# Underwater acoustics

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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, its contents and its boundaries. The water may be in the ocean, a lake, a river or a tank. Typical frequencies associated with underwater acoustics are between 10 Hz and 1 MHz. The propagation of sound in the ocean at frequencies lower than 10 Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1 MHz are rarely used because they are absorbed very quickly. Underwater acoustics is sometimes known as hydroacoustics.

In physics, sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.

Water is a transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's streams, lakes, and oceans, and the fluids of most living organisms. It is vital for all known forms of life, even though it provides no calories or organic nutrients. Its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. Water is the name of the liquid state of H2O at standard ambient temperature and pressure. It forms precipitation in the form of rain and aerosols in the form of fog. Clouds are formed from suspended droplets of water and ice, its solid state. When finely divided, crystalline ice may precipitate in the form of snow. The gaseous state of water is steam or water vapor. Water moves continually through the water cycle of evaporation, transpiration (evapotranspiration), condensation, precipitation, and runoff, usually reaching the sea.

A mechanical wave is a wave that is an oscillation of matter, and therefore transfers energy through a medium. While waves can move over long distances, the movement of the medium of transmission—the material—is limited. Therefore, the oscillating material does not move far from its initial equilibrium position. Mechanical waves transport energy. This energy propagates in the same direction as the wave. Any kind of wave has a certain energy. Mechanical waves can be produced only in media which possess elasticity and inertia.

## Contents

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.

Sonar is a technique that uses sound propagation 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 for robot navigation, and SODAR 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.

A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.

Acoustical oceanography is the use of underwater sound to study the sea, its boundaries and its contents.

## History

Underwater sound has probably been used by marine animals for millions of years. The science of underwater acoustics began in 1490, when Leonardo da Vinci wrote the following, [1]

Leonardo di ser Piero da Vinci, more commonly Leonardo da Vinci, was an Italian polymath of the Renaissance whose areas of interest included invention, drawing, painting, sculpture, architecture, science, music, mathematics, engineering, literature, anatomy, geology, astronomy, botany, paleontology and cartography. He is widely considered one of the greatest painters of all time, despite perhaps only 15 of his paintings having survived. The Mona Lisa is the most famous of his works and the most popular portrait ever made. The Last Supper is the most reproduced religious painting of all time and his Vitruvian Man drawing is regarded as a cultural icon as well. Leonardo's paintings and preparatory drawings—together with his notebooks, which contain sketches, scientific diagrams, and his thoughts on the nature of painting—compose a contribution to later generations of artists rivalled only by that of his contemporary Michelangelo.

"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. [2] They measured a sound speed of 1435 metres per second over a 17 kilometre(Km) distance, providing the first quantitative measurement of sound speed in water. [3] 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.

Jean-Daniel Colladon was a Swiss physicist.

The Swiss are the citizens of Switzerland or people of Swiss ancestry.

A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate causes of phenomena, and usually frame their understanding in mathematical terms. Physicists work across a wide range of research fields, spanning all length scales: from sub-atomic and particle physics, through biological physics, to cosmological length scales encompassing the universe as a whole. The field generally includes two types of physicists: experimental physicists who specialize in the observation of physical phenomena and the analysis of experiments, and theoretical physicists who specialize in mathematical modeling of physical systems to rationalize, explain and predict natural phenomena. Physicists can apply their knowledge towards solving practical problems or to developing new technologies.

The sinking of Titanic in 1912 and the start of World War I provided the impetus for the next wave of progress in underwater acoustics. Systems for detecting icebergs and U-boats 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 Langevin and in Britain by A B Wood and associates. [4] The development of both active ASDIC 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.

RMS Titanic was a British passenger liner that sank in the North Atlantic Ocean in 1912 after the ship struck an iceberg during her maiden voyage from Southampton to New York City. Of the estimated 2,224 passengers and crew aboard, more than 1,500 died, making it one of modern history's deadliest peacetime commercial marine disasters. RMS Titanic was the largest ship afloat at the time she entered service and was the second of three Olympic-class ocean liners operated by the White Star Line. She was built by the Harland and Wolff shipyard in Belfast. Thomas Andrews, chief naval architect of the shipyard at the time, died in the disaster.

