Underwater acoustics is the study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water and its boundaries. The field of underwater acoustics is closely related to a number of other fields of acoustic study, including bioacoustics, acoustic signal processing, transduction, acoustical oceanography and physical acoustics. Sound is a disturbance of mechanical energy that propagates through matter as a longitudinal wave, and therefore is a mechanical wave. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... Bioacoustics is a cross-disciplinary science, which investigates sound production and reception in animals, including man, the biological acoustically-borne information transfer and its propagation in elastic media. ... A transducer is a device, usually electrical or electronic, that converts one type of energy to another. ...

History

The study of the propagation of sound underwater began in 1490. Leonardo Da Vinci wrote,^[1] The Mona Lisa Leonardo di ser Piero da Vinci (April 15, 1452 – May 2, 1519) was an Italian polymath: scientist, mathematician, engineer, inventor, anatomist, painter, sculptor, architect, musician, and writer. ...

"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."

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 1827, 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. 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.^[2] The result they obtained was within 5% of currently accepted values. Jean-Daniel Colladon (born on 15 December 1802 in Geneva and died on 30 June 1893), was a Swiss physicist. ... Articles with similar titles include physician, a person who practices medicine. ... Jacques Charles François Sturm (September 29, 1803 - December 18, French German extraction, was born at Geneva. ... Leonhard Euler, one of the greatest mathematicians of all time A mathematician is a person whose primary area of study and research is the field of mathematics. ... Year 1827 (MDCCCXXVII) was a common year starting on Monday (link will display the full calendar) of the Gregorian Calendar (or a common year starting on Wednesday of the 12-day slower Julian calendar). ... Lake Geneva or Lake Léman (French Lac Léman, le Léman, or Lac de Genève) is the second largest freshwater lake in Central Europe (after Lake Balaton). ...

The sinking of the Titanic and the start of World War I provided the impetus for the next wave of progress in underwater acoustics. 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 Langevin and in Britain by A B Wood. ^[3] The development of both active and 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. For other uses, see Titanic. ... “The Great War ” redirects here. ... 1912 (MCMXII) was a leap year starting on Monday in the Gregorian calendar (or a leap year starting on Tuesday in the 13-day-slower Julian calendar). ... 1914 (MCMXIV) was a common year starting on Thursday (see link for calendar). ... The F70 type frigates (here, La Motte-Picquet) are fitted with VDS (Variable Depth Sonar) type DUBV43 or DUBV43C towed sonars SONAR (SOund Navigation And Ranging) â€” or sonar â€” is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. ... Albert Einstein, Paul Ehrenfest, Paul Langevin, Heike Kamerlingh Onnes, and Pierre Weiss at Ehrenfests home in Leiden Paul Langevin (January 23, 1872 â€“ December 19, 1946) was a prominent French physicist who developed Langevin dynamics and the Langevin equation. ... The F70 type frigates (here, La Motte-Picquet) are fitted with VDS (Variable Depth Sonar) type DUBV43 or DUBV43C towed sonars SONAR (SOund Navigation And Ranging) â€” or sonar â€” is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. ... USS Los Angeles A submarine is a specialized watercraft that can operate underwater. ... Polish wz. ...

In 1919, the first scientific paper on underwater acoustics was published ^[4], 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 transmission loss measurements.

The next two decades saw the development of several applications of underwater acoustics. The fathometer, or depth sounder, was developed commercially during the 1920s. By the 1930s, sonar systems incorporating piezoelectric transducers made from synthetics were being used for passive listening systems and for active echo-ranging systems. These systems were used to good effect against German U-boats during World War II. Cabin display of a commercial or oceanographic fathometer sonar A fishfinder is a type of Fathometer, both being specialized types of echo sounding systems, a type of Active SONAR. (Sounding is the measurement of water depth, a historical nautical term of very long usage. ... Piezoelectricity is the ability of certain crystals to produce a voltage when subjected to mechanical stress. ... A transducer is a device that converts one type of energy to another, or responds to a physical parameter. ... Combatants Allied powers: China France Great Britain Soviet Union United States and others Axis powers: Germany Italy Japan and others Commanders Chiang Kai-shek Charles de Gaulle Winston Churchill Joseph Stalin Franklin Roosevelt Adolf Hitler Benito Mussolini Hideki Tōjō Casualties Military dead: 17,000,000 Civilian dead: 33,000...

Theory

Underwater sound

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 ear or a hydrophone, as changes in pressure. A pierced human ear. ... A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ...

