Fishermen also use active sonar and echo sounder technology to determine water depth , bottom contour , and bottom composition . Companies such as eSonar , Raymarine UK , Marport Canada , Wesmar , Furuno , Krupp , and Simrad make a variety of sonar and acoustic instruments for the deep sea commercial fishing industry . For example , net sensors take various underwater measurements and transmit the information back to a receiver on board a vessel . Each sensor is equipped with one or more acoustic transducers depending on its specific function . Data is transmitted from the sensors using wireless acoustic telemetry and is received by a hull mounted hydrophone . The analog signal s are decoded and converted by a digital acoustic receiver into data which is transmitted to a bridge computer for graphical display on a high resolution monitor . Echo sounding . Echo sounding is a process used to determine the depth of water beneath ships and boats . A type of active sonar , echo sounding is the transmission of an acoustic pulse directly downwards to the seabed , measuring the time between transmission and echo return , after having hit the bottom and bouncing back to its ship of origin . The acoustic pulse is emitted by a transducer which receives the return echo as well . The depth measurement is calculated by multiplying the speed of sound in water ( averaging 1 , 500 meters per second ) by the time between emission and echo return . The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echo - sounders but , because their function is slightly different from the initial model of the echo - sounder , have been given different terms . Net location . The net sounder is an echo sounder with a transducer mounted on the headline of the net rather than on the bottom of the vessel . Nevertheless , to accommodate the distance from the transducer to the display unit , which is much greater than in a normal echo - sounder , several refinements have to be made . Two main types are available . The first is the cable type in which the signals are sent along a cable . In this case there has to be the provision of a cable drum on which to haul , shoot and stow the cable during the different phases of the operation . The second type is the cable less net - sounder – such as Marport ’ s Trawl Explorer - in which the signals are sent acoustically between the net and hull mounted receiver / hydrophone on the vessel . In this case no cable drum is required but sophisticated electronics are needed at the transducer and receiver . The display on a net sounder shows the distance of the net from the bottom ( or the surface ) , rather than the depth of water as with the echo - sounder ' s hull - mounted transducer . Fixed to the headline of the net , the footrope can usually be seen which gives an indication of the net performance . Any fish passing into the net can also be seen , allowing fine adjustments to be made to catch the most fish possible . In other fisheries , where the amount of fish in the net is important , catch sensor transducers are mounted at various positions on the cod - end of the net . As the cod - end fills up these catch sensor transducers are triggered one by one and this information is transmitted acoustically to display monitors on the bridge of the vessel . The skipper can then decide when to haul the net . Modern versions of the net sounder , using multiple element transducers , function more like a sonar than an echo sounder and show slices of the area in front of the net and not merely the vertical view that the initial net sounders used . The sonar is an echo - sounder with a directional capability that can show fish or other objects around the vessel . ROV and UUV . Small sonars have been fitted to Remotely Operated Vehicles ( ROV ) and Unmanned Underwater Vehicles ( UUV ) to allow their operation in murky conditions . These sonars are used for looking ahead of the vehicle . The Long - Term Mine Reconnaissance System is an UUV for MCM purposes . Vehicle location . Sonars which act as beacons are fitted to aircraft to allow their location in the event of a crash in the sea . Short and Long Baseline sonars may be used for caring out the location , such as LBL . Scientific applications . Biomass estimation . Detection of fish , and other marine and aquatic life , and estimation their individual sizes or total biomass using active sonar techniques . As the sound pulse travels through water it encounters objects that are of different density or acoustic characteristics than the surrounding medium , such as fish , that reflect sound back toward the sound source . These echoes provide information on fish size , location , abundance and behavior . Data is usually processed and analysed using a variety of software such as Echoview . See Also : Hydroacoustics and Fisheries Acoustics . Wave measurement . An upward looking echo sounder mounted on the bottom or on a platform may be used to make measurements of wave height and period . From this statistics of the surface conditions at a location can be derived . Water velocity measurement . Special short range sonars have been developed to allow measurements of water velocity . Bottom type assessment . Sonars have been developed that can be used to characterise the sea bottom into , for example , mud , sand , and gravel . Relatively simple sonars such as echo sounders can be promoted to seafloor classification systems via add - on modules , converting echo parameters into sediment type . Different algorithms exist , but they are all based on changes in the energy or shape of the reflected sounder pings . Advanced substrate classification analysis can be achieved using calibrated ( scientific ) echosounders and parametric or fuzzy - logic analysis of the acoustic data ( See : Acoustic Seabed Classification ) Bottom topography measurement . Side - scan sonar s can be used to derive maps of the topography of an area by moving the sonar across it just above the bottom . Low frequency sonars such as GLORIA have been used for continental shelf wide surveys while high frequency sonars are used for more detailed surveys of smaller areas . Sub - bottom profiling . Powerful low frequency echo - sounders have been developed for providing profiles of the upper layers of the ocean bottom . Synthetic aperture sonar . Various synthetic aperture sonars have been built in the laboratory and some have entered use in mine - hunting and search systems . An explanation of their operation is given in synthetic aperture sonar . Parametric sonar . Parametric sources use the non - linearity of water to generate the difference frequency between two high frequencies . A virtual end - fire array is formed . Such a projector has advantages of broad bandwidth , narrow beamwidth , and when fully developed and carefully measured it has no obvious sidelobes : see Parametric array . Its major disadvantage is very low efficiency of only a few percent . P . J . Westervelt ' s seminal 1963 JASA paper summarizes the trends involved . Harm to marine mammals . Research has shown that active sonar can lead to mass strandings marine mammals . Beaked whales , the most common casualty of the strandings , have been shown to be highly sensitive to mid - frequency active sonar . Other marine mammals such as the blue whale also flee away from the source of the sonar , while naval activity was suggested to be the most probable cause of a mass stranding of dolphins . The US Navy , which part - funded some of studies , said the findings only showed behavioural responses to sonar , not actual harm , but " will evaluate the effectiveness of [ their ] marine mammal protective measures in light of new research findings . " Some marine animals , such as whales and dolphins , use echolocation systems , sometimes called " biosonar " to locate predators and prey . It is conjectured that active sonar transmitters could confuse these animals and interfere with basic biological functions such as feeding and mating .