World War I, also known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars", it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history. It is also one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the resulting 1918 influenza pandemic caused another 50 to 100 million deaths worldwide.

An iceberg is a large piece of freshwater ice that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Small bits of disintegrating icebergs are called "growlers" or "bergy bits".

In 1919, the first scientific paper on underwater acoustics was published, [5] theoretically describing the refraction of sound waves produced by temperature and salinity gradients in the ocean. The range predictions of the paper were experimentally validated by propagation loss measurements.

In underwater acoustics, propagation loss is a measure of the reduction in sound intensity as the sound propagates away from an underwater sound source. It is defined as the difference between the source level and the received sound pressure level.

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 transducers made 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 II by 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.

## Theory

### Sound waves in water, bottom of sea

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. These waves may be man-made or naturally generated.

### Speed of sound, density and impedance

The speed of sound ${\displaystyle c\,}$ (i.e., the longitudinal motion of wavefronts) is related to frequency ${\displaystyle f\,}$ and wavelength ${\displaystyle \lambda \,}$ of a wave by ${\displaystyle c=f\cdot \lambda }$.

This is different from the particle velocity ${\displaystyle u\,}$, which refers to the motion of molecules in the medium due to the sound, and relates the plane wave pressure ${\displaystyle p\,}$ to the fluid density ${\displaystyle \rho \,}$ and sound speed ${\displaystyle c\,}$ by ${\displaystyle p=c\cdot u\cdot \rho }$.

The product of ${\displaystyle c}$ and ${\displaystyle \rho \,}$ from the above formula 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 ${\displaystyle I=q^{2}/(\rho c)\,}$, where ${\displaystyle q\,}$ is the root mean square acoustic pressure.

At 1 kHz, the wavelength in water 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. [6] (see 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) [6] and magnesium sulfate (c. 10 kHz-100 kHz). [7]

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.

### Sound reflection and scattering

#### Boundary interactions

Both the water surface and bottom are reflecting and scattering boundaries.

##### Surface

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. [8]

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 ${\displaystyle R=-e^{-2k^{2}h^{2}sin^{2}A}}$, where h is the rms wave height. [9]

A further complication is the presence of wind generated bubbles or fish close to the sea surface. [10] The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves. [11]

##### Seabed

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 [12] and by Buckingham. [13]

#### At target

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 strength of various simple shapes as a function of angle of sound incidence. More complex shapes may be approximated by combining these simple ones. [1]

### 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.This is an important thing that happens in water, because the speed of sound travel in water with velocity regular. In the sea the vertical gradients are generally much larger than the horizontal ones. Combining this with a tendency towards increasing sound speed at increasing depth, due to the increasing pressure in the deep sea, causes a reversal of the sound speed gradient in the thermocline, creating 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.

At equator and temperate latitudes in 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 kilometres without interaction with the sea surface or the seabed. Another phenomenon in the deep sea is the formation of sound focusing 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 focusing. 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 ${\displaystyle I_{s}}$ is the far field intensity of the source referred to a point 1 m from its acoustic centre and ${\displaystyle I_{r}}$ is the intensity at the receiver, then the propagation loss is given by [1] ${\displaystyle PL=10\log(I_{s}/I_{r})}$. In this equation ${\displaystyle I_{r}}$ is not the true acoustic intensity at 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, [14] giving ${\displaystyle PL=20\log(p_{s}/p_{r})}$, where ${\displaystyle p_{s}}$ is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and ${\displaystyle p_{r}}$ 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. [15] If the source and receiver are both in water, the difference is small.

#### Propagation modelling

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. [16] 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. [17] Various empirical and analytical formulae have also been derived from measurements that are useful approximations. [18]

#### Reverberation

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 fish and other biota. For an acoustic signal to be detected easily, it must exceed the reverberation level as well as the background noise level.

#### Doppler shift

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 easily observed in active sonar systems, particularly narrow-band 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.

#### Intensity fluctuations

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.

