Underwater acoustics

Underwater acoustics, also termed hydroacoustics, investigates the transmission of sound through water and the interaction of these mechanical waves with the water medium, its constituents, and its boundaries. This aquatic environment can include oceans, lakes, rivers, or tanks. Frequencies typically studied in underwater acoustics range from 10 Hz to 1 MHz. Sound propagation in the ocean below 10 Hz generally requires significant penetration into the seabed, while frequencies exceeding 1 MHz are seldom employed due to rapid absorption.

Utilizing sonar technology, hydroacoustics is primarily applied to monitor underwater physical and biological attributes. It facilitates the determination of water body depth (bathymetry), in addition to identifying the presence, absence, abundance, distribution, size, and behavior of aquatic flora and fauna. Hydroacoustic sensing encompasses either 'passive acoustics,' which involves listening for existing sounds, or active acoustics, which entails emitting a sound and detecting its echo, leading to the common device designation of echo sounder or echosounder.

Shipping generates noise from various sources, which can be categorized into propeller-induced noise, machinery-generated noise, and noise resulting from the hull's movement through water. The significance of each category varies depending on factors such as the specific ship type.

A primary source of hydroacoustic noise originating from fully submerged lifting surfaces is the unsteady, separated turbulent flow near the trailing edge. This flow induces pressure fluctuations on the surface and generates unsteady oscillatory flow in the immediate wake. The interaction between the surface and the ocean establishes a turbulent boundary layer (TBL) around the surface, within which fluctuating velocity and pressure fields produce the noise.

Underwater acoustics is intrinsically linked to several other acoustic disciplines, such as sonar, transduction, signal processing, acoustical oceanography, bioacoustics, and physical acoustics.

History

Marine animals have likely utilized underwater sound for millions of years. The scientific study of underwater acoustics commenced in 1490 with Leonardo da Vinci's observations:

“Should you halt your vessel, immerse one end of a long tube into the water, and place the other extremity to your ear, you will detect distant ships.”

Isaac Newton's 1687 work, Mathematical Principles of Natural Philosophy, presented the inaugural mathematical analysis of sound. A significant advancement in underwater acoustics followed in 1826, when Swiss physicist Daniel Colladon and French mathematician Charles Sturm conducted an experiment on Lake Geneva. They quantified the time delay between a light flash and the sound of a submerged ship's bell, detected via an underwater listening horn. Their measurement of sound speed in water was 1435 meters per second over a 17-kilometer distance, marking the first quantitative determination of this parameter. This result closely approximated (within 2%) currently accepted values. Subsequently, in 1877, Lord Rayleigh authored Theory of Sound, which laid the foundation for modern acoustic theory.

The 1912 sinking of the Titanic and the onset of World War I spurred subsequent advancements in underwater acoustics. This period saw the development of systems for detecting icebergs and U-boats. From 1912 to 1914, numerous echolocation patents were issued in Europe and the U.S., culminating in Reginald A. Fessenden's echo-ranger in 1914. Concurrently, pioneering research was conducted in France by Paul Langevin and in Britain by A. B. Wood and his colleagues. The war, driven by the widespread deployment of submarines, accelerated the development of both active ASDIC and passive sonar (SOund Navigation And Ranging). Further innovations in underwater acoustics included the creation of acoustic mines.

The inaugural scientific publication on underwater acoustics appeared in 1919, providing a theoretical description of sound wave refraction caused by oceanic temperature and salinity gradients. Experimental measurements of propagation loss subsequently validated the paper's range predictions.

The subsequent two decades witnessed the emergence of numerous underwater acoustic applications. The fathometer, also known as a depth sounder, underwent commercial development in the 1920s. Initially, transducers utilized natural materials; however, by the 1930s, sonar systems began incorporating piezoelectric transducers fabricated from synthetic materials, enabling both passive listening and active echo-ranging capabilities. These systems proved highly effective during World War II, employed by both submarine and anti-submarine forces. Significant advancements in underwater acoustics were achieved, subsequently summarized in the 1946 publication series, Physics of Sound in the Sea.

Post-World War II, the Cold War significantly propelled sonar system development, fostering advancements in both the theoretical and practical comprehension of underwater acoustics, often facilitated by computer-based methodologies.

