Underwater_acoustics - Wikiparency

# Underwater acoustics **Underwater acoustics** is the study of the production, propagation and reception of [[Sound|sound]] in [[Water|water]], and of the instruments — sonar, hydrophones, acoustic modems — built on that physics. It is the branch of [[Acoustics]] that has no easy substitute: light and radio are absorbed by seawater within tens of meters, so sound is the only signal that carries useful information across an ocean basin, and every field that needs to see, talk or listen underwater — naval sonar, marine biology, oceanography, seismic exploration — is built on it. The microsim below traces a fan of rays from a source at a chosen depth through the ocean's [[Speed_of_sound|sound-speed]] profile. Move the source deeper or shallower, widen the launch angles, or switch between the Munk mid-latitude profile and simplified summer and winter profiles, and watch which rays stay trapped near the axis of the deep sound channel and which strike the surface or the seabed and lose their energy there. The field's founding puzzle — why some sounds carry across an entire ocean while others die out in a few kilometers — turns on a single fact: the speed of sound in the sea is not constant. It falls as water cools with depth and then rises again as pressure compresses it, and a wave that meets a change in speed bends away from the faster medium exactly as light bends at a lens. That bending, not the sound's loudness, decides how far it travels, and it is the subject of underwater acoustics' central object, the deep sound channel. ## History Leonardo da Vinci is credited with an early observation that a tube placed in water and held to the ear carries the sound of a distant ship, though the anecdote is far better attested to his notebooks' spirit than to a dated, verified passage.[^leonardo-cn] The first quantitative measurement came in 1826, when the Swiss physicist Jean-Daniel Colladon, working with the mathematician Charles-François Sturm, struck a submerged bell in Lake Geneva at the same instant as a gunpowder flash on one boat, and timed the bell's sound as it reached a second boat about 10 miles (16 km) away; from that delay they found a sound speed in fresh water at 8 °C of 1,435 m/s, within about 3 m/s of the modern accepted value — showing directly that sound travels faster through water than through air.[^colladon] The loss of the *Titanic* in 1912 gave the search for underwater ranging new urgency, and Reginald Fessenden built a moving-armature oscillator that could send and receive echo signals underwater, testing it for iceberg detection in 1914; the same drive to find icebergs and detect submarines in the First World War then led Paul Langevin to build a working piezoelectric transducer for underwater sound.[^fessenden][^langevin] A second wartime advance came in 1944, when researchers including Maurice Ewing found that a small explosive charge fired at the axis of the deep ocean's speed minimum could be detected thousands of kilometers away by a single hydrophone — the discovery of the channel that Cold War-era oceanography would use for long-range detection and later, in reverse, to track whale calls across ocean basins.[^munk] ## Theory ### Sound waves in water, bottom of sea In the water column, sound propagates as a longitudinal [[Wave|wave]] of alternating compression and rarefaction, the same mechanism that carries it through air, only faster and with far less loss to viscosity. The seabed is a different kind of medium: unlike water, a sediment or rock bottom is rigid enough to carry shear as well as compressional waves, so a downward-going wave that strikes it can convert part of its energy into a wave type the water column cannot support, one route by which the seabed drains energy out of the underwater sound field. ### Speed of sound, density and impedance The speed of sound in seawater is about 1,540 m/s, compared with about 1,480 m/s in fresh water and 331 m/s in air at 0 °C — sound moves roughly four and a half times faster in the sea than in the air above it.[^upv1-t171] [[Acoustic_impedance|Acoustic impedance]] is the product of a medium's density and its sound speed; seawater's density of roughly 1,025 kg/m³ times its sound speed gives an impedance near 1.58 × 10⁶ kg/(m²·s), thousands of times water's impedance in air but a small fraction of a steel ship's hull, which is why a hull rings audibly under sonar insonification and a person swimming directly beneath a boat can hear its engine clearly. ### Absorption of sound Sound loses energy to the medium itself as it travels, and in seawater that loss depends strongly on frequency: high frequencies fade within kilometers, while very low frequencies below a few hundred hertz can cross an ocean basin. Two chemical relaxation processes drive most of the loss — boric acid near 1 kHz and magnesium sulfate near tens of kilohertz — on top of the same viscous absorption that attenuates sound in fresh water, and the combined [[Acoustic_attenuation|absorption]] coefficient is the basis of every sonar range prediction.