#### Non-linearity

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 sinusoidal waves 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, e.g. by Westerfield.

## 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.

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 octaves or thirds of octave (dB re 1 μPa)
• spectral density (mean square pressure per unit bandwidth) in micropascals-squared per hertz (dB re 1 μPa²/Hz)

The scale for acoustic pressure in water differs from that used for sound in air. In air the reference pressure is 20 μPa rather than 1 μPa. For the same numerical value of SPL, the intensity of a plane wave (power per unit area, proportional to mean square sound pressure divided by acoustic impedance) in air is about 202×3600 = 1 440 000 times higher than in water. Similarly, the intensity is about the same if the SPL is 61.6 dB higher in the water.

### Sound speed

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. [19] The speed of sound in water increases with increasing pressure, temperature and salinity. [20] [21] The maximum speed in pure water under atmospheric pressure is attained at about 74 °C; sound travels slower in hotter water after that point; the maximum increases with pressure. [22] On-line calculators can be found at Technical Guides – Speed of Sound in Sea-Water and Technical Guides – Speed of Sound in Pure Water.

### Absorption

Many measurements have been made of sound absorption in lakes and the ocean [6] [7] (see Technical Guides – Calculation of absorption of sound in seawater for an on-line calculator).

### 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. 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. [23] At the lowest frequencies, from about 0.1 Hz to 10 Hz, ocean turbulence and microseisms are the primary contributors to the noise background. [24] 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). [25]

Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes, [26] rainfall on the surface, and biological activity. Biological sources include cetaceans (especially blue, fin and sperm whales), [27] [28] certain types of fish, and snapping shrimp.

Rain can produce high levels of ambient noise. However the numerical relationship between rain rate and ambient noise level is difficult to determine because measurement of rain rate is problematic at sea.

### Reverberation

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. [29] 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. [30] Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman. [31] The under-surface of ice can produce strong reverberation when it is rough, see for example Milne. [32]

### Bottom loss

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. [33] 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. In shallow water bottom loss often has the dominant impact on long range propagation. At low frequencies sound can propagate through the sediment then back into the water.

## Underwater hearing

### Comparison with airborne sound levels

As with 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: [34]

• difference in reference pressure: 1 μPa (one micropascal, or one millionth of a pascal) instead of 20 μPa. [14]
• difference in interpretation: there are two schools of thought, one maintaining that pressures should be compared directly, and 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. [35]

### Human 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. [36] This corresponds to a sound intensity 5.4 dB, or 3.5 times, higher than the threshold in air (see Measurements above).

#### Safety thresholds

High levels of underwater sound create a potential hazard to human divers. [37] Guidelines for exposure of human divers to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre. [38] 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. Diver aversion to low frequency sound is dependent upon sound pressure level and center frequency. [39]

### Other species

#### Aquatic mammals

Dolphins and other toothed whales are known for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz. [35] [40] 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. [41]

High levels of underwater sound create a potential hazard to marine and amphibious animals. [35] The effects of exposure to underwater noise are reviewed by Southall et al. [42]

#### Fish

The hearing sensitivity of fish is reviewed by Ladich and Fay. [43] The hearing threshold of the soldier fish, is 0.32 mPa (50 dB re 1 μPa) at 1.3 kHz, whereas the lobster has a hearing threshold of 1.3 Pa at 70 Hz (122 dB re 1 μPa). [41] The effects of exposure to underwater noise are reviewed by Popper et al. [44]

## Applications of underwater acoustics

### Sonar

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.

### 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. A related application is underwater remote control, in which acoustic telemetry is used to remotely actuate a switch or trigger an event. A prominent example of underwater remote control are acoustic releases, devices that are used to return sea floor deployed instrument packages or other payloads to the surface per remote command at the end of a deployment. Acoustic communications form an active field of research [45] [46] 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 beamforming and diversity combining.

