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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 and its boundaries. The water may be in the ocean, a lake 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.

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,^[1]

"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."Template:Vague

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 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.

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.

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 transmission loss measurements.

The next two decades saw the development of several applications of underwater acoustics. The fathometer, or depth sounder, was developed commercially during the 1920s. 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

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 $c$ (i.e., the longitudinal motion of wavefronts) is related to frequency $f$ and wavelength $λ$ of a wave by $c = f \cdot λ$ .

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

The product of $c$ and $ρ$ 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 $I = q^{2} / (ρ c)$ , where $q$ 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-500 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.

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 $R = - e^{- 2 k^{2} h^{2} s i n^{2} A}$ , where h is the rms wave height.^[8]

A further complication is the presence of wind generated bubbles or fish close to the sea surface.^[9] The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves.^[10]

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 ^[11] and by Buckingham.^[12]

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. 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 $I_{s}$ is the far field intensity of the source referred to a point 1 m from its acoustic centre and $I_{r}$ is the intensity at the receiver, then the propagation loss is given by^[1] $P L = 10 l o g (I_{s} / I_{r})$ . In this equation $I_{r}$ 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,^[13] giving $P L = 20 l o g (p_{s} / p_{r})$ , where $p_{s}$ is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and $p_{r}$ 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.^[14] 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.^[15] 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.^[16] Various empirical and analytical formulae have also been derived from measurements that are useful approximations.^[17]

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 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 per hertz (dB re 1 μPa²/Hz)

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.^[18] The speed of sound in water increases with increasing pressure, temperature and salinity.^[19]^[20] 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.^[21] 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] ^[22] (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.^[13]
difference in interpretation: there are two schools of thought, one maintaining that pressures should be compared directly, and that the other that one should first convert to the intensity of an equivalent plane wave.
difference in hearing sensitivity: any comparison with (A-weighted) sound in air needs to take into account the differences in hearing sensitivity, either of a human diver or other animal.^[35]

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] Dolphins and other toothed whales are renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz.^[35]^[37] 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.^[38] By comparison the most sensitive fish is the soldier fish, whose threshold is 0.32 mPa (50 dB re 1 μPa) at 1.3 kHz, whereas the lobster has a hearing threshold of 1.3 Pa at 70 Hz (122 dB re 1 μPa).^[38]

Safety thresholds

High levels of underwater sound create a potential hazard to marine and amphibious animals as well as to human divers.^[35]^[39] Guidelines for exposure of human divers and marine mammals to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre.^[40] 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.^[41]

Applications of underwater acoustics

Sonar

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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 ^[42]^[43] 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

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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.^[44] 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

Mining Engineer (Excluding Oil ) Truman from Alma, loves to spend time knotting, largest property developers in singapore developers in singapore and stamp collecting. Recently had a family visit to Urnes Stave Church. 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.^[45] Lightning strikes can also be detected.^[46]Acoustic thermometry of ocean climate (ATOC) uses low frequency sound to measure the global ocean temperature.

Oceanography

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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

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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 fishes 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.^[47]

Pistol shrimp create sonoluminescent cavitation bubbles that reach up to Template:Convert ^[48]

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.^[49]