Theoretical Principles

Acoustic Wave Propagation in Aquatic Environments

Underwater sound waves are characterized by alternating phases of compression and rarefaction within the water medium. These pressure fluctuations, resulting from compressions and rarefactions, are detectable by various receivers, including the human auditory system or hydrophones. Such waves can originate from anthropogenic or natural sources.

Sound Speed, Density, and Acoustic Impedance

The speed of sound, denoted as ${\displaystyle c\,}$ (representing the longitudinal propagation of wavefronts), is mathematically linked to the wave's frequency ${\displaystyle f\,}$ and wavelength ${\displaystyle \lambda \,}$ through the equation ${\displaystyle c=f\cdot \lambda }$.

This differs from the particle velocity, ${\displaystyle u\,}$, which describes the displacement of individual molecules within the medium caused by the sound wave. The plane wave pressure, ${\displaystyle p\,}$, is related to the fluid density ${\displaystyle \rho \,}$ and the sound speed ${\displaystyle c\,}$ by the formula ${\displaystyle p=c\cdot u\cdot \rho }$.

The characteristic acoustic impedance is defined as the product of ${\displaystyle c}$ and ${\displaystyle \rho \,}$, derived from the preceding formula. Acoustic intensity, representing the acoustic power (energy per second) transmitted across a unit area, is expressed for a plane wave by the average intensity formula: ${\displaystyle I=q^{2}/(\rho c)\,}$, where ${\displaystyle q\,}$ denotes the root mean square acoustic pressure.

At a frequency of 1 kHz, the corresponding wavelength in water approximates 1.5 meters. The term "sound velocity" is occasionally employed; however, this usage is imprecise, as the quantity in question is a scalar, correctly referred to as "sound speed."

The significant impedance disparity between air and water, approximately 3600, combined with the scale of surface roughness, causes the sea surface to function as an almost ideal sound reflector for frequencies under 1 kHz. The speed of sound in water is 4.4 times greater than in air, and the density ratio is approximately 820.

Sound Absorption

Low-frequency sound exhibits minimal absorption. The primary cause of sound attenuation in freshwater, and at high frequencies (exceeding 100 kHz) in seawater, is viscosity. At lower frequencies in seawater, further attenuation arises from the ionic relaxation of boric acid (up to approximately 10 kHz) and magnesium sulfate (ranging from approximately 10 kHz to 100 kHz).

Sound absorption can occur due to energy losses at fluid boundaries. Near the ocean surface, these losses may manifest within a bubble layer or ice. Conversely, at the seabed, sound can permeate and be absorbed by the sediment.

Sound Reflection and Scattering

Boundary Interactions

Both the water's surface and the seabed act as boundaries that reflect and scatter sound.

Surface Characteristics

In numerous contexts, the sea-air interface is considered an ideal reflector. The substantial impedance contrast significantly impedes energy transmission across this boundary. Acoustic pressure waves reflecting from the sea surface undergo a phase reversal, commonly described as either a "pi phase change" or a "180-degree phase change." Mathematically, this phenomenon is represented by assigning a reflection coefficient of -1 to the sea surface, rather than +1.

At high frequencies (exceeding approximately 1 kHz) or under rough sea conditions, a portion of the incident sound undergoes scattering. This effect is accounted for by employing a reflection coefficient with a magnitude less than one. For instance, near normal incidence, the reflection coefficient is expressed as ${\displaystyle R=-e^{-2k^{2}h^{2}\sin ^{2}A}}$, where h denotes the root-mean-square (rms) wave height.

The presence of wind-generated bubbles or fish near the sea surface introduces additional complexity. These bubbles can form plumes that both absorb a portion of the incident and scattered sound and contribute to sound scattering.

Seabed Characteristics

The acoustic impedance mismatch between water and the seabed is typically less pronounced and more intricate than that at the surface. This mismatch is contingent upon the types of bottom materials and the depths of their respective layers. Theories, such as those proposed by Biot and Buckingham, have been formulated to predict sound propagation within the seabed under these conditions.

Target Interaction

The reflection of sound from a target, particularly when its dimensions significantly exceed the acoustic wavelength, is influenced by its size, shape, and impedance relative to water. Formulas have been established to determine the target strength of various basic geometric forms as a function of the sound's angle of incidence. More intricate shapes can be approximated by synthesizing these simpler configurations.