The speed of sound in water exceeds that in air by a factor of 4.4 and the density ratio is about 820. Approximate values for fresh water and seawater, respectively, at atmospheric pressure are 1450 and 1500 m/s for the sound speed, and 1000 and 1030 kg/m³ for the density.^[5] The speed of sound in water increases with increasing pressure, temperature and salinity.^{[6] [7]} An on-line calculator can be found at Technical Guides - Speed of Sound in Sea-Water. The speed of sound is a term used to describe the velocity of sound waves passing through an elastic medium. ... For the village on the Isle of Wight, see Freshwater, Isle of Wight. ... Annual mean sea surface salinity for the World Ocean. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ... Fig. ... Annual mean sea surface salinity for the World Ocean. ...

The large impedance contrast between air and water (the ratio is about 3600) means that the sea surface behaves as an almost perfect reflector of sound at frequencies below 1 kHz. Absorption of low frequency sound is also very weak.^{[8] [9] [10]} (see Technical Guides - Calculation of absorption of sound in seawater for an on-line calculator). These facts, combined with a tendency for increasing sound speed with increasing depth due to the increasing pressure in the deep sea reverses the sound speed gradient in the thermocline creating an efficient waveguide at the depth corresponding to the minimum sound speed. At equatorial and temperate latitudes 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 the SOFAR (sound fixing and ranging) channel, permitting guided propagation of underwater sound for thousands of kilometres without interaction with the sea surface or the seabed. The term deep sea refers to those areas of oceans to which little or no light penetrates (the aphotic zone). ... In acoustics, the sound speed gradient is the rate of change of the speed of sound with depth in the ocean,[1] or height in the Earths atmosphere. ... The thermocline is a layer within a body of water where the temperature changes rapidly with depth. ... This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... In geography, temperate latitudes of the globe lie between the tropics and the polar circles. ... Latitude, denoted φ, gives the location of a place on Earth north or south of the Equator. ... A kilometre (American spelling: kilometer) (symbol: km) is a unit of length equal to 1000 metres (from the Greek words khilia = thousand and metro = count/measure). ...

Measurements

Sound in water is measured using a hydrophone, which is the underwater equivalent of a microphone. A hydrophone measures pressure fluctuations, and these are usually converted to sound pressure level (SPL), which is a logarithmic measure of the mean square acoustic pressure. As with airborne sound, SPL 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:^[11] A hydrophone is a sound-to-electricity transducer for use in water or other liquids, analogous to a microphone for air. ... A microphone, sometimes referred to as a mike or mic (both IPA pronunciation: ), is an acoustic to electric transducer or sensor that converts sound into an electrical signal. ... The use of water pressure - the Captain Cook Memorial Jet in Lake Burley Griffin in Canberra, Australia. ... It has been suggested that this article or section be merged into Sound pressure. ... Sound pressure p (or acoustic pressure) is the measurement in pascals of the Root Mean Square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. ... The decibel is a dimensionless unit (like percent) that is a measure of ratios on a logarithmic scale. ...

• difference in reference pressure: 1 μPa (one micropascal, or one millionth of a pascal) instead of 20 μPa.^[12]
• 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 intensity of 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.^[13]

Measurements are usually reported in one of three forms The pascal (symbol: Pa) is the SI derived unit of pressure or stress (also: Youngs modulus and tensile strength). ... One millionth is equal to 0. ... The pascal (symbol: Pa) is the SI unit of pressure. ... In physics, intensity is a measure of the time-averaged energy flux. ... An audiogram is a graphical representation of how well a certain person can perceive different sound frequencies. ... The A-weighting curve is one of a family of curves defined in IEC179 and various other standards for use in sound level meters. ...

• RMS acoustic pressure in micropascals (or dB re 1 μPa)
• RMS acoustic pressure in a specified bandwidth, usually octaves or thirds of octave (dB re 1 μPa)
• spectral density (mean square pressure per unit bandwidth) in micropascals per hertz (dB re 1 μPa²/Hz)

RMS may refer to any of the following: Railway Mail Service, US mail transportation service until the mid-20th century Royal Mail Ship or Steamer, ship prefix for vessels that carry mail under contract to the British Royal Mail Royal Melbourne Show Royal Meteorological Society Royal Society of Miniature Painters... Sound pressure p (or acoustic pressure) is the measurement in pascals of the Root Mean Square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. ... For the numerical computation software, see GNU Octave. ... In applied mathematics and physics, the spectral density is a general concept applied to a signal which may have any physical dimensions or none at all. ... The hertz (symbol: Hz) is the SI unit of frequency. ...

Ambient noise

Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by the signal processing used and partly by the level of background noise. Signal processing is the processing, amplification and interpretation of signals, and deals with the analysis and manipulation of signals. ...

