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.
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.
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. 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. The result they obtained was within about 2% of currently accepted values. In 1877 Lord Rayleigh wrote the Theory of Sound and established modern acoustic theory.
The sinking of Titanic in 1912 and the start of World War 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. 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.
In 1919, the first scientific paper on underwater acoustics was published, 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.
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.
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.
This is different from the particle velocity , which refers to the motion of molecules in the medium due to the sound, and relates the plane wave pressure to the fluid density and sound speed by .
The product of and 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 , where 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 low frequency sound is weak. (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) and magnesium sulfate (c. 10 kHz-100 kHz).
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.
Both the water surface and bottom are reflecting and scattering boundaries.
For many purposes the sea-air surface can be thought of as a perfect reflector. The impedance contrast is so great that little energy is able to cross this boundary. Acoustic pressure waves reflected from the sea surface experience a reversal in phase, often stated as either a “pi phase change” or a “180 deg phase change”. This is represented mathematically by assigning a reflection coefficient of minus 1 instead of plus one to the sea surface.
At high frequency (above about 1 kHz) or when the sea is rough, some of the incident sound is scattered, and this is taken into account by assigning a reflection coefficient whose magnitude is less than one. For example, close to normal incidence, the reflection coefficient becomes , where h is the rms wave height.
A further complication is the presence of wind generated bubbles or fish close to the sea surface. The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves.
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  and by Buckingham.
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.
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 is the far field intensity of the source referred to a point 1 m from its acoustic centre and is the intensity at the receiver, then the propagation loss is given by . In this equation is not the true acoustic 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, giving , where is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and is the RMS pressure at the receiver position.
These two definitions are not exactly equivalent because the characteristic impedance at the receiver may be different from that at the source. Because of this, the use of the intensity definition leads to a different sonar equation to the definition based on a pressure ratio. If the source and receiver are both in water, the difference is small.
The propagation of sound through water is described by the wave equation, with appropriate boundary conditions. A number of models have been developed to simplify propagation calculations. These models include ray theory, normal mode solutions, and parabolic equation simplifications of the wave equation. 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. Various empirical and analytical formulae have also been derived from measurements that are useful approximations.
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.
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.
Though acoustic propagation modelling generally predicts a constant received sound level, in practice there are both temporal and spatial fluctuations. These may be due to both small and large scale environmental phenomena. These can include sound speed profile fine structure and frontal zones as well as internal waves. Because in general there are multiple propagation paths between a source and receiver, small phase changes in the interference pattern between these paths can lead to large fluctuations in sound intensity.
In water, especially with air bubbles, the change in density due to a change in pressure is not exactly linearly proportional. As a consequence for a sinusoidal wave input additional harmonic and subharmonic frequencies are generated. When two 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.
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 :-
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.
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. The speed of sound in water increases with increasing pressure, temperature and salinity. 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. On-line calculators can be found at Technical Guides – Speed of Sound in Sea-Water and Technical Guides – Speed of Sound in Pure Water.
Many measurements have been made of sound absorption in lakes and the ocean  (see Technical Guides – Calculation of absorption of sound in seawater for an on-line calculator).
Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by the signal 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. At the lowest frequencies, from about 0.1 Hz to 10 Hz, ocean turbulence and microseisms are the primary contributors to the noise background. 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).
Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes, rainfall on the surface, and biological activity. Biological sources include cetaceans (especially blue, fin and sperm whales), 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.
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. 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. Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman. The under-surface of ice can produce strong reverberation when it is rough, see for example Milne.
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. 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.
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:
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. This corresponds to a sound intensity 5.4 dB, or 3.5 times, higher than the threshold in air (see Measurements above).
High levels of underwater sound create a potential hazard to human divers. Guidelines for exposure of human divers to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre. 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.
Dolphins and other toothed whales are known for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz. 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.
The hearing sensitivity of fish is reviewed by Ladich and Fay. 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). The effects of exposure to underwater noise are reviewed by Popper et al.
Sonar is the name given to the acoustic equivalent of radar. Pulses of sound are used to probe the sea, and the echoes are then processed to extract information about the sea, its boundaries and submerged objects. An alternative use, known as passive sonar, attempts to do the same by listening to the sounds radiated by underwater objects.
The need for underwater acoustic 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  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. 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 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.
Acoustic sensors can be used to monitor the sound made by wind and precipitation. For example, an acoustic rain gauge is described by Nystuen. Lightning strikes can also be detected. Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature.
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.
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.