Sound Propagation

Underwater acoustic propagation is contingent upon numerous variables. The trajectory of sound propagation is dictated by the sound speed gradients present in the water. These gradients modify the sound wave through processes of refraction, reflection, and dispersion. Within marine environments, vertical gradients typically surpass horizontal ones in magnitude. This, coupled with the tendency for sound speed to increase with depth due to rising pressure in the deep ocean, results in a reversal of the sound speed gradient within the thermocline. This reversal establishes an effective waveguide at the depth corresponding to the minimum sound speed. The resulting sound speed profile can generate areas of diminished sound intensity, termed "Shadow Zones," and regions of heightened intensity, known as "Caustics." These phenomena can be identified using ray tracing methodologies.

In oceanic equatorial and temperate regions, elevated surface temperatures can counteract the pressure effect, resulting in a sound speed minimum at depths of several hundred meters. This minimum facilitates the formation of a unique conduit, termed the deep sound channel or SOFAR (sound fixing and ranging) channel, which enables guided underwater sound propagation over thousands of kilometers without interacting with the sea surface or seabed. A distinct deep-sea phenomenon involves the creation of sound focusing regions, known as convergence zones. Here, sound originating from a near-surface source is refracted downwards and subsequently upwards. The horizontal range from the source at which this phenomenon manifests is contingent upon the positive and negative sound speed gradients. Furthermore, a surface duct can develop in both deep and moderately shallow waters under conditions of upward refraction, such as those caused by cold surface temperatures. Propagation within this duct occurs through successive reflections off the water's surface.

Underwater sound propagation generally entails a reduction in sound intensity with increasing range, although focusing effects can occasionally lead to intensity gains. Propagation loss (also termed transmission loss) quantifies the decrease in sound intensity between two points, typically the sound source and a distant receiver. If ${\displaystyle I_{s}}$ represents the far-field intensity of the source, referenced at 1 meter from its acoustic center, and ${\displaystyle I_{r}}$ denotes the intensity at the receiver, then propagation loss is calculated as ${\displaystyle {\mathit {PL}}=10\log(I_{s}/I_{r})}$. In this formula, ${\displaystyle I_{r}}$ signifies not the actual acoustic intensity at the receiver (which is a vector quantity), but rather a scalar value equivalent 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 possessing the same root-mean-square (RMS) pressure as the actual acoustic field. At shorter ranges, spreading primarily accounts for propagation loss, whereas at longer ranges, absorption and/or scattering losses become dominant.

An alternative definition can be formulated using pressure rather than intensity, expressed as ${\displaystyle {\mathit {PL}}=20\log(p_{s}/p_{r})}$. In this context, ${\displaystyle p_{s}}$ represents the root mean square (RMS) acoustic pressure in the projector's far-field, normalized to a standard distance of 1 meter. Conversely, ${\displaystyle p_{r}}$ denotes the RMS pressure at the receiver's location.

These two definitions are not precisely equivalent due to potential differences in characteristic impedance between the receiver and the source. Consequently, employing the intensity-based definition results in a distinct sonar equation compared to one derived from a pressure ratio. However, if both the source and receiver are situated in water, this discrepancy is minimal.

Propagation Modeling

The transmission of sound through aquatic environments is governed by the wave equation, subject to specific boundary conditions. To streamline propagation computations, several models have been devised, encompassing ray theory, normal mode solutions, and parabolic equation approximations of the wave equation. Each solution set typically offers validity and computational efficiency within a restricted frequency and range domain, potentially incorporating additional constraints. Ray theory is particularly suitable for short-range and high-frequency scenarios, whereas other solutions demonstrate superior performance at long ranges and low frequencies. Furthermore, various empirical and analytical formulas, derived from experimental measurements, provide valuable approximations.

Reverberation

Transient acoustic events generate a decaying background that can persist significantly longer than the initial signal. This background phenomenon, termed reverberation, arises from scattering off irregular boundaries and from biological entities such as fish and other aquatic organisms. For an acoustic signal to be readily detectable, its intensity must surpass both the reverberation level and the ambient background noise.