The background noise present in the ocean, or ambient noise, has many different sources and varies with location and frequency.^[14] At the lowest frequencies, from about 0.1 Hz to 10 Hz, ocean turbulence and microseisms are the primary contributors to the noise background.^[15] 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 noise of 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). In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... Seismology (from the Greek seismos = earthquake and logos = word) is the scientific study of earthquakes and the propagation of elastic waves through the Earth. ... Johnson-Nyquist noise (sometimes thermal noise, Johnson noise or Nyquist noise) is the noise generated by the equilibrium fluctuations of the electric current inside an electrical conductor, which happens without any applied voltage, due to the random thermal motion of the charge carriers (the electrons). ... A decade on a graphical logarithmic scale represents multiplication by 10 from the previous value. ... In music, an octave (sometimes abbreviated 8ve) is the interval between one musical note and another with half or double its frequency. ...

Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes^[16], rainfall on the surface, and biological activity. Biological sources include cetaceans (especially blue, fin and sperm whales),^[17][18] certain types of fish, and snapping shrimp. Suborders Mysticeti Odontoceti Archaeoceti (extinct) (see text for families) The order Cetacea (IPA: , L. cetus, whale) includes whales, dolphins and porpoises. ... Binomial name Balaenoptera musculus (Linnaeus, 1758) Blue Whale range The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales. ... Binomial name Balaenoptera physalus (Linneus, 1758) Fin Whale range The Fin Whale (Balaenoptera physalus), also called the Finback Whale, is a mammal that belongs to the baleen whales suborder. ... Binomial name Physeter macrocephalus Linnaeus, 1758 Sperm Whale range (in blue) The Sperm Whale (Physeter macrocephalus) is the largest of all toothed whales and is the largest toothed animal alive, measuring up to 18 metres (60 ft) long. ... Genera Alpheopsis Alpheus Amphibetaeus Arete Aretopsis Athanas Athanopsis Automate Bannereus Batella Bermudacaris Betaeus Betaeopsis Coronalpheus Coutieralpheus Deioneus Fenneralpheus Hamalpheus Leptalpheus Leslibetaeus Metabetaeus Metalpheus Mohocaris Notalpheus Nennalpheus Orygmalpheus Parabetaeus Pomagnathus Potamalpheops Prionalpheus Pterocaris Racilius Salmoneus Stenalpheops Synalpheus Thuylamea Vexillipar Yagerocaris Alpheidae is a family of caridean snapping shrimp characterized by having...

Reverberation

Transient sounds result in a decaying background that can be of much larger duration than the original transient signal. This background is known as reverberation. For an acoustic signal to be detected easily, it must exceed the reverberation level as well as the background noise level. In telecommunication, noise level is the noise power, usually relative to a reference. ...

Underwater hearing

Hearing sensitivity

The lowest audible SPL for a human diver with normal hearing is about 67 dB re 1 μPa, with greatest sensitivity occurring at frequencies around 1 kHz ^[19]. Dolphins and other toothed whales are renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz ^[20][13]. Several species have hearing thresholds between 30 and 50 dB re 1 μPa in this frequency range. For example the hearing threshold of the killer whale occurs at an RMS acoustic pressure of 0.02 mPa (and frequency 15 kHz), corresponding to an SPL threshold of 26 dB re 1 μPa.^[21] 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 lobster has a hearing threshold of 1.26 Pa at 70 Hz (122 dB re 1 μPa).^[21] SPL is a three-letter abbreviation with multiple meanings, including: Sad Paki Loser Sound pressure level Scottish Premier League Standard PHP Library Sun Public License SugarCRM Public License SPL notation: Sentence Plan Language - a notation used in natural language processing Senior Patrol Leader SPL (computer science) short for Set Priority... This article is about the dolphin mammal. ... Families See text The toothed whales (systematic name Odontoceti) form a suborder of the cetaceans. ... Threshold of hearing is the sound pressure level SPL of 20 µPa (micropascal) = 2 × 10-5 Pascal (Pa). ... Binomial name Orcinus orca Linnaeus, 1758 Orca range (in blue) The orca (Orcinus orca), commonly known as the killer whale, and sometimes called the grampus, is the largest member of the oceanic dolphin family. ... RMS may refer to any of the following: Railway Mail Service, US mail transportation service until the mid-20th century Royal Mail Ship or Steamer, ship prefix for vessels that carry mail under contract to the British Royal Mail Royal Melbourne Show Royal Meteorological Society Royal Society of Miniature Painters... Sound pressure p (or acoustic pressure) is the measurement in pascals of the Root Mean Square (RMS) pressure deviation (from atmospheric pressure) caused by a sound wave passing through a fixed point. ... Subfamilies and Genera Neophoberinae Acanthacaris Thymopinae Nephropsis Nephropides Thymops Thymopsis Nephropinae Homarus Nephrops Homarinus Metanephrops Eunephrops Thymopides Clawed lobsters comprise a family (Nephropidae, sometimes also Homaridae) of large marine crustaceans. ...