References

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External links

Template:Hydroacoustics Template:Physical oceanography

↑ ^1.0 ^1.1 ^1.2 Urick, Robert J. Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill, 1983.
↑ C. S. Clay & H. Medwin, Acoustical Oceanography (Wiley, New York, 1977)
↑ Annales de Chimie et de Physique 36 [2] 236 (1827)
↑ 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).
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↑ ^6.0 ^6.1 ^6.2 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).
↑ 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).
↑ H. Medwin & C. S. Clay, Fundamentals of Acoustical Oceanography (Academic, Boston, 1998).
↑ D. E. Weston & P. A. Ching, Wind effects in shallow-water transmission, J. Acoust. Soc. Am. 86, 1530-1545 (1989).
↑ 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)
↑ N Chotiros, Biot Model of Sound Propagation in Water Saturated Sand. J. Acoust. Soc. Am. 97, 199 (1995)
↑ 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).
↑ ^13.0 ^13.1 C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).
↑ M. A. Ainslie, The sonar equation and the definitions of propagation loss, J. Acoust. Soc. Am. 115, 131-134 (2004).
↑ F. B. Jensen, W. A. Kuperman, M. B. Porter & H. Schmidt, Computational Ocean Acoustics (AIP Press, NY, 1994).
↑ C. H. Harrison, Ocean propagation models, Applied Acoustics 27, 163-201 (1989).
↑ L. M. Brekhovskikh & Yu. P. Lysanov, Fundamentals of Ocean Acoustics, 3rd edition (Springer-Verlag, NY, 2003).
↑ A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989).
↑ Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. 70, 807-812 (1982).
↑ 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)
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In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
↑ 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).
↑ G. M. Wenz, Acoustic ambient noise in the ocean: spectra and sources, J. Acoust. Soc. Am. 34, 1936-1956 (1962).
↑ S. C. Webb, The equilibrium oceanic microseism spectrum, J. Acoust. Soc. Am. 92, 2141-2158 (1992).
↑ R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. 24, 478-480 (1952).
↑ 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).
↑ 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).
↑ Ocean Noise and Marine Mammals, National Research Council of the National Academies (The National Academies Press, Washington DC, 2003).
↑ R Chapman and J Harris, Surface backscattering Strengths Measured with Explosive Sound Sources. J. Acoust. Soc. Am. 34, 547 (1962)
↑ K Mackenzie, Bottom Reverberation for 530 and 1030 cps Sound in Deep Water. J. Acoust. Soc. Am. 36, 1596 (1964)
↑ 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)
↑ A. Milne, Underwater Backscattering Strengths of Arctic Pack Ice. J. Acoust. Soc. Am. 36, 1551 (1964)
↑ MGS Station Data Listing and Report Catalog, Nav Oceanog Office Special Publication 142, 1974
↑ D.M.F. Chapman, D.D. Ellis, The elusive decibel - thoughts on sonars and marine mammals, Can. Acoust. 26(2), 29–31 (1996)
↑ ^35.0 ^35.1 ^35.2 W. J. Richardson, C. R. Greene, C. I. Malme and D. H. Thomson, Marine Mammals and Noise (Academic Press, San Diego, 1995).
↑ 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).
↑ W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).
↑ ^38.0 ^38.1 D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005)
↑ One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
↑ NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, NURC Special Publication NURC-SP-2006-008, September 2006
↑ One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
↑ D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4-27 (2000).
↑ M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003.
↑ Underwater Acoustic Positioning Systems, P.H. Milne 1983, ISBN 0-87201-012-0
↑ J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, 18(10), 1640-1657 (2001).
↑ R. D. Hill, Investigation of lightning strikes to water surfaces, J. Acoust. Soc. Am. 78, 2096-2099 (1985).
↑ 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.
↑ One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang
↑ 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

[Urick-1] 1.0 ^1.1 ^1.2 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] One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang

[FrancoisGarrison1982b-6] 6.0 ^6.1 ^6.2 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).

[FrancoisGarrison1982a-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] H. Medwin & C. S. Clay, Fundamentals of Acoustical Oceanography (Academic, Boston, 1998).

[9] D. E. Weston & P. A. Ching, Wind effects in shallow-water transmission, J. Acoust. Soc. Am. 86, 1530-1545 (1989).

[10] 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)

[11] N Chotiros, Biot Model of Sound Propagation in Water Saturated Sand. J. Acoust. Soc. Am. 97, 199 (1995)

[12] 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).

[Morfey-13] 13.0 ^13.1 C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).

[Ainslie2004-14] M. A. Ainslie, The sonar equation and the definitions of propagation loss, J. Acoust. Soc. Am. 115, 131-134 (2004).

[15] F. B. Jensen, W. A. Kuperman, M. B. Porter & H. Schmidt, Computational Ocean Acoustics (AIP Press, NY, 1994).

[16] C. H. Harrison, Ocean propagation models, Applied Acoustics 27, 163-201 (1989).