Doppler Shift

When an underwater object moves relative to an underwater receiver, the frequency of the detected sound deviates from the frequency of the sound emitted (or reflected) by the object. This alteration in frequency is termed a Doppler shift. This shift is readily observable in active sonar systems, especially narrow-band configurations, because the known transmitter frequency allows for the calculation of relative motion between the sonar and the target. In some instances, the frequency of radiated noise (a tonal signal) may also be known, enabling similar calculations for passive sonar. For active systems, the frequency change is approximately 0.69 Hz per knot per kHz, while for passive systems, it is half this value due to one-way propagation. An approaching target corresponds to an increase in the observed frequency.

Intensity Fluctuations

While acoustic propagation models typically forecast a consistent received sound level, practical observations reveal both temporal and spatial variations. These fluctuations can be attributed to environmental phenomena across various scales, including fine structures in sound speed profiles, frontal zones, and internal waves. Given that multiple propagation paths generally exist between a source and a receiver, minor phase shifts within the interference patterns among these paths can induce substantial fluctuations in sound intensity.

Non-linearity

In aqueous environments, particularly those containing air bubbles, the change in density resulting from pressure variations exhibits a non-linear relationship. Consequently, when a sinusoidal wave is introduced, additional harmonic and subharmonic frequencies emerge. The introduction of two sinusoidal waves results in the generation of sum and difference frequencies. This conversion process intensifies with higher source levels. Due to this non-linearity, sound speed is contingent upon pressure amplitude, causing larger pressure variations to propagate more rapidly than smaller ones. Consequently, a sinusoidal waveform progressively transforms into a sawtooth pattern, characterized by a rapid ascent and a gradual decay. This phenomenon is leveraged in parametric sonar applications, and theoretical frameworks, such as those proposed by Westerfield, have been developed to elucidate it.

Measurements

Underwater sound is quantified using a hydrophone, an instrument analogous to an aerial microphone. Hydrophones detect pressure fluctuations, which are typically translated into sound pressure level (SPL), a logarithmic metric representing the mean square acoustic pressure.

Acoustic measurements are generally presented in one of two formats:

• Root mean square (RMS) acoustic pressure, expressed in pascals, or as sound pressure level (SPL), quantified in decibels relative to 1 μPa.
• Spectral density, defined as the mean square pressure per unit bandwidth, is reported in pascals squared per hertz (dB re 1 μPa²/Hz).

The reference scale for acoustic pressure in aqueous environments diverges from that employed for airborne sound. Specifically, the reference pressure in air is 20 μPa, contrasting with 1 μPa in water. Consequently, for an identical numerical SPL value, the intensity of a plane wave (defined as power per unit area and proportional to the mean square sound pressure divided by acoustic impedance) in air is approximately 20²×3600 = 1,440,000 times greater than in water. Conversely, equivalent intensities are observed when the SPL in water is approximately 61.6 dB higher.

The ISO 18405 standard, published in 2017, delineates terminology and expressions pertinent to underwater acoustics, encompassing methodologies for calculating underwater sound pressure levels.

Sound speed

Under atmospheric pressure, the approximate sound speeds in fresh water and seawater are 1450 m/s and 1500 m/s, respectively, with corresponding densities of 1000 kg/m³ and 1030 kg/m³. The propagation speed of sound in water escalates with increments in pressure, temperature, and salinity. In pure water at atmospheric pressure, the maximum sound speed is achieved at approximately 74 °C; beyond this temperature, sound propagates more slowly in hotter water. This maximum speed further increases with elevated pressure.

Absorption

Extensive measurements have been conducted to quantify sound absorption in both lacustrine and oceanic environments.

Ambient noise

Acoustic signals can be measured only if their amplitude surpasses a minimum threshold. This threshold is partially dictated by the signal processing methodology employed and partially by the ambient background noise level. Ambient noise refers to the component of received noise that remains unaffected by the characteristics of the source, receiver, or platform. Consequently, it specifically excludes phenomena such as reverberation and towing noise.