[^urick] ### Sound reflection and scattering A pulse that meets the sea surface, the seabed, a submarine hull, or a school of fish partly reflects and partly scatters, and the scattered return is exactly what a fish-finder or a sonar operator reads as a target. Fisheries and naval acousticians alike discovered during the Second World War that even open water scatters sound strongly at certain depths, a layer that migrates toward the surface at night and back down by day — later identified as swarms of small fish and zooplankton and named the deep scattering layer.[^urick] ### Propagation of sound Because the speed of sound falls with depth in the upper few hundred meters (as the water cools) and then rises again with depth (as pressure compresses it), there is a depth of minimum speed — the axis of the deep sound channel, also called the SOFAR (Sound Fixing and Ranging) channel. Rays leaving a source near that axis at a shallow enough angle bend back toward it whenever they wander into faster water above or below, by the same Snell's-law logic that guides light in an optical fiber, and so a wave trapped on the axis never touches the lossy surface or seabed and can be heard across thousands of kilometers.[^munk] (Wikitube has not yet built a standalone article on the underlying refraction law; the physics is Snell's law of refraction applied to a continuously varying sound speed.) In the North Pacific and North Atlantic the axis typically sits near 1,000–1,300 m; in the model curve first published by Walter Munk in 1974, the sound speed *c(z)* follows *c*₁[1 + ε(η + e^−η − 1)], with η = 2(*z* − *z*₁)/*B*, *c*₁ = 1,500 m/s, *z*₁ = *B* = 1,300 m and ε = 0.00737 — the curve the microsim's shader field draws.[^munk] *Try: move the source depth from near the surface down to the channel axis near 1,000–1,300 m, widen the launch-angle spread, and switch the profile between the Munk mid-latitude curve and the simplified summer and winter cases to see which rays stay trapped on the axis and which run into the surface or seabed.* ## Measurements ### Sound speed Because the sound field bends around whatever speed profile is actually present, oceanographers need the speed accurately as a function of temperature, salinity and depth rather than a single average number. The nine-term empirical equation published by Kenneth Mackenzie in 1981, fitted to laboratory and field data, remains a standard reference formula for computing sound speed from measured temperature, salinity and depth throughout the world ocean.[^mackenzie] ### Absorption Measured absorption coefficients in seawater are compiled into standard tables and formulas (following Thorp and later Francois–Garrison) that every sonar-range calculation and marine-mammal noise-exposure estimate draws on, because the same 20 dB of source level buys vastly different range at 100 Hz than at 100 kHz.[^urick] ### Ambient noise Even a silent sea is not quiet: wind and breaking waves near the surface, distant shipping, seismic activity, rain, and marine life all add their own noise, and the shape of that background as a function of frequency was first compiled into a family of curves by Gordon Wenz in 1962. Wind and wave noise dominates above a few kilohertz; distant shipping is the main source in a band roughly 20–300 Hz; and biological sources — from the clicks of snapping shrimp to the calls of large whales — can dominate locally at almost any frequency.[^wenz] ### Reverberation Instead of a single clean echo, an active sonar pulse often returns a long, decaying tail of energy scattered back from the sea surface, the seabed and the volume of water in between; naval sonar operators call this [[Reverberation|reverberation]], by direct analogy with the way sound lingers in a room after Sabine's own reverberation time. In shallow water reverberation, not the ambient noise floor, is usually what limits how weak a target echo a sonar can still detect.[^urick] ### Bottom loss The fraction of a downward-going wave's energy that a sonar recovers after it strikes bottom depends heavily on the seabed's material: a hard rock or sand bottom reflects strongly and returns a fair amount of what a signal loses on each bounce in shallow water, while soft mud absorbs much more, and the loss also grows as the grazing angle gets shallower.[^urick] ## Underwater hearing ### Comparison with airborne sound levels Underwater and airborne decibels are not directly comparable numbers. By convention, sound levels in water are expressed relative to a reference pressure of 1 micropascal, while sound levels in air are referenced to 20 micropascals — a level chosen near the threshold of human hearing in air. Because of that reference-pressure difference together with water's much higher density and sound speed, a sound wave of the same physical intensity carries a value about 61.5 dB higher when expressed in the underwater (re 1 µPa) convention than in the airborne (re 20 µPa) one — 26 dB of the gap from the reference pressures alone, the remaining 35.5 dB from the density and speed difference between the two media — so a bare [[Decibel|decibel]] number is meaningless without stating which medium and reference pressure it uses.