- Underwater acoustics
The field of underwater acoustics is closely related to a number of other fields of acoustic study, including
sonar, transduction, acoustic signal processing, acoustical oceanography, bioacoustics, and physical acoustics.
Underwater sound has probably been used by marine animals for millions of years. The science of underwater acoustics began in 1490, when
Leonardo Da Vinciwrote,Urick, Robert J. "Principles of Underwater Sound, 3rd Edition." New York. McGraw-Hill, 1983. ] :"If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you."In 1687 Isaac Newton wrote his Mathematical Principles of Natural Philosophy which included the first mathematical treatment of sound. The next major step in the development of underwater acoustics was made by Daniel Colladon, a Swiss physicist, and Charles Sturm, a French mathematician. In 1826, on Lake Geneva, they measured the elapsed time between a flash of light and the sound of a submerged ship's bell heard using an underwater listening horn.C. S. Clay & H. Medwin, Acoustical Oceanography (Wiley, New York, 1977) They measured a sound speed of 1435 meters per second over a 17 kilometer distance, providing the first quantitative measurement of sound speed in water.Annales de Chimie et de la Physique 36  236 (1827) The result they obtained was within about 2 % of currently accepted values. In 1877 Lord Rayleigh wrote the Theory of Sound and established modern acoustic theory.
The sinking of "Titanic" in 1912 and the start of
World War Iprovided the impetus for the next wave of progress in underwater acoustics. Anti-submarine listening systems were developed. Between 1912 and 1914, a number of echolocation patents were granted in Europe and the U.S., culminating in Reginald A. Fessenden's echo-ranger in 1914. Pioneering work was carried out during this time in France by Paul Langevinand in Britain by A B Wood and associates. A. B. Wood, "From the Board of Invention and Research to the Royal Naval Scientific Service", Journal of the Royal Naval Scientific Service Vol 20, No 4, pp 1-100 (185-284). The development of both active ASDICand passive sonar(SOund Navigation And Ranging) proceeded apace during the war, driven by the first large scale deployments of submarines. Other advances in underwater acoustics included the development of acoustic mines.
In 1919, the first scientific paper on underwater acoustics was published [ cite journal|author= H. Lichte| date = 1919 | title = On the influence of horizontal temperature layers in sea water on the range of underwater sound signals | journal =Physik. Z. | volume = 17 | number = 385] , theoretically describing the refraction of sound rays produced by temperature and salinity gradients in the ocean. The range predictions of the paper were experimentally validated by
The next two decades saw the development of several applications of underwater acoustics. The
fathometer, or depth sounder, was developed commercially during the 1920s. Originally natural materials were used for the transducers, but by the 1930s sonar systems incorporating piezoelectric transducersmade from synthetic materials were being used for passive listening systems and for active echo-ranging systems. These systems were used to good effect during World War IIby both submarines and anti-submarine vessels. Many advances in underwater acoustics were made which were summarised later in the series Physics of Sound in the Sea, published in 1946.
After World War II, the development of sonar systems was driven largely by the
Cold War, resulting in advances in the theoretical and practical understanding of underwater acoustics, aided by computer-based techniques.
ound waves in water
A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by a receiver, such as the human
earor a hydrophone, as changes in pressure. These waves may be man-made or naturally generated.
peed of sound, density and impedance
speed of sound(i.e., the longitudinal motion of wavefronts) is related to frequency and wavelength of a wave by .
This is different from the particle velocity , which refers to the motion of molecules in the medium due to the sound, and relates the plane wave the pressure to the fluid density and sound speed by .
The term in brackets is known as the characteristic acoustic impedance. The acoustic power (energy per second) crossing unit area is known as the intensity of the wave and for a plane wave the average intensity is given by , where is the
root mean squareacoustic pressure.
At 1 kHz, the wavelength is about 1.5 m. Sometimes the term "sound velocity" is used but this is incorrect as the quantity is a scalar.
The large impedance contrast between air and water (the ratio is about 3600) and the scale of surface roughness means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1 kHz. Sound speed in water exceeds that in air by a factor of 4.4 and the density ratio is about 820.