ANSI/ASA S1.1-2013, published by the American National Standards Institute (ANSI), is the current American National Standard on Acoustical Terminology. ANSI S1.1 was first published in 1960 and has its roots in a 1942 standard published by the American Standards Association, the predecessor of ANSI. It includes the following sections
Oscillation, vibration, and shock
Transmission and propagation
Transducers and linear systems
Acoustical apparatus and instruments
Sonics and ultrasonic testing
Physiological and psychological acoustics
Musical acousticsASA Silver Medal
The ASA Silver Medal is an award presented by the Acoustical Society of America to individuals, without age limitation, for contributions to the advancement of science, engineering, or human welfare through the application of acoustic principles or through research accomplishments in acoustics. The medal is awarded in a number of categories depending on the technical committee responsible for making the nomination.
Recipients of the medal are listed below.Acoustic seabed classification
Acoustic seabed classification is the partitioning of a seabed acoustic image into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics, underwater acoustics and benthic habitat mapping. Seabed classification is one route to characterizing the seabed and its habitats. Seabed characterization makes the link between the classified regions and the seabed physical, geological, chemical or biological properties. Acoustic seabed classification is possible using a wide range of acoustic imaging systems including multibeam echosounders, sidescan sonar, single-beam echosounders, interferometric systems and sub-bottom profilers. Seabed classification based on acoustic properties can be divided into two main categories; surficial seabed classification and sub-surface seabed classification. Sub-surface imaging technologies use lower frequency sound to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).Acoustical Society of America
The Acoustical Society of America (ASA) is an international scientific society founded in 1929 dedicated to generating, disseminating and promoting the knowledge of acoustics and its practical applications. The Society is primarily a voluntary organization of about 7500 members and attracts the interest, commitment, and service of many professionals.Acoustical engineering
Acoustical engineering (also known as acoustic 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.
One goal of acoustical engineering can be the reduction of unwanted noise, which is referred to as noise control. Unwanted noise can have significant impacts on animal and human health and well-being, reduce attainment by students in schools, and cause hearing loss. Noise control principles are implemented into technology and design in a variety of ways, including control by redesigning sound sources, the design of noise barriers, sound absorbers, suppressors, and buffer zones, and the use of hearing protection (earmuffs or earplugs).
But acoustical engineering is not just about noise control; it also covers positive uses of sound, from the use of ultrasound in medicine to the programming of digital sound synthesizers, and from designing a concert hall to enhance the sound of an orchestra to specifying a railway station's sound system so announcements are intelligible.Acoustics
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.
Hearing is one of the most crucial means of survival in the animal world, and speech is one of the most distinctive characteristics of human development and culture. Accordingly, the science of acoustics spreads across many facets of human society—music, medicine, architecture, industrial production, warfare and more. Likewise, animal species such as songbirds and frogs use sound and hearing as a key element of mating rituals or marking territories. Art, craft, science and technology have provoked one another to advance the whole, as in many other fields of knowledge. Robert Bruce Lindsay's 'Wheel of Acoustics' is a well accepted overview of the various fields in acoustics. Acoustic music is a genre of music using instruments that produce sound solely through acoustic means, without electronic amplification.Albert Beaumont Wood
Albert Beaumont Wood DSc (1890 – 19 July 1964), better known as A B Wood, was a British physicist, known for his pioneering work in the field of underwater acoustics and sonar. Wood is a physicist best known for his work on developing sonar (known at that time as 'ASDICS') in the UK from the First World War until after the Second World War.Bioacoustics
Bioacoustics is a cross-disciplinary science that combines biology and acoustics. Usually it refers to the investigation of sound production, dispersion and reception in animals (including humans). This involves neurophysiological and anatomical basis of sound production and detection, and relation of acoustic signals to the medium they disperse through. The findings provide clues about the evolution of acoustic mechanisms, and from that, the evolution of animals that employ them.
In underwater acoustics and fisheries acoustics the term is also used to mean the effect of plants and animals on sound propagated underwater, usually in reference to the use of sonar technology for biomass estimation. The study of substrate-borne vibrations used by animals is considered by some a distinct field called biotremology.European Conference on Underwater Acoustics
The European Conference on Underwater Acoustics (ECUA) was a conference on underwater acoustics that took place in Europe every two years, until 2012, when it was held in Edinburgh (Scotland), and organized by the Institute of Acoustics. Previous editions took place in Delft (Netherlands, 2004), Algarve, (Portugal, 2006) and Paris (France, 2008) and Istanbul (Turkey, 2010).Fred Tappert
Frederick Drach Tappert (April 21, 1940 – January 9, 2002) was an American physicist whose primary contributions were in underwater acoustics. He is noted for the development of the parabolic equation model and split-step Fourier algorithm for electromagnetic and ocean acoustic propagation.
Fred Tappert was born in April 1940 to Rev. Dr. Theodore Gerhardt Tappert and Helen Carson Tappert. As a child, Fred lived with his family on the campus of the Lutheran Theological Seminary in the Germantown neighborhood of Northwest Philadelphia. He attended Philadelphia's Central High School, and Pennsylvania State University. Growing up, his father "often mentioned the satisfaction that would result from the pursuit of knowledge for its own sake."