[17] L. M. Brekhovskikh & Yu. P. Lysanov, Fundamentals of Ocean Acoustics, 3rd edition (Springer-Verlag, NY, 2003).

[18] A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989).

[19] Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. 70, 807-812 (1982).

[20] 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)

[21] One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang

[FrancoisGarrison1982a.<ref_name_=-22] 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).

[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)

[Richardson1995-35] 35.0 ^35.1 ^35.2 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] W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).

[SimmondsMacLennan2005-38] 38.0 ^38.1 D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005)

[39] One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang

[40] NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures, NURC Special Publication NURC-SP-2006-008, September 2006

[41] One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang

[42] D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4-27 (2000).

[43] M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003.

[44] Underwater Acoustic Positioning Systems, P.H. Milne 1983, ISBN 0-87201-012-0

[45] J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, 18(10), 1640-1657 (2001).

[46] R. D. Hill, Investigation of lightning strikes to water surfaces, J. Acoust. Soc. Am. 78, 2096-2099 (1985).

[47] 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.

[48] One of the biggest reasons investing in a Singapore new launch is an effective things is as a result of it is doable to be lent massive quantities of money at very low interest rates that you should utilize to purchase it. Then, if property values continue to go up, then you'll get a really high return on funding (ROI). Simply make sure you purchase one of the higher properties, reminiscent of the ones at Fernvale the Riverbank or any Singapore landed property Get Earnings by means of Renting

In its statement, the singapore property listing - website link, government claimed that the majority citizens buying their first residence won't be hurt by the new measures. Some concessions can even be prolonged to chose teams of consumers, similar to married couples with a minimum of one Singaporean partner who are purchasing their second property so long as they intend to promote their first residential property. Lower the LTV limit on housing loans granted by monetary establishments regulated by MAS from 70% to 60% for property purchasers who are individuals with a number of outstanding housing loans on the time of the brand new housing purchase. Singapore Property Measures - 30 August 2010 The most popular seek for the number of bedrooms in Singapore is 4, followed by 2 and three. Lush Acres EC @ Sengkang

Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

Extending the tax exemption would help. The exemption, which may be as a lot as $2 million per family, covers individuals who negotiate a principal reduction on their existing mortgage, sell their house short (i.e., for lower than the excellent loans), or take part in a foreclosure course of. An extension of theexemption would seem like a common-sense means to assist stabilize the housing market, but the political turmoil around the fiscal-cliff negotiations means widespread sense could not win out. Home Minority Chief Nancy Pelosi (D-Calif.) believes that the mortgage relief provision will be on the table during the grand-cut price talks, in response to communications director Nadeam Elshami. Buying or promoting of blue mild bulbs is unlawful.

A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

There are various methods to go about discovering the precise property. Some local newspapers (together with the Straits Instances ) have categorised property sections and many local property brokers have websites. Now there are some specifics to consider when buying a 'new launch' rental. Intended use of the unit Every sale begins with 10 p.c low cost for finish of season sale; changes to 20 % discount storewide; follows by additional reduction of fiftyand ends with last discount of 70 % or extra. Typically there is even a warehouse sale or transferring out sale with huge mark-down of costs for stock clearance. Deborah Regulation from Expat Realtor shares her property market update, plus prime rental residences and houses at the moment available to lease Esparina EC @ Sengkang