Absorption of sound
Absorption of low frequency sound is weak.R. E. Francois & G. R. Garrison, Sound absorption based on ocean measurements. Part II: Boric acid contribution and equation for total absorption, J. Acoust. Soc. Am. 72, 1879-1890 (1982). M. A. Ainslie and J. G. McColm (1998), A simplified formula for viscous and chemical absorption in sea water, J. Acoust. Soc. Am. 103, 1671-1672 (1998). (see [http://www.npl.co.uk/acoustics/techguides/seaabsorption/ Technical Guides - Calculation of absorption of sound in seawater] for an on-line calculator). The main cause of sound attenuation in fresh water, and at high frequency in sea water (above 100 kHz) is
viscosity. Important additional contributions at lower frequency in seawater are associated with the ionic relaxation of boric acid(up to c. 10 kHz) and magnesium sulfate(up to c. 100 kHz)R. E. Francois and G. R. Garrison, Sound absorption based on ocean measurements. Part I: Pure water and magnesium sulfate contributions, J. Acoust. Soc. Am. 72, 896-907 (1982). .
Sound may be absorbed by losses at the fluid boundaries. Near the surface of the sea losses can occur in a bubble layer or in ice, while at the bottom sound can penetrate into the sediment and be absorbed.
ound Reflection and Scattering
Both the water surface and bottom are reflecting and scattering boundaries.
For many purposes the sea-air surface can be thought of as a perfect reflector. The impedance contrast is so great that little energy is able to cross this boundary. Acoustic pressure waves reflected from the sea surface experience a reversal in phase, often stated as either a “pi phase change” or a “180 deg phase change”. This is represented mathematically by assigning a reflection coefficient of minus 1 instead of plus one to the sea surface.
At high frequency (above about 1 kHz) or when the sea is rough, some of the incident sound is scattered, and this is taken into account by assigning a reflection coefficient whose magnitude is less than one. For example, close to normal incidence, the reflection coefficient becomes , where "h" is the rms wave height. [H. Medwin & C. S. Clay, Fundamentals of Acoustical Oceanography (Academic, Boston, 1998).]
A further complication is the presence of wind generated bubbles or fish close to the sea surface. [D. E. Weston & P. A. Ching, Wind effects in shallow-water transmission, J. Acoust. Soc. Am. 86, 1530-1545 (1989).] . The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves. [G. V. Norton & J. C. Novarini, On the relative role of sea-surface roughness and bubble plumes in shallow-water propagation in the low-kilohertz region, J. Acoust. Soc. Am. 110, 2946-2955 (2001)] .
The acoustic impedance mismatch between water and the bottom is generally much less than at the surface and is more complex. It depends on the bottom material types and depth of the layers. Theories have been developed for predicting the sound propagation in the bottom in this case, for example by Biot [N Chotiros, Biot Model of Sound Propagation in Water Saturated Sand. J. Acoust. Soc. Am. 97, 199 (1995) ] and by Buckingham. [M. J. Buckingham, Wave propagation, stress relaxation, and grain-to-grain shearing in saturated, unconsolidated marine sediments, J. Acoust. Soc. Am. 108, 2796-2815 (2000).]
The reflection of sound at a target whose dimensions are large compared with the acoustic wavelength depends on its size and shape as well as the impedance of the target relative to that of water. Formulae have been developed for the
target strengthof various simple shapes as a function of angle of sound incidence. More complex shapes may be approximated by combining these simple ones.
Propagation of sound
Underwater acoustic propagation depends on many factors. The direction of sound propagation is determined by the sound speed gradients in the water. In the sea the vertical gradients are generally much larger than the horizontal ones. These facts, combined with a tendency for increasing sound speed with increasing depth due to the increasing pressure in the
deep seareverses the sound speed gradientin the thermoclinecreating an efficient waveguide at the depth corresponding to the minimum sound speed. The sound speed profile may cause regions of low sound intensity called "Shadow Zones" and regions of high intensity called "Caustics". These may be found by ray tracing methods.