Underwater navigation and tracking is a common requirement for exploration and work by divers, ROV, autonomous underwater vehicles (AUV), manned submersibles and submarines alike. Unlike most radio signals which are quickly absorbed, sound propagates far underwater and at a rate that can be precisely measured or estimated. [47] It can thus be used to measure distances between a tracked target and one or multiple reference of baseline stations precisely, and triangulate the position of the target, sometimes with centimeter accuracy. Starting in the 1960s, this has given rise to underwater acoustic positioning systems which are now widely used.

### Seismic exploration

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, vibroseis and explosives.

### Weather and climate observation

Acoustic sensors can be used to monitor the sound made by wind and precipitation. For example, an acoustic rain gauge is described by Nystuen. [48] Lightning strikes can also be detected. [49] Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature.

### Oceanography

Large scale ocean features can be detected by acoustic tomography. Bottom characteristics can be measured by side-scan sonar and sub-bottom profiling.

### Marine biology

Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from microplankton to the blue whale. Echo sounders are often used to provide data on marine life abundance, distribution, and behavior information. Echo sounders, also referred to as hydroacoustics is also used for fish location, quantity, size, and biomass.

Acoustic telemetry is also used for monitoring fish and marine wildlife. An acoustic transmitter is attached to the fish (sometimes internally) while an array of receivers listen to the information conveyed by the sound wave. This enables the researchers to track the movements of individuals in a small-medium scale. [50]

Pistol shrimp create sonoluminescent cavitation bubbles that reach up to 5,000 K (4,700 °C) [51]

### Particle physics

A neutrino is 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. [52]

## Related Research Articles

Acoustics is the branch of physics that deals with the study of all mechanical waves in gases, liquids, and solids including topics such as vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics technology may be called an acoustical engineer. The application of acoustics is present in almost all aspects of modern society with the most obvious being the audio and noise control industries.

Echo sounding is a type of sonar used to determine the depth of water by transmitting sound waves into water. The time interval between emission and return of a pulse is recorded, which is used to determine the depth of water along with the speed of sound in water at the time. This information is then typically used for navigation purposes or in order to obtain depths for charting purposes. Echo sounding can also refer to hydroacoustic "echo sounders" defined as active sound in water (sonar) used to study fish. Hydroacoustic assessments have traditionally employed mobile surveys from boats to evaluate fish biomass and spatial distributions. Conversely, fixed-location techniques use stationary transducers to monitor passing fish.

Room acoustics describes how sound behaves in an enclosed space.

Acoustical engineering is the branch of engineering dealing with sound and vibration. It is the application of acoustics, the science of sound and vibration, in technology. Acoustical engineers are typically concerned with the design, analysis and control of sound.

Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).

Computational aeroacoustics is a branch of aeroacoustics that aims to analyze the generation of noise by turbulent flows through numerical methods.

Hydroacoustics is the study and application of sound in water. Hydroacoustics, using sonar technology, is most commonly used for monitoring of underwater physical and biological characteristics.

Acoustic waves are a type of longitudinal waves that propagate by means of adiabatic compression and decompression. Longitudinal waves are waves that have the same direction of vibration as their direction of travel. Important quantities for describing acoustic waves are sound pressure, particle velocity, particle displacement and sound intensity. Acoustic waves travel with the speed of sound which depends on the medium they're passing through.

A parametric array, in the field of acoustics, is a nonlinear transduction mechanism that generates narrow, nearly side lobe-free beams of low frequency sound, through the mixing and interaction of high frequency sound waves, effectively overcoming the diffraction limit associated with linear acoustics. The main side lobe-free beam of low frequency sound is created as a result of nonlinear mixing of two high frequency sound beams at their difference frequency. Parametric arrays can be formed in water, air, and earth materials/rock.

In acoustics, the sound speed gradient is the rate of change of the speed of sound with distance, for example with depth in the ocean, or height in the Earth's atmosphere. A sound speed gradient leads to refraction of sound wavefronts in the direction of lower sound speed, causing the sound rays to follow a curved path. The radius of curvature of the sound path is inversely proportional to the gradient.