[49] 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

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-Hi there, I am Alyson Boon although it is not the title on my birth certification. Kentucky is where I've always been residing. Distributing manufacturing is exactly where her primary income comes from. The preferred hobby for him and his children is to perform lacross and he'll be beginning something else along with it.<br><br>Review my weblog: free psychic reading ([http://kard.dk/?p=24252 Recommended Internet page])
+[[Image:SCM4 fig11.jpg|thumb|right|300px|Output of a computer model of underwater acoustic propagation in a simplified ocean environment.]]'''Underwater acoustics''' is the study of the propagation of [[sound]] in [[water]] and the interaction of the [[mechanical wave]]s that constitute sound with the water and its boundaries. The water may be in the ocean, a lake or a tank. Typical frequencies associated with underwater acoustics are between 10 [[Hertz|Hz]] and 1 [[MHz]].  The propagation of sound in the ocean at frequencies lower than 10&nbsp;Hz is usually not possible without penetrating deep into the seabed, whereas frequencies above 1&nbsp;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]], [[Transducer|transduction]], [[acoustic signal processing]], [[acoustical oceanography]], [[bioacoustics]], and [[physical acoustics]].
+==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,<ref name="Urick">Urick, Robert J. ''Principles of Underwater Sound, 3rd Edition.'' New York. McGraw-Hill, 1983.</ref>
+:"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."{{vague|without motors, say what sound he must have been talking about|date=December 2013}}
+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 [[Jean-Daniel Colladon|Daniel Colladon]], a [[Swiss]] [[physicist]], and [[Charles Sturm]], a [[France|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.<ref>C. S. Clay & H. Medwin, Acoustical Oceanography (Wiley, New York, 1977)</ref> They measured a sound speed of 1435 metres per second over a 17 kilometre distance, providing the first quantitative measurement of sound speed in water.<ref>[[Annales de Chimie et de Physique]] 36 [2] 236 (1827)</ref> 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 [[RMS Titanic|''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 [[iceberg]]s and [[U-boat]]s were developed. Between 1912 and 1914, a number of [[Sonar|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 [[Albert Beaumont Wood|A B Wood]] and associates.<ref>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).</ref> 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 [[Naval mine|mines]].
+In 1919, the first scientific paper on underwater acoustics was published,<ref>{{cite journal|author= H. Lichte| year = 1919 | title = On the influence of horizontal temperature layers in sea water on the range of underwater sound signals | journal =Physik. Z. | volume = 17 | issue = 385}}</ref> 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 [[transmission loss]] measurements.
+The next two decades saw the development of several applications of underwater acoustics. The [[fathometer]], or depth sounder, was developed commercially during the 1920s. 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===
+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]] <math>c \,</math> (i.e., the longitudinal motion of wavefronts) is related to frequency <math>f \,</math> and wavelength <math>\lambda \,</math> of a wave by <math>c = f \cdot \lambda</math>.
+This is different from the particle velocity <math>u \,</math>, which refers to the motion of molecules in the medium due to the sound, and relates the plane wave the pressure <math>p \,</math> to the fluid density <math>\rho \,</math> and sound speed <math>c \,</math> by <math>p = c \cdot u \cdot \rho</math>.
+The product of <math>c</math> and <math>\rho \,</math> from the above formula is known as the [[acoustic impedance|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 <math>I = q^2/(\rho c) \,</math>, where <math>q \,</math> is the [[root mean square]] acoustic pressure.
+At 1&nbsp;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&nbsp;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.<ref name = "FrancoisGarrison1982b">R. E. Francois & G. R. Garrison, Sound absorption based on ocean measurements. Part II: Boric acid contribution and equation for total absorption, [[Journal of the Acoustical Society of America|J. Acoust. Soc. Am.]] '''72''', 1879-1890 (1982).</ref>  (see [http://www.npl.co.uk/acoustics/techguides/seaabsorption/ 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&nbsp;kHz) is [[viscosity]].  Important additional contributions at lower frequency in seawater are associated with the ionic relaxation of [[boric acid]] (up to c. 10&nbsp;kHz)<ref name="FrancoisGarrison1982b"/> and [[magnesium sulfate]] (c. 10&nbsp;kHz-500&nbsp;kHz).<ref name = "FrancoisGarrison1982a">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).</ref>