[^dosits] ### Human hearing Sound reaches a human ear underwater mostly by conduction through the skull rather than through the normal air-conduction path of the outer and middle ear, and the water's far higher impedance than air makes it difficult for a swimmer to localize a sound's direction the way they can in air. ### Other species Marine mammals evolved hearing, and in the case of toothed whales and dolphins active biosonar, matched to a medium where sound is the primary long-range sense; their acoustic behavior is treated at [[Animal_echolocation|animal echolocation]]. ## Applications of underwater acoustics ### Sonar [[Sonar]] times a pulse's round trip to a target and back — range equals the speed of sound times half the travel time — to detect [[Submarine|submarines]], mines and fish, or, run passively, listens for the sound a target itself radiates and reads a moving target's [[Doppler_effect|Doppler shift]] for its closing speed. Both modes inherit every limit above: the sound-speed profile that bends the beam, the absorption that sets its maximum range, and the ambient noise or reverberation that sets the weakest echo it can still detect. ### Underwater communication Because radio is absorbed by seawater within meters, [[Submarine|submarines]], autonomous underwater vehicles and divers exchange data with acoustic modems that trade off bit rate against range in the same way a sonar trades resolution against penetration — and the same multipath bending that helps a SOFAR signal travel far also scrambles the timing of a communication pulse's many bottom- and surface-bounced arrivals. ### Underwater navigation and tracking A vehicle or a tagged animal can be tracked from long-baseline or ultra-short-baseline arrays of hydrophones that time an acoustic pulse's arrival at several known positions and triangulate a fix, the underwater equivalent of a satellite-navigation solution built from sound instead of radio. ### Seismic exploration Marine seismic surveys tow an array of airguns and hydrophones and read the timing and strength of sound reflected from rock layers below the seabed to map oil, gas and mineral deposits, using the same reflection physics that returns an echo from a submarine hull. ### Weather and climate observation Long-range hydrophone arrays first built to listen for submarines have been repurposed to track ocean temperature by timing how long a signal takes to cross a known path — because the speed of sound in water rises with temperature, the travel time itself becomes a thermometer averaged over thousands of kilometers of ocean. ### Acoustical oceanography Echo sounders and multibeam sonar systems map the seafloor's shape and the water column's structure — including the deep scattering layer and internal waves — by the same pulse–echo principle a fish-finder uses, at a scale that spans a single lake bed to an entire ocean basin. ### Marine biology Fisheries scientists estimate how much fish biomass occupies a volume of water from how strongly it scatters an echo sounder's pulse, and passive listening arrays track whale and dolphin calls across ocean basins by the same long-range [[Wave|propagation]] that the deep sound channel gives a distant naval hydrophone. ### Particle physics Deep, dark, acoustically quiet water has also been proposed and tested as a medium for detecting the faint thermoacoustic pulse a very high-energy neutrino interaction would leave behind, an application far removed from sonar but built on the same propagation physics. ### Other applications Underwater acoustics also underlies ocean-going gliders' station-keeping, subsea infrastructure inspection, and the tracking of icebergs and sea ice, each a variation on timing or listening to sound in water. ## Minnesota *This section is specific to Wikitube.* Minnesota's underwater acoustics is a freshwater story rather than an oceanic one. The University of Minnesota Duluth's Large Lakes Observatory operates a suite of acoustic survey instruments for work on [[Lake_Superior]] and other large lakes: a 240 kHz Reson SeaBat 7101 multibeam sonar rated to 400 m depth for hydrographic mapping, a dual-frequency (100/400 kHz) EdgeTech CHIRP sidescan sonar rated to 300 m, a hand-deployable EdgeTech 424 CHIRP sub-bottom profiler for seismic-reflection surveys of lake-bottom sediment, and a 28 kHz Knudsen echo sounder, deployed from the research vessel *Blue Heron* or from smaller boats on remote inland lakes.