equatorial and temperate latitudesin the ocean the surface temperature is high enough to reverse the pressure effect, such that a sound speed minimum occurs at depth of a few hundred metres. The presence of this minimum creates a special channel known as Deep Sound Channel, previously known as the SOFAR(sound fixing and ranging) channel, permitting guided propagation of underwater sound for thousands of kilometreswithout interaction with the sea surface or the seabed. Another phenomenon in the deep sea is the formation of sound focussing areas known as Convergence Zones. In this case sound is refracted downward from a near-surface source and then back up again. The horizontal distance from the source at which this occurs depends on the positive and negative sound speed gradients. A surface duct can also occur in both deep and moderately shallow water when there is upward refraction, for example due to cold surface temperatures. Propagation is by repeated sound bounces off the surface.
In general, as sound propagates underwater there is a reduction in the sound intensity over increasing ranges, though in some circumstances a gain can be obtained due to focussing. "Propagation loss" (sometimes referred to as "transmission loss") is a quantitative measure of the reduction in sound intensity between two points, normally the sound source and a distant receiver. If is the far field intensity of the source referred to a point 1 m from its acoustic centre and is the intensity at the receiver, then the propagation loss is given by .In this equation is not the true
acoustic intensityat the receiver, which is a vector quantity, but a scalar equal to the equivalent plane wave intensity (EPWI) of the sound field. The EPWI is defined as the magnitude of the intensity of a plane wave of the same RMS pressure as the true acoustic field. At short range the propagation loss is dominated by spreading while at long range it is dominated by absorption and/or scattering losses.
An alternative definition is possible in terms of pressure instead of intensity, giving , where is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and is the RMS pressure at the receiver position.
These two definitions are not exactly equivalent because the characteristic impedance at the receiver may be different from that at the source. Because of this, the use of the intensity definition leads to a different sonar equation to the definition based on a pressure ratio.M. A. Ainslie, The sonar equation and the definitions of propagation loss, J. Acoust. Soc. Am. 115, 131-134 (2004). If the source and receiver are both in water, the difference is small.
The propagation of sound through water is described by the wave equation, with appropriate boundary conditions. A number of models have been developed to simplify propagation calculations. These models include ray theory, normal mode solutions, and parabolic equation simplifications of the wave equation. [F. B. Jensen, W. A. Kuperman, M. B. Porter & H. Schmidt, Computational Ocean Acoustics (AIP Press, NY, 1994).] Each set of solutions is generally valid and computationally efficient in a limited frequency and range regime, and may involve other limits as well. Ray theory is more appropriate at short range and high frequency, while the other solutions function better at long range and low frequency. [C. H. Harrison, Ocean propagation models, Applied Acoustics 27, 163-201 (1989).] Various empirical and analytical formulae have also been derived from measurements that are useful approximations. [L. M. Brekhovskikh & Yu. P. Lysanov, Fundamentals of Ocean Acoustics, 3rd edition (Springer-Verlag, NY, 2003).]
Transient sounds result in a decaying background that can be of much larger duration than the original transient signal. The cause of this background, known as reverberation, is partly due to scattering from rough boundaries and partly due to scattering from
fishand other biota. For an acoustic signal to be detected easily, it must exceed the reverberation levelas well as the background noise level.
If an underwater object is moving relative to an underwater receiver, the frequency of the received sound is different from that of the sound radiated (or reflected) by the object. This change in frequency is known as a
Doppler shift. The shift can be observed in active sonarsystems, particularly narrowband ones, because the transmitter frequency is known, and the relative motion between sonar and object can be calculated. Sometimes the frequency of the radiated noise (a tonal) may also be known, in which case the same calculation can be done for passive sonar. For active systems the change in frequency is 0.69 Hz per knot per kHz and half this for passive systems as propagation is only one way. The shift corresponds to an increase in frequency for an approaching target.
Though acoustic propagation modelling generally predicts a constant received sound level, in practice there are both temporal and spatial fluctuations. These may be due to both small and large scale environmental phenomena. These can include sound speed profile fine structure and frontal zones as well as internal waves. Because in general there are multiple propagation paths between a source and receiver, small phase changes in the interference pattern between these paths can lead to large fluctuations in sound intensity.
