underwater acoustics

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UNDERWATERACOUSTICSRobert E. RandallOcean Engineering Program Civil Engineering Department Texas A&M University College Station, Texas, USA

CONTENTSINTRODUCTION, 383 Early History, 383 Underwater Acoustic System Categories, 385 Underwater Sound Fundamentals, 387 Decibel Scales, 388 SONAR EQUATIONS, 389 Active Sonar Equation, 390 Passive Sonar Equation, 391 Transient Form of the Sonar Equations, 392 PROPERTIES OF TRANSDUCER ARRAYS, 394 Array Gain, 395 Receiving Directivity Index, 397 Transducer Responses, 398 Beam Pattern, 399 Special Arrays, 401 UNDERWATER SOUND PROJECTOR, 403 Explosives as Sound Projectors, 405 UNDERWATER SOUND PROPAGATION, 408 Spreading Laws, 409 Cylindrical Spreading, 409 Multiple Constant-Gradient Layers, 422 TRANSMISSION LOSS MODELS, 426 Transmission Loss for Ray Diagrams, 426 Sea Surface and Bottom Loss, 427


Underwater Acoustics


Transmission Loss Model for Mixed Layer Sound Channel, 428 Deep Sound Channel Transmission Loss Model, 428 Arctic Propagation, 429 Transmission Loss Models for Shallow Water, 429 AMBIENT NOISE, 430 SCA'I~ERING AND REVERBERATION LEVEL, 434 TARGET STRENGTH, 439 RADIATED NOISE LEVELS, 440 SELF NOISE LEVELS, 443 Sources of Self Noise, 443 Flow Noise, 446 DETECTION THRESHOLD, 447 UNDERWATER ACOUSTIC APPLICATIONS, 449 Seismic Exploration, 449 Analysis of Seismic Reflection Data, 450 Acoustic Position Reference System for Offshore Dynamic Positioning, 452 Short Baseline System, 455 Acoustic Depth Sounders, 457 Side-Scan Sonar, 459 Subbottom Profiling, 465 Acoustic Positioning and Navigation, 466 Acoustic Doppler Measurements, 467 REFERENCES, 468


Early HistoryIn the 15th century, Leonardo da Vinci stated that if a ship is stopped and a person places one end of a long tube in the water and places the other end to the ear, then the person can hear other ships at a great distance. Placing a second tube similarly to the other ear provided the opportunity to estimate the direction to the ship. The first mathematical treatment of sound theory was completed by Newton in 1687, and it related the propagation of sound in fluids to physical properties of density and elasticity. Bernoulli, Euler, LaGrange, d'Alembert, and Fourier all contributed to the theory of sound during the 18th and 19th centuries. In 1827, Colladon and Sturm measured the speed of sound in water using a light flash coupled with the sounding of an underwater bell to obtain 4707 ft/s at 8~ Rayleigh



published his famous "Theory of Sound" in 1877, and it is republished as Strutt (1945). The magnetostrictive and piezoelectric effects, discovered respectively by Joule in 1840 and Curie in 1880, are used to produce transducers that generate and receive underwater sound. Fessenden developed the first high powered underwater sound source in 1912 and later developed the first commercial application for an underwater acoustic device in which a foghorn and underwater bell were used to determine distance from shore. He also designed a moving coil transducer, called the Fessenden oscillator, for echo ranging. During World War I (1914-18), a system of underwater echo-ranging was developed under the acronym ASDIC (Allied Submarine Devices Investigation Committee). The principle of echo-ranging was that a pulse of sound was transmitted into the water, and any reflection (echo) from a submarine was received by a hydrophone. The received signal was heard on headphones, and the time delay between transmission and reception was used as a measure of the range of the submarine. If sound transmission could be made directional, then target directions could be determined. In the United States, Hayes pioneered the field of passive sonar arrays at the New London Experiment Station, currently the New London Division of the Naval Underwater Warfare Center (NUWC), in New London, Connecticut. In the United States, the term ASDIC was replaced by SONAR, which is an acronym for Sound Navigation and Ranging. Langevin used the piezoelectric effect in underwater sound equipment to detect submarine echoes at distances as great as 1500 m. In 1925, the Submarine Signal Company coined the word fathometer, a trademark of the Raytheon Company, that was an acoustic device used by ships in the US and Great Britain for depth sounding. Adequate sonar systems were developed and produced in US in 1935, and US ships were equipped with underwater listening and echo ranging equipment. In 1937, Spilhaus invented the bathythermograph that measures the temperature versus depth of water. This device was installed on all submarines to measure the temperature profile of the ocean to assist in the determination of characteristics of sound propagation and sonar detection. Surface vessels were equipped in 1938 with echo ranging equipment and operators searched in bearing with headphones and a loudspeaker. Submarines were equipped with line hydrophone arrays with headphones for listening. After WWlI, underwater sound applications greatly expanded. Signal processing developments have been greatly advanced and target noise has been greatly reduced. Some underwater acoustic applications since then are: 9 Fishing aids (locating commercial fish). 9 Ocean engineering/oceanography (telemetry of data, acoustic Doppler current meter, acoustic release mechanism, vertical echo sounders, dynamic positioning systems). 9 Geophysical research (oil exploration).