Safety thresholds

High levels of underwater sound create a potential hazard to marine and amphibious animals as well as to human divers ^[13]. Guidelines for exposure of human divers and marine mammals to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre^[22]. (Available from [1]). 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

Main article: Sonar

The F70 type frigates (here, La Motte-Picquet) are fitted with VDS (Variable Depth Sonar) type DUBV43 or DUBV43C towed sonars SONAR (SOund Navigation And Ranging) â€” or sonar â€” is a technique that uses sound propagation under water (primarily) to navigate, communicate or to detect other vessels. ...

Underwater communication

The need for underwater acoustic telemetry exists 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 ^[23][24] 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 velocity 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 beamforming and diversity combining. Beamforming is a signal processing technique used to make a collection of fixed simple antennas act like a single, highly focused, movable antenna. ... In telecommunications, a diversity scheme refers to a method for improving the reliability of a message signal by utilizing two or more communication channels with different characteristics. ...

Seismic exploration

Seismic exploration involves the use of low frequency sound to probe deep into the seabed. Sound sources used include airguns, vibroseis and explosives. Vibroseis is a method used in exploration seismology to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by an impulsive source such as explosives or weight-drop trucks. ... This article is concerned solely with chemical explosives. ...

Main article: Reflection seismology

Seismic reflection data Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earths subsurface from reflected seismic waves. ...

Weather observation

Acoustic sensors can be used to monitor the sound made by wind and precipitation. For example, an acoustic rain gauge is desrcibed by Nystuen.^[25] This article or section does not cite any references or sources. ...

Oceanography

Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature. Acoustic Thermometry of Ocean Climate (ATOC) is an idea to observe the state of the worlds oceans, and the ocean climate in particular, using long-range acoustic transmissions. ...

The study of marine life, from microplankton to the blue whale, uses bioacoustics. Photomontage of plankton organisms Plankton are any drifting organism that inhabits the water column of oceans, seas, and bodies of fresh water. ... Binomial name Balaenoptera musculus (Linnaeus, 1758) Blue Whale range The Blue Whale (Balaenoptera musculus) is a marine mammal belonging to the suborder of baleen whales. ... Bioacoustics is a cross-disciplinary science, which investigates sound production and reception in animals, including man, the biological acoustically-borne information transfer and its propagation in elastic media. ...

Particle physics

Acoustic detection of ultra-high energy neutrinos [2] The neutrino is an elementary particle. ...

References

1. ^ Urick, Robert J. Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill, 1983.
2. ^ Annales de Chimie et de la Physique 36 [2] 236 (1827)
3. ^ 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).
4. ^ H. Lichte (1919). "On the influence of horizontal temperature layers in sea water on the range of underwater sound signals". Physik. Z. 17. 
5. ^ A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989).
6. ^ Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. 70, 807-812 (1982).
7. ^ 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)
8. ^ 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).
9. ^ 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).
10. ^ 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).
11. ^ D.M.F. Chapman, D.D. Ellis, The elusive decibel - thoughts on sonars and marine mammals, Can. Acoust, 1998
12. ^ C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).
13. ^ ^{a b c} W. J. Richardson, C. R. Greene, C. I. Malme and D. H. Thomson, Marine Mammals and Noise (Academic Press, San Diego, 1995).
14. ^ G. M. Wenz, Acoustic ambient noise in the ocean: spectra and sources, J. Acoust. Soc. Am. 34, 1936-1956 (1962).
15. ^ S. C. Webb, The equilibrium oceanic microseism spectrum, JASA 92, 2141-2158 (1992).
16. ^ R. S. Dietz and M. J. Sheehy, Transpacific detection of myojin volcanic explosions by underwater sound. Bulletin of the Geological Society 2: 942-956 (1954).
17. ^ 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).
18. ^ Ocean Noise and Marine Mammals, National Research Council of the National Academies (The National Academies Press, Washington DC, 2003).
19. ^ 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).
20. ^ W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).
21. ^ ^{a b} D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005)
22. ^ NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, NURC Special Publication NURC-SP-2006-008, September 2006
23. ^ D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4-27 (2000).
24. ^ M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003.
25. ^ J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, 18(10), 1640-1657 (2001).