In water, especially with air bubbles, the change in density due to a change in pressure is not exactly linearly proportional. As a consequence for a sinusoidal wave input additional harmonic and subharmonic frequencies are generated. When two sinusoids are input sum and difference frequencies are generated. The conversion process is greater at high source levels than small ones. Because of the non-linearity there is a dependence of sound speed on the pressure amplitude so that large changes travel faster than small ones. Thus a sinusoidal waveform gradually becomes a sawtooth one with a steep rise and a gradual tail. Use is made of this phenomenon in parametric sonar and theories have been developed to account for this, eg by Westerfield.
Sound in water is measured using a
hydrophone, which is the underwater equivalent of a microphone. A hydrophone measures pressurefluctuations, and these are usually converted to sound pressure level(SPL), which is a logarithmic measure of the mean square acoustic pressure.
Measurements are usually reported in one of three forms :-
* RMS acoustic pressure in micropascals (or dB re 1 μPa)
* RMS acoustic pressure in a specified bandwidth, usually
octavesor thirds of octave (dB re 1 μPa)
spectral density(mean square pressure per unit bandwidth) in micropascals per hertz(dB re 1 μPa²/Hz)
Approximate values for
fresh waterand seawater, respectively, at atmospheric pressure are 1450 and 1500 m/s for the sound speed, and 1000 and 1030 kg/m³ for the density.A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989). The speed of sound in water increases with increasing pressure, temperatureand salinity.Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. 70, 807-812 (1982). C. C. Leroy, The speed of sound in pure and neptunian water, in Handbook of Elastic Properties of Solids, Liquids and Gases, edited by Levy, Bass & Stern, Volume IV: Elastic Properties of Fluids: Liquids and Gases (Academic Press, 2001) On-line calculators can be found at [http://www.npl.co.uk/acoustics/techguides/soundseawater/ Technical Guides - Speed of Sound in Sea-Water] and [http://www.npl.co.uk/acoustics/techguides/soundpurewater/ Technical Guides - Speed of Sound in Pure Water] .
Many measurements have been made of sound absorption in lakes and the ocean R. E. Francois and G. R. Garrison, Sound absorption based on ocean measurements. Part I: Pure water and magnesium sulfate contributions, J. Acoust. Soc. Am. 72, 896-907 (1982). (see [http://www.npl.co.uk/acoustics/techguides/seaabsorption/ Technical Guides - Calculation of absorption of sound in seawater] for an on-line calculator).
Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by the
signal processingused and partly by the level of background noise. Ambient noise is that part of the received noise that is independent of the source, receiver and platform characteristics. This it excludes reverberation and towing noise for example.
The background noise present in the ocean, or ambient noise, has many different sources and varies with location and frequency.G. M. Wenz, Acoustic ambient noise in the ocean: spectra and sources, J. Acoust. Soc. Am. 34, 1936-1956 (1962). At the lowest frequencies, from about 0.1 Hz to 10 Hz, ocean
turbulenceand microseisms are the primary contributors to the noise background.S. C. Webb, The equilibrium oceanic microseism spectrum, J. Acoust. Soc. Am. 92, 2141-2158 (1992). Typical noise spectrum levels decrease with increasing frequency from about 140 dB re 1 μPa²/Hz at 1 Hz to about 30 dB re 1 μPa²/Hz at 100 kHz. Distant ship traffic is one of the dominant noise sources in most areas for frequencies of around 100 Hz, while wind-induced surface noise is the main source between 1 kHz and 30 kHz. At very high frequencies, above 100 kHz, thermal noiseof water molecules begins to dominate. The thermal noise spectral level at 100 kHz is 25 dB re 1 μPa²/Hz. The spectral density of thermal noise increases by 20 dB per decade (approximately 6 dB per octave).
Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoesR. S. Dietz and M. J. Sheehy, Transpacific detection of myojin volcanic explosions by underwater sound. Bulletin of the Geological Society 2 942-956 (1954)., rainfall on the surface, and biological activity. Biological sources include
cetaceans (especially blue, fin and sperm whales),M. A. McDonald, J. A. Hildebrand & S. M. Wiggins, Increases in deep ocean ambient noise in the Northeast Pacific west of San Nicolas Island, California, J. Acoust. Soc. Am. 120, 711-718 (2006).Ocean Noise and Marine Mammals, National Research Council of the National Academies (The National Academies Press, Washington DC, 2003). certain types of fish, and snapping shrimp.