Tappert began his scientific career in the field of plasma physics, receiving his Ph.D. from Princeton University in 1967. His dissertation, entitled "Kinetic theory of equilibrium plasmas", was supervised by Edward A. Frieman, then Associate Director of the Princeton Plasma Physics Laboratory.
He was a member of the technical staff of Bell Telephone Labs from
1967 to 1974. Among his notable accomplishments there was a collaboration with Akira Hasegawa on optical solitons which underpinned later advances in fiber-optic communication technology.
Following his years at Bell Labs, Tappert was a Senior Research Scientist at the Courant Institute of New York University from
1974 to 1978. He moved to Coral Gables, Florida, in 1978 to join the faculty of the University of Miami, where he had a joint appointment in the Department of Physics on the main campus and in the Department of Applied Marine Physics at the Rosenstiel School of Marine and Atmospheric Science (RSMAS).
In 2001, he was awarded the Department of the Navy's Superior Public Service Award, the citation of which noted, "Professor Tappert's introduction of the parabolic equation propagation model in 1974 started a revolution in the
underwater acoustics modeling community. ... It is, in large part, a tribute to Professor Tappert's superb efforts that today the PE model is the de facto standard full wave propagation model in underwater acoustics and that, in a practical sense, he is thought of as the 'father of the PE model'."
Tappert was posthumously awarded the Pioneer in Underwater Acoustics Medal by the Acoustical Society of America, "for application of the parabolic equation to underwater acoustic propagation."
The 145th Annual Meeting of the Acoustical Society, in 2003, featured a memorial session dedicated to Frederick Tappert on the subject of "Propagation Phenomena and the Parabolic Equation."J. Lamar Worzel
J. Lamar (Joe) Worzel (February 21, 1919 – December 26, 2008) was an American geophysicist known for his important contributions to underwater acoustics, underwater photography, and gravity measurements at sea.Lloyd's mirror
Lloyd's mirror is an optics experiment that was first described in 1834 by Humphrey Lloyd in the Transactions of the Royal Irish Academy. Its original goal was to provide further evidence for the wave nature of light, beyond those provided by Thomas Young and Augustin-Jean Fresnel. In the experiment, light from a monochromatic slit source reflects from a glass surface at a small angle and appears to come from a virtual source as a result. The reflected light interferes with the direct light from the source, forming interference fringes. It is the optical wave analogue to a sea interferometer.Passive acoustics
Passive acoustics is the action of listening for sounds, often at specific frequencies or for purposes of specific analyses.
As applied to underwater acoustics, also termed hydroacoustics or sonar, passive acoustics can be used to listen for underwater explosions, earthquakes, volcanic eruptions, sounds produced by fish and other animals, vessel activity or aquatic detecting equipment (as in hydroacoustics to track fish).Pioneers of Underwater Acoustics Medal
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.Propagation loss
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.Sound speed gradient
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.When the sun warms the Earth's surface, there is a negative temperature gradient in atmosphere. The speed of sound decreases with decreasing temperature, so this also creates a negative sound speed gradient. The sound wave front travels faster near the ground, so the sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. The opposite effect happens when the ground is covered with snow, or in the morning over water, when the sound speed gradient is positive. In this case, sound waves can be refracted from the upper levels down to the surface.In underwater acoustics, speed of sound depends on pressure (hence depth), temperature, and salinity of seawater, thus leading to vertical speed gradients similar to those that exist in atmospheric acoustics. However, when there is a zero sound speed gradient, values of sound speed have the same "isospeed" in all parts of a given water column (there is no change in sound speed with depth). The same effect happens in an isothermal atmosphere with the ideal gas assumption.Structural acoustics
Structural acoustics is the study of the mechanical waves in structures and how they interact with and radiate into adjacent media. The field of structural acoustics is often referred to as vibroacoustics in Europe and Asia. People that work in the field of structural acoustics are known as structural acousticians. The field of structural acoustics can be closely related to a number of other fields of acoustics including noise, transduction, underwater acoustics, and physical acoustics.Transmission loss
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.
It is a terminology frequently used in optics and acoustics. Measures of TL are very important in the industry of acoustic devices such as mufflers and sonars.Ultrasonics (journal)
Ultrasonics is a bimonthly peer reviewed scientific journal published by Elsevier and covering research on theory and application of ultrasonics in physics, biology, chemistry, medicine, underwater acoustics, industry, materials characterization, control, and other disciplines. The journal was established in 1963 and the editor-in-chief is Prof. Zhongqing Su (The Hong Kong Polytechnic University).