[^llo] The same pulse–echo and reflection physics that maps the deep sound channel in the open ocean maps a lake bed and its buried sediment layers in Minnesota's own waters. ## See also - [[Sonar]] - [[Animal_echolocation]] - [[Sound]] - [[Speed_of_sound]] - [[Acoustic_impedance]] - [[Reverberation]] - [[PORTAL_Acoustics]] Connects to: [[Ultrasound]] · [[Acoustic_attenuation]] ## References [^leonardo-cn]: The story that Leonardo da Vinci noted a tube in water carries the sound of a distant ship circulates widely in underwater-acoustics histories but is not traced here to a specific, dated primary passage in his notebooks; treated as a well-known but loosely sourced anecdote. *Citation needed* for the exact notebook folio, if one exists. [^colladon]: Colladon, Jean-Daniel; Sturm, Charles-François (1827). "Mémoire sur la compression des liquides et sur la vitesse du son dans l'eau" ["Memoir on the compressibility of liquids and the speed of sound in water"], read to the Académie des Sciences. The 1826 Lake Geneva date is also given in *Ultrasound Physics and its Application in Medicine* (2024), Preface, p. vii — on the [[PORTAL_Acoustics]] book shelf. Experimental detail (bell and gunpowder flash timed across roughly 10 miles/16 km; measured speed 1,435 m/s in fresh water at 8 °C, about 3 m/s from the modern value) per Discovery of Sound in the Sea (DOSITS), University of Rhode Island, "The First Studies of Underwater Acoustics: The 1800s." https://dosits.org/people-and-sound/history-of-underwater-acoustics/the-first-studies-of-underwater-acoustics-the-1800s/ [^fessenden]: Science History Institute. "Reginald Fessenden and the Invention of Sonar." https://www.sciencehistory.org/stories/magazine/reginald-fessenden-and-the-invention-of-sonar/ ; Engineering and Technology History Wiki (ETHW). "Fessenden's underwater ice finder." https://ethw.org/Fessenden%27s_underwater_ice_finder [^langevin]: *Ultrasound Physics and its Application in Medicine* (2024), Preface, pp. vii–viii: Paul Langevin's piezoelectric transducer, developed during the First World War to detect icebergs and enemy submarines. On the [[PORTAL_Acoustics]] book shelf. [^munk]: Munk, Walter H. (1974). "Sound channel in an exponentially stratified ocean, with application to SOFAR," *Journal of the Acoustical Society of America* 55, 220–226. The 1944 discovery that an explosive charge on the channel axis could be heard across an ocean basin is recounted in the underwater-acoustics literature that follows from this paper; treated here as a widely reported historical claim rather than independently verified against a primary 1944 report. [^upv1-t171]: University Physics Volume 1 (OpenStax, 2016), Chapter 17 "Sound," §17.2 "Speed of Sound," Table 17.1: air 331 m/s, fresh water 1,480 m/s, sea water 1,540 m/s, human tissue 1,540 m/s. [^urick]: Urick, Robert J. (1983). *Principles of Underwater Sound* (3rd ed.). McGraw-Hill. Chapters 5–6 (absorption, ray bending and the deep sound channel), and the general treatment of reverberation, bottom loss, and the deep scattering layer that this article follows. [^mackenzie]: Mackenzie, Kenneth V. (1981). "Nine-term equation for sound speed in the oceans," *Journal of the Acoustical Society of America* 70 (3), 807–812. https://doi.org/10.1121/1.386920 [^wenz]: Wenz, Gordon M. (1962). "Acoustic ambient noise in the ocean: spectra and sources," *Journal of the Acoustical Society of America* 34 (12), 1936–1956. https://doi.org/10.1121/1.1909155 [^dosits]: Discovery of Sound in the Sea (DOSITS), University of Rhode Island. "How does sound in air differ from sound in water?" https://dosits.org/science/sounds-in-the-sea/how-does-sound-in-air-differ-from-sound-in-water/ — states the 1 µPa vs. 20 µPa reference pressures and the resulting 61.5 dB offset (26 dB from the reference pressures, 35.5 dB from the density and sound-speed difference). [^llo]: University of Minnesota Duluth, Swenson College of Science and Engineering, Large Lakes Observatory. "Geophysical Tools." https://scse.d.umn.edu/large-lakes-observatory/research-services/analytical-facilities/geophy-tools  **Microsim — three.js (Wikitube framework):** *Underwater acoustics* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/acoustics/Underwater_acoustics.html" data-title="Underwater acoustics"></div> *Built from `MICROSIM_GUIDE/specs/acoustics/sims/Underwater_acoustics.json`; part of the [[PORTAL_Acoustics|Acoustics portal]] spine (section sims and See-also variants).*   **Microsim — three.js (Wikitube framework):** *Underwater acoustics* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/flight/Underwater_acoustics.html" data-title="Underwater acoustics"></div> *Built from `MICROSIM_GUIDE/specs/variants/Underwater_acoustics.json`; part of the [[Aviation]] · [[Avionics]] flight set.*   *Linked from the [[Avionics]] hub, section X26, Sonar.*  ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Underwater_acoustics) : [Wikitube](https://en.wikitube.io/wiki/Underwater_acoustics) - skeleton pinned to revision 1368892725 (2026-09-11).