In acoustics, microbaroms, also known as the "voice of the sea", are a class of atmospheric infrasonic waves generated in marine storms by a non-linear interaction of ocean surface waves with the atmosphere. They typically have narrow-band, nearly sinusoidal waveforms with amplitudes up to a few microbars, and wave periods near 5 seconds. Due to low atmospheric absorption at these low frequencies, microbaroms can propagate thousands of kilometers in the atmosphere, and can be readily detected by widely separated instruments on the Earth's surface.

Underwater acoustic communication is a technique of sending and receiving messages below water. There are several ways of employing such communication but the most common is by using hydrophones. Underwater communication is difficult due to factors such as multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges. Compared to terrestrial communication, underwater communication has low data rates because it uses acoustic waves instead of electromagnetic waves.

Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. Rayleigh waves are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

Most sonar systems are monostatic, in that the transmitter and receiver are in the same place. Bistatic sonar describes when the transmitter and receiver(s) are separated by a distance large enough to be comparable to the distance to the target.

Acoustic attenuation is a measure of the energy loss of sound propagation in media. Most media have viscosity, and are therefore not ideal media. When sound propagates in such media, there is always thermal consumption of energy caused by viscosity. For inhomogeneous media, besides media viscosity, acoustic scattering is another main reason for removal of acoustic energy. Acoustic attenuation in a lossy medium plays an important role in many scientific researches and engineering fields, such as medical ultrasonography, vibration and noise reduction.

A sound speed profile shows the speed of sound in water at different vertical levels. It has two general representations:

1. tabular form, with pairs of columns corresponding to ocean depth and the speed of sound at that depth, respectively.
2. a plot of the speed of sound in the ocean as a function of depth, where the vertical axis corresponds to the depth and the horizontal axis corresponds to the sound speed. By convention, the horizontal axis is placed at the top of the plot, and the vertical axis is labeled with values which increase from top to bottom, thus reproducing visually the ocean from its surface downward.

The Pioneers of Underwater Acoustics Medal is awarded by the Acoustical Society of America in recognition of "an outstanding contribution to the science of underwater acoustics, as evidenced by publication of research results in professional journals or by other accomplishments in the field". The award was named in honor of H. J. W. Fay, Reginald Fessenden, Harvey Hayes, G. W. Pierce, and Paul Langevin.

Transmission loss (TL) in general describes the accumulated decrease in intensity of a waveform energy as a wave propagates outwards from a source, or as it propagates through a certain area or through a certain type of structure.