Many measurements have been made of sea surface, bottom and volume reverberation. Empirical models have sometimes been derived from these. A commonly used expression for the band 0.4 to 6.4 kHz is that by Chapman and Harris. [R Chapman and J Harris, Surface backscattering Strengths Measured with Explosive Sound Sources. J. Acoust. Soc. Am. 34, 547 (1962)] It is found that a sinusoidal waveform is spread in frequency due to the surface motion. For bottom reverberation a Lambert's Law is found often to apply approximately, for example see Mackenzie [K Mackenzie, Bottom Reverberation for 530 and 1030 cps Sound in Deep Water. J. Acoust. Soc. Am. 36, 1596 (1964)] . Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman [J. R. Marshall and R. P. Chapman, Reverberation from a Deep Scattering Layer Measured with Explosive Sound Sources. J. Acoust. Soc. Am. 36, 164 (1964)] . The under-surface of ice can produce strong reverberation when it is rough, see for example Milne [ A. Milne, Underwater Backscattering Strengths of Arctic Pack Ice. J. Acoust. Soc. Am. 36, 1551 (1964) ] .
Bottom loss has been measured as a function of grazing angle for many frequencies in various locations, for example those by the US Marine Geophysical Survey [ MGS Station Data Listing and Report Catalog, Nav Oceanog Office Special Publication 142, 1974 ] . The loss depends on the sound speed in the bottom (which is affected by gradients and layering) and by roughness. Graphs have been produced for the loss to be expected in particular circumstances.
Comparison with airborne sound levels
airborne sound, sound pressure level underwater is usually reported in units of decibels, but there are some important differences that make it difficult (and often inappropriate) to compare SPL in water with SPL in air. These differences include:D.M.F. Chapman, D.D. Ellis, The elusive decibel - thoughts on sonars and marine mammals, Can. Acoust. 26(2), 29–31 (1996)
* difference in reference pressure: 1 μPa (one micropascal, or one
millionthof a pascal) instead of 20 μPa.C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).
* difference in interpretation: there are two schools of thought, one maintaining that pressures should be compared directly, and that the other that one should first convert to the
intensityof an equivalent plane wave.
* difference in hearing sensitivity: any comparison with (
A-weighted) sound in air needs to take into account the differences in hearing sensitivity, either of a human diver or other animal.W. J. Richardson, C. R. Greene, C. I. Malme and D. H. Thomson, Marine Mammals and Noise (Academic Press, San Diego, 1995).
The lowest audible
SPLfor a human diver with normal hearing is about 67 dB re 1 μPa, with greatest sensitivity occurring at frequencies around 1 kHz S. J. Parvin, E. A. Cudahy & D. M. Fothergill, Guidance for diver exposure to underwater sound in the frequency range 500 to 2500 Hz, Underwater Defence Technology (2002).. Dolphinsand other toothed whalesare renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).. Several species have hearing thresholds between 30 and 50 dB re 1 μPa in this frequency range. For example the hearing thresholdof the killer whaleoccurs at an RMS acoustic pressureof 0.02 mPa (and frequency 15 kHz), corresponding to an SPL threshold of 26 dB re 1 μPa.D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005) By comparison the most sensitive fish is the soldier fish, whose threshold is 0.32 mPa (50 dB re 1 μPa) at 1.3 kHz, whereas the lobsterhas a hearing threshold of 1.3 Pa at 70 Hz (122 dB re 1 μPa).
High levels of underwater sound create a potential hazard to marine and amphibious animals as well as to human divers . Guidelines for exposure of human divers and marine mammals to underwater sound are reported by the SOLMAR project of the
NATO Undersea Research Centre[http://solmar.nurc.nato.int/solmar/PDF/NURC-SP-2006-008.pdf NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures] , NURC Special Publication NURC-SP-2006-008, September 2006. Human divers exposed to SPL above 154 dB re 1 μPa in the frequency range 0.6 to 2.5 kHz are reported to experience changes in their heart rate or breathing frequency.