Oceanic background noise, also known as ambient noise, originates from diverse sources and exhibits variability based on geographical location and frequency. Within the lowest frequency range, approximately 0.1 Hz to 10 Hz, ocean turbulence and microseisms constitute the principal contributors to the ambient noise spectrum. Characteristically, noise spectrum levels diminish with rising frequency, ranging from approximately 140 dB re 1 μPa²/Hz at 1 Hz to about 30 dB re 1 μPa²/Hz at 100 kHz. Distant maritime traffic represents a predominant noise source in many regions for frequencies around 100 Hz, whereas wind-generated surface noise serves as the primary contributor between 1 kHz and 30 kHz. At extremely high frequencies, exceeding 100 kHz, the thermal noise generated by water molecules becomes the dominant factor. The spectral level of thermal noise at 100 kHz measures 25 dB re 1 μPa§4^{5§/Hz. The spectral density of thermal noise exhibits an increase of 20 dB per decade, which approximates 6 dB per octave.}

Transient acoustic sources also contribute to the overall ambient noise. Such sources encompass intermittent geological phenomena, including earthquakes and submarine volcanoes, as well as surface rainfall and various forms of biological activity. Notable biological contributors comprise cetaceans (specifically blue, fin, and sperm whales), particular species of fish, and snapping shrimp.

Precipitation can generate substantial levels of ambient noise. Nevertheless, establishing a precise numerical correlation between rainfall rate and ambient noise level proves challenging, primarily due to the inherent difficulties in accurately measuring rainfall at sea.

Reverberation

Extensive measurements have been conducted on sea surface, bottom, and volume reverberation. Empirical models have occasionally been developed from these observations. A widely adopted formula for the 0.4 to 6.4 kHz frequency range is attributed to Chapman and Harris. Surface motion causes the frequency spreading of sinusoidal waveforms. For bottom reverberation, Lambert's Law frequently provides an approximate description, as noted by Mackenzie. Volume reverberation typically manifests in distinct layers, whose depths fluctuate with the diurnal cycle, a phenomenon documented by Marshall and Chapman. A rough ice undersurface can generate significant reverberation, as exemplified by Milne's work.

Bottom Loss

Bottom loss has been quantified across a range of frequencies and grazing angles in diverse geographical areas, including studies conducted by the US Marine Geophysical Survey. This loss is contingent upon the sound velocity within the seabed, which is influenced by gradients and stratification, as well as the bottom's roughness. Predictive graphs illustrating expected loss under specific conditions have been developed. In shallow water environments, bottom loss frequently exerts the primary influence on long-range sound propagation. At lower frequencies, sound is capable of traversing through the sediment before re-entering the water column.

Underwater Hearing

Comparison with Airborne Sound Levels

Similar to airborne sound, underwater sound pressure level (SPL) is conventionally expressed in decibels; however, inherent discrepancies render direct comparisons between aquatic and aerial SPL challenging and frequently unsuitable. These distinctions encompass:

• A divergent reference pressure: 1 μPa (one micropascal, equivalent to one millionth of a pascal) is utilized, contrasting with the 20 μPa standard for air.
• A disparity in interpretative methodologies: two primary perspectives exist, one advocating for direct pressure comparisons, while the other posits the necessity of initial conversion to the intensity of an equivalent plane wave.
• A variance in auditory sensitivity: any comparison involving (A-weighted) airborne sound must account for the differing hearing sensitivities of either a human diver or other animal species.

Human Hearing

Auditory Sensitivity

For a human diver possessing normal hearing, the minimum audible SPL is approximately 67 dB re 1 μPa, with peak sensitivity observed at frequencies near 1 kHz. This level corresponds to a sound intensity 5.4 dB, or 3.5 times, greater than the airborne threshold.

Safety Thresholds

Elevated underwater sound levels pose a potential risk to human divers. The SOLMAR project, conducted by the NATO Undersea Research Centre, has published guidelines for human diver exposure to underwater sound. Reports indicate that human divers subjected to SPL exceeding 154 dB re 1 μPa within the 0.6 to 2.5 kHz frequency range may exhibit alterations in heart rate or respiratory frequency. Diver aversion to low-frequency sound is contingent upon both the sound pressure level and the center frequency.

Other Species

Aquatic Mammals

Dolphins and other odontocetes are recognized for their exceptional auditory acuity, particularly within the 5 to 50 kHz frequency band. Multiple species exhibit hearing thresholds ranging from 30 to 50 dB re 1 μPa in this spectrum. For instance, the killer whale's hearing threshold is observed at an RMS acoustic pressure of 0.02 mPa (at a frequency of 15 kHz), which correlates to an SPL threshold of 26 dB re 1 μPa.