Underwater Acoustics


9 Underwater communications (surface ships, submarines, divers, remotely operated vehicles). 9 Navigation (depth sounders, beacons, transponders, acoustic speedometers, upward-looking depth sounders for navigating under ice). 9 Underwater search and hydrographic surveying (side scan sonar, subbottom profilers, depth sounders). 9 Coastal processes and Dredging (sediment thickness and characteristics, acoustic flow meters). 9 Acoustic tracking ranges (surface ships, submarines, torpedoes). Sonar systems that occur in nature are found in porpoises, whales, and bats for navigation. Porpoises and bats also use their natural sonar capability for search, detection, and localization of food sources. A valuable source of reference texts have been developed [ 1, 5, 7, 8, 13, 16, 17, 19, 24, 29, 37, 38, 40--43, 50].

Underwater Acoustic System CategoriesUnderwater acoustic systems can be divided into four categories such as active sonar systems, seismic systems, underwater communications and navigation systems, and passive systems. Examples of these systems are 1. Active Sonar Systems 9 Active echo ranging sonar is used by ships, submarines, helicopter, fixed underwater installations, and sonobuoys to locate submarine targets. These sonars use a short pulse of sound that is transmitted into the water by a sound projector (transducer). For reception, the same transducer or a second transducer is used as a hydrophone to receive the returning sound signal (echo). Fixed transducer arrays such as line, conformal, cylindrical, and spherical are used as well as towed line arrays. 9 Torpedoes use moderately high frequencies to echo range on targets and then steer on reflected signals. 9 Depth sounders send short pulses downward and time the bottom return. 9 Side-scan sonars are used for mapping ocean terrain at fight angles to a ship' s track. 9 Subbottom profilers are used for showing features of the ocean subbottom directly beneath the ship's track. Frequently, the transducers for side-scan and subbottom sonars are contained together in a h y d r o d y n a m i c a l l y designed tow body to collect seafloor and subbottom information simultaneously. 9 Fish finding aids are forward or side looking active sonars for spotting fish schools. These sonars may also use multiple beams. 9 Diver hand held sonars are for diver location of underwater objects.



9 Position marking beacons transmit sound signals repeatedly. 9 Position marking transponders transmit sound only when interrogated. 9 Acoustic flow meters and wave height sensors are used to measure flow rate and wave height, respectively. Acoustic Doppler current profilers (ADCP) measure ocean currents. 9 Fluid levels in tanks are frequently measured using active sonar systems. 9 Sonobuoy is a floating buoy that is equipped to send and receive acoustic signals. It is used as the link between an aircraft and underwater explosive source when used to track submarines. 2. Seismic Systems 9 Seismic profilers are used to explore the ocean subbottom, or sub-floor. The acoustic pulses used are basically unidirectional pressure pulses that are generated by explosive charges, underwater arcs (sparkers), compressed air release (air guns), and electromagnetic devices (thumpers). These seismic devices produce results that show the geological features of the ocean floor. 3. Underwater Communications and Telemetry Systems and Navigation 9 Underwater telephone is a device used to communicate between a surface ship and a submarine or between two submarines. 9 Diver communications use a full face mask that allows the diver to speak normally underwater and a throat microphone to acquire speech signals. A transducer is used to transmit the signal. The same transducer is used to receive, and the signal is passed to the diver via an ear piece. 9 Telemetry systems transmit data from a submerged instrument to the surface. 9 Doppler navigation uses a pair of transducers pointing obliquely downward to obtain speed over the bottom from the Doppler shift of the bottom returns. A pulsed Doppler system uses only a single transducer. 9 Acoustic tracking ranges are used for tracking submarines, surface ships, and torpedoes. 4. Passive Systems 9 Passive sonars use a hydrophone array that detects acoustic radiation from another vessel or object (e.g., line, conformal, cylindrica


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