Applications of underwater acoustics
Sonar is the name given to the acoustic equivalent of
radar. Pulses of sound are used to probe the sea, and the echoes are then processed to extract information about the sea, its boundaries and submerged objects. An alternative use, known as "passive sonar", attempts to do the same by listening to the sounds radiated by underwater objects.
The need for underwater acoustic
telemetryexists in applications such as data harvesting for environmental monitoring, communication with and between manned and unmanned underwater vehicles, transmission of diver speech, etc. Acoustic communications form an active field of research D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4-27 (2000).M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003. with significant challenges to overcome, especially in horizontal, shallow-water channels. Compared with radio telecommunications, the available bandwidth is reduced by several orders of magnitude. Moreover, the low speed of sound causes multipath propagation to stretch over time delay intervals of tens or hundreds of milliseconds, as well as significant Doppler shifts and spreading. Often acoustic communication systems are not limited by noise, but by reverberation and time variability beyond the capability of receiver algorithms. The fidelity of underwater communication links can be greatly improved by the use of hydrophone arrays, which allow processing techniques such as adaptive beamformingand diversity combining.
Seismic exploration involves the use of low frequency sound (< 100 Hz) to probe deep into the seabed. Despite the relatively poor resolution due to their long wavelength, low frequency sounds are preferred because high frequencies are heavily attenuated when they travel through the seabed. Sound sources used include airguns,
Weather and climate observation
Acoustic sensors can be used to monitor the sound made by
windand precipitation. For example, an acoustic rain gaugeis described by Nystuen.J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, 18(10), 1640-1657 (2001). Lightning strikes can also be detected. R. D. Hill, Investigation of lightning strikes to water surfaces, J. Acoust. Soc. Am. 78, 2096-2099 (1985).Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature.
Large scale ocean features can be detected by acoustic tomography. Bottom characteristics can be measured by
side-scan sonarand sub-bottom profiling.
Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from microplankton to the
neutrinois a fundamental particle that interacts very weakly with other matter. For this reason, it requires detection apparatus on a very large scale, and the ocean is sometimes used for this purpose. In particular, it is thought that ultra-high energy neutrinos in seawater can be detected acoustically. [S. Bevan, S. Danaher, J. Perkin, S. Ralph, C. Rhodes, L. Thompson, T. Sloane, D. Waters and The ACoRNE Collaboration, Simulation of ultra high energy neutrino induced showers in ice and water, Astroparticle PhysicsVolume 28, Issue 3, November 2007, Pages 366-379]
Ocean Tracking Network
* [http://oalib.hlsresearch.com/ Ocean Acoustics Library]
* [http://www.isvr.soton.ac.uk/FDAG/uaua.HTM Ultrasonics and Underwater Acoustics]
* [http://www.pmel.noaa.gov/vents/acoustics.html Monitoring the global ocean through underwater acoustics]
* [http://sonar-fs.lboro.ac.uk/newsite/index.php Underwater Acoustics Research]
* [http://www.apl.washington.edu/projects/ASA-UATC/index.php ASA Underwater Acoustics Technical Committee]
* [http://oceanlink.island.net/oinfo/acoustics/acoustics.html An Ocean of Sound]
* [http://www.arl.nus.edu.sg/web/research/acomms Underwater Acoustic Communications]
* [http://acomms.whoi.edu/ Acoustic Communications Group] at the
Woods Hole Oceanographic Institution
* [http://www.oceanexplorer.noaa.gov/explorations/sound01/sound01.html Sound in the Sea]
* [http://www.physics.sfsu.edu/~bland/research/uarg.html SFSU Underwater Acoustics Research Group]
* [http://www.dosits.org/ Discovery of Sound in the Sea]
* [http://www.km.kongsberg.com/KS/WEB/NOKBG0240.nsf/AllWeb/FF57C18363FAD917C1256A7E002B9F2F?OpenDocument Acoustic underwater positioning and navigation systems]
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