Elevated underwater sound levels present a potential hazard to both marine and amphibious fauna. Southall et al. have comprehensively reviewed the impacts of exposure to underwater noise.

Fish

Ladich and Fay have reviewed the auditory sensitivity of fish. The soldier fish exhibits a hearing threshold of 0.32 mPa (50 dB re 1 μPa) at 1.3 kHz. Popper et al. have examined the effects of exposure to underwater noise.

Crustaceans

The lobster possesses a hearing threshold of 1.3 Pa at 70 Hz, corresponding to 122 dB re 1 μPa.

Aquatic Birds

Observations indicate that several aquatic avian species respond to underwater sound within the 1–4 kHz range, aligning with their optimal airborne hearing sensitivities. Seaducks and cormorants have been conditioned to react to sounds between 1–4 kHz, exhibiting lowest hearing thresholds (highest sensitivity) of 71 dB re 1 μPa for cormorants and 105 dB re 1 μPa for seaducks. Diving avian species display distinct morphological variations in their auditory apparatus compared to terrestrial counterparts, implying specific adaptations of the ear in diving birds for aquatic environments.

Applications of Underwater Acoustics

Sonar

Sonar represents the acoustic counterpart to radar technology. This system employs sound pulses to survey marine environments, subsequently processing the resulting echoes to derive data concerning the ocean, its geological features, and any submerged entities. An alternative methodology, termed passive sonar, achieves similar objectives by detecting sounds emitted from underwater objects.

Underwater Communication

The imperative for underwater acoustic telemetry is evident across various applications, including data acquisition for environmental surveillance, communication protocols for both crewed and uncrewed underwater vehicles, and the transmission of diver vocalizations. A closely associated application is underwater remote control, where acoustic telemetry facilitates the remote activation of switches or the initiation of specific events. A notable instance of underwater remote control involves acoustic releases, which are devices designed to retrieve instrument packages or other payloads deployed on the seafloor to the surface upon receiving a remote command at the conclusion of a mission. Acoustic communication remains a dynamic area of research, confronting substantial challenges, particularly within horizontal, shallow-water channels.

In contrast to radio telecommunications, the available bandwidth for underwater acoustic communication is diminished by several orders of magnitude. The inherently low propagation speed of sound contributes to multipath propagation, extending over time delay intervals ranging from tens to hundreds of milliseconds, alongside pronounced Doppler shifts and spectral spreading. Frequently, acoustic communication systems are constrained not by ambient noise, but by reverberation and temporal variability that exceed the processing capabilities of current receiver algorithms. The reliability of underwater communication links can be substantially enhanced through the deployment of hydrophone arrays, which enable advanced processing methodologies such as adaptive beamforming and diversity combining.

Underwater Navigation and Tracking

Underwater navigation and tracking constitute essential requirements for exploration and operational activities conducted by divers, remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), crewed submersibles, and submarines. Distinct from most radio signals, which experience rapid attenuation in water, sound propagates extensively underwater at a rate that can be accurately measured or estimated. Consequently, sound can be utilized to precisely determine distances between a tracked object and one or more reference or baseline stations, thereby enabling the triangulation of the target's position, occasionally achieving centimeter-level accuracy. This principle led to the development of underwater acoustic positioning systems, widely adopted since the 1960s.

Seismic Exploration

Seismic exploration employs low-frequency sound (typically below 100 Hz) to penetrate deep into the seabed. Although these long wavelengths result in comparatively lower resolution, low-frequency sounds are favored because higher frequencies undergo significant attenuation during their propagation through the seafloor. Common sound sources utilized in this process include airguns, vibroseis systems, and explosives.

Weather and Climate Observation

Acoustic sensors offer capabilities for monitoring sounds generated by atmospheric phenomena such as wind and precipitation; for instance, Nystuen describes an acoustic rain gauge. Additionally, lightning strikes are detectable through acoustic means. The Acoustic Thermometry of Ocean Climate (ATOC) project utilizes low-frequency sound to assess global ocean temperatures.

Acoustical Oceanography

Acoustical oceanography involves the application of underwater sound for the comprehensive study of the marine environment, its geological interfaces, and its constituent elements.