## References

1. Urick, Robert J. Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill, 1983.
2. C. S. Clay & H. Medwin, Acoustical Oceanography (Wiley, New York, 1977)
3. Annales de Chimie et de Physique 36 [2] 236 (1827)
4. 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).
5. H. Lichte (1919). "On the influence of horizontal temperature layers in sea water on the range of underwater sound signals". Phys. Z. 17 (385).
6. 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).
7. 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).
8. Ainslie, M. A. (2010). Principles of Sonar Performance Modeling. Berlin: Springer. p36
9. H. Medwin & C. S. Clay, Fundamentals of Acoustical Oceanography (Academic, Boston, 1998).
10. D. E. Weston & P. A. Ching, Wind effects in shallow-water transmission, J. Acoust. Soc. Am. 86, 1530–1545 (1989).
11. 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)
12. N Chotiros, Biot Model of Sound Propagation in Water Saturated Sand. J. Acoust. Soc. Am. 97, 199 (1995)
13. 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).
14. C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).
15. M. A. Ainslie, The sonar equation and the definitions of propagation loss, J. Acoust. Soc. Am. 115, 131–134 (2004).
16. F. B. Jensen, W. A. Kuperman, M. B. Porter & H. Schmidt, Computational Ocean Acoustics (AIP Press, NY, 1994).
17. C. H. Harrison, Ocean propagation models, Applied Acoustics 27, 163–201 (1989).
18. L. M. Brekhovskikh & Yu. P. Lysanov, Fundamentals of Ocean Acoustics, 3rd edition (Springer-Verlag, NY, 2003).
19. A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989).
20. Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. 70, 807–812 (1982).
21. 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)
22. Wilson, Wayne D. (26 Jan 1959). "Speed of Sound in Distilled Water as a Function of Temperature and Pressure". J. Acoust. Soc. Am. 31 (8): 1067–1072. Bibcode:1959ASAJ...31.1067W. doi:10.1121/1.1907828 . Retrieved 11 February 2012.
23. G. M. Wenz, Acoustic ambient noise in the ocean: spectra and sources, J. Acoust. Soc. Am. 34, 1936–1956 (1962).
24. S. C. Webb, The equilibrium oceanic microseism spectrum, J. Acoust. Soc. Am. 92, 2141–2158 (1992).
25. R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. 24, 478–480 (1952).
26. 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).
27. 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).
28. Ocean Noise and Marine Mammals, National Research Council of the National Academies (The National Academies Press, Washington DC, 2003).
29. R Chapman and J Harris, Surface backscattering Strengths Measured with Explosive Sound Sources. J. Acoust. Soc. Am. 34, 547 (1962)
30. K Mackenzie, Bottom Reverberation for 530 and 1030 cps Sound in Deep Water. J. Acoust. Soc. Am. 36, 1596 (1964)
31. 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)
32. A. Milne, Underwater Backscattering Strengths of Arctic Pack Ice. J. Acoust. Soc. Am. 36, 1551 (1964)
33. MGS Station Data Listing and Report Catalog, Nav Oceanog Office Special Publication 142, 1974
34. D.M.F. Chapman, D.D. Ellis, The elusive decibel – thoughts on sonars and marine mammals, Can. Acoust. 26(2), 29–31 (1996)
35. W. J. Richardson, C. R. Greene, C. I. Malme and D. H. Thomson, Marine Mammals and Noise (Academic Press, San Diego, 1995).
36. 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).
37. Steevens CC, Russell KL, Knafelc ME, Smith PF, Hopkins EW, Clark JB (1999). "Noise-induced neurologic disturbances in divers exposed to intense water-borne sound: two case reports". Undersea Hyperb Med. 26 (4): 261–5. PMID   10642074 . Retrieved 2009-03-31.
38. NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, NURC Special Publication NURC-SP-2006-008, September 2006
39. Fothergill DM, Sims JR, Curley MD (2001). "Recreational scuba divers' aversion to low-frequency underwater sound". Undersea Hyperb Med. 28 (1): 9–18. PMID   11732884 . Retrieved 2009-03-31.
40. W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).
41. D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005)
42. Southall, B. L., Bowles, A. E., Ellison, W. T., Finneran, J. J., Gentry, R. L., Greene, C. R., ... & Richardson, W. J. (2007). Marine Mammal Noise Exposure Criteria Aquatic Mammals.
43. Ladich, F., & Fay, R. R. (2013). Auditory evoked potential audiometry in fish. Reviews in fish biology and fisheries, 23(3), 317-364.
44. Popper, A. N., Hawkins, A. D., Fay, R. R., Mann, D. A., Bartol, S., Carlson, T. J., ... & Løkkeborg, S. (2014). ASA S3/SC1. 4 TR-2014 Sound exposure guidelines for fishes and sea turtles: A technical report prepared by ANSI-Accredited standards committee S3/SC1 and registered with ANSI. Springer.
45. D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4–27 (2000).
46. M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003.
47. Underwater Acoustic Positioning Systems, P.H. Milne 1983, ISBN   0-87201-012-0
48. J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, 18(10), 1640–1657 (2001).
49. R. D. Hill, Investigation of lightning strikes to water surfaces, J. Acoust. Soc. Am. 78, 2096–2099 (1985).
50. Moore, A., T. Storeton-West, I. C. Russell, E. C. E. Potter, and M. J. Challiss. 1990. A technique for tracking Atlantic salmon (Salmo salar L.) smolts through estuaries. International Council for the Ex- ploration of the Sea, C.M. 1990/M: 18, Copenhagen.
51. D. Lohse, B. Schmitz & M. Versluis (2001). "Snapping shrimp make flashing bubbles". Nature . 413 (6855): 477–478. Bibcode:2001Natur.413..477L. doi:10.1038/35097152. PMID   11586346.
52. 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 Physics Volume 28, Issue 3, November 2007, Pages 366–379