History

Significant interest in the development of echo-ranging systems emerged subsequent to the sinking of the RMS Titanic in 1912. The underlying principle posited that by transmitting a sound wave forward from a vessel, a reflected echo from the submerged section of an iceberg could provide an early warning of potential collisions. Furthermore, by directing an identical beam downwards, the depth of the ocean floor could be precisely determined.

The inaugural practical deep-ocean echo sounder was developed by Harvey C. Hayes, a physicist affiliated with the U.S. Navy. This innovation enabled, for the first time, the creation of a quasi-continuous topographical profile of the ocean floor along a ship's trajectory. Hayes conducted the initial such profiling aboard the U.S.S. Stewart, a Navy destroyer, during its voyage from Newport to Gibraltar between June 22 and 29, 1922, accumulating 900 deep-ocean soundings within that week.

The German survey ship Meteor, employing a sophisticated echo sounder, conducted multiple transects across the South Atlantic from the equator to Antarctica between 1925 and 1927, acquiring soundings at intervals of 5 to 20 miles. This endeavor yielded the inaugural detailed cartography of the Mid-Atlantic Ridge, revealing it as a rugged mountain range rather than the smooth plateau some scientists had hypothesized. Subsequently, both naval and research vessels have consistently utilized echo sounders during their maritime operations.

Notable contributors to the field of acoustical oceanography include:

• Leonid Brekhovskikh
• Walter Munk
• Herman Medwin
• John L. Spiesberger
• C.C. Leroy
• David E. Weston
• D. Van Holliday
• Charles Greenlaw

Instrumentation

The initial and most prevalent application of sound and sonar technology for investigating marine properties involves the utilization of a rainbow echo sounder to ascertain water depth. Such sounders were instrumental in mapping extensive areas of the Santa Barbara Harbor seafloor until 1993.

Fathometers are instruments designed to measure water depths. Their operation involves electronically transmitting sound from vessels and subsequently receiving the acoustic waves reflected from the ocean floor. A calibrated paper chart within the fathometer records these depth measurements.

Technological advancements, particularly the emergence of high-resolution sonars in the latter half of the 20th century, enabled not only the detection but also the classification and imaging of submerged objects. Contemporary vessels and robotic submarines frequently deploy Remotely Operated Vehicles (ROVs) equipped with electronic sensors and cameras, providing oceanographers with clear and precise visual data. Sonars can also generate "images" by reflecting sound off marine environments. Furthermore, acoustic reflections from marine fauna often yield valuable data for comprehensive studies of animal behavior.

Passive Acoustic Monitoring

Hydrophones serve as passive listening devices in acoustical oceanography, facilitating the construction of an acoustic representation of the submerged soundscape.

Marine Biology

Given its superior propagation characteristics, underwater sound is an invaluable tool for investigating marine life, encompassing organisms from microplankton to baleen whales. Echo sounders frequently furnish data concerning marine organism abundance, spatial distribution, and behavioral patterns. These hydroacoustic instruments are also employed for determining fish location, population quantity, individual size, and overall biomass.

Acoustic telemetry is additionally utilized for the surveillance of fish and other marine wildlife. This method involves affixing an acoustic transmitter to an individual fish (occasionally internally), while a network of receivers detects the information encoded within the emitted sound wave. This methodology permits researchers to monitor the movements of individual organisms within localized to mesoscale environments.

Pistol shrimp generate sonoluminescent cavitation bubbles capable of attaining temperatures up to 5,000 K (4,700 °C).

Particle Physics

Neutrinos are fundamental particles characterized by their extremely weak interaction with other matter. Consequently, their detection necessitates exceptionally large-scale apparatus, for which the ocean occasionally serves as a medium. Specifically, it is hypothesized that ultra-high energy neutrinos within seawater can be acoustically detected.

Additional Applications

Further applications encompass:

• Rain rate measurement
• Wind speed measurement
• Global thermometry
• Monitoring of ocean-atmospheric gas exchange
• Surveillance Towed Array Sensor System
• Acoustic Doppler current profiler for water velocity measurement
• Acoustic camera
• Liquid sound
• Passive acoustic monitoring

• Echo sounder – Method of measuring the depth of waterPages displaying short descriptions of redirect targets
• Ocean exploration – Part of oceanography describing the exploration of ocean surfaces
• Refraction (sound) – Change of direction of propagation due to variation of velocity

Notes

References

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