+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.
+At high frequency (above about 1&nbsp;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 <math>R=-e^{-2 k^{2} h^{2} sin^2A}</math>, where ''h'' is the [[Root mean square|rms]] wave height.<ref>H. Medwin & C. S. Clay, Fundamentals of Acoustical Oceanography (Academic, Boston, 1998).</ref>
+A further complication is the presence of wind generated bubbles or fish close to the sea surface.<ref>D. E. Weston & P. A. Ching, Wind effects in shallow-water transmission, J. Acoust. Soc. Am. 86, 1530-1545 (1989).</ref>  The bubbles can also form [[plume (hydrodynamics)|plume]]s that absorb some of the incident and scattered sound, and scatter some of the sound themselves.<ref>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)</ref>
+=====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 <ref>N Chotiros, Biot Model of Sound Propagation in Water Saturated Sand. J. Acoust. Soc. Am. '''97''', 199 (1995)</ref> and by Buckingham.<ref>M. J. Buckingham, Wave propagation, stress relaxation, and grain-to-grain shearing in saturated, unconsolidated marine sediments, [[Journal of the Acoustical Society of America|J. Acoust. Soc. Am.]] '''108''', 2796-2815 (2000).</ref>
+====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.<ref name="Urick"/>
+===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. 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 channel|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 <math>I_s</math> is  the far field intensity of the source referred to a point 1 m from its acoustic centre and <math>I_r</math> is the intensity at the receiver, then the propagation loss is given by<ref name="Urick"/> <math>PL=10log (I_s/I_r)</math>.
+In this equation <math>I_r</math> is not the true [[acoustic intensity]] at the receiver, which is a [[Vector (geometric)|vector]] quantity, but a [[Scalar (mathematics)|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,<ref name="Morfey"/> giving <math>PL=20 log (p_s/p_r)</math>, where <math>p_s</math>  is the RMS acoustic pressure in the far-field of the projector, scaled to a standard distance of 1 m, and <math>p_r</math>  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.<ref name = "Ainslie2004">M. A. Ainslie, The sonar equation and the definitions of propagation loss, J. Acoust. Soc. Am. '''115''', 131-134 (2004).</ref>  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 [[Fred Tappert|parabolic equation]] simplifications of the wave equation.<ref>F. B. Jensen, W. A. Kuperman, M. B. Porter & H. Schmidt, Computational Ocean Acoustics (AIP Press, NY, 1994).</ref> 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.<ref>C. H. Harrison, Ocean propagation models, Applied Acoustics '''27''', 163-201 (1989).</ref>  Various empirical and analytical formulae have also been derived from measurements that are useful approximations.<ref>L. M. Brekhovskikh & Yu. P. Lysanov, Fundamentals of Ocean Acoustics, 3rd edition (Springer-Verlag, NY, 2003).</ref>
+====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 (ecology)|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 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 [[Tonality|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&nbsp;Hz per [[knot (unit)|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 :-
+* [[Root mean square|RMS]] acoustic pressure in micropascals (or dB re 1 μPa)
+* RMS acoustic pressure in a specified [[Bandwidth (signal processing)|bandwidth]], usually [[octaves]] or thirds of octave (dB re 1 μPa)
+* [[spectral density]] (mean square pressure per unit bandwidth) in micropascals per [[hertz]] (dB re 1 μPa²/Hz)
+===Sound speed===
+Approximate values for [[fresh water]] and [[seawater]], respectively, at atmospheric pressure are 1450 and 1500&nbsp;m/s for the sound speed, and 1000 and 1030&nbsp;kg/m³ for the density.<ref>A. D. Pierce, Acoustics: An Introduction to its Physical Principles and Applications (American Institute of Physics, New York, 1989).</ref>  The speed of sound in water increases with increasing [[pressure]], [[temperature]] and [[salinity]].<ref>Mackenzie, Nine-term equation for sound speed in the oceans, J. Acoust. Soc. Am. '''70''', 807-812 (1982).</ref><ref>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)</ref>  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.<ref>{{cite journal|last=Wilson|first=Wayne D.|title=Speed of Sound in Distilled Water as a Function of Temperature and Pressure|journal=J. Acoust. Soc. Am.|date=26 Jan 1959|volume=31|issue=8|pages=1067–1072|doi=10.1121/1.1907828|url=http://asadl.org/jasa/resource/1/jasman/v31/i8/p1067_s1|accessdate=11 February 2012|bibcode = 1959ASAJ...31.1067W }}</ref>
+On-line calculators can be found at [http://www.npl.co.uk/acoustics/techguides/soundseawater/ Technical Guides - Speed of Sound in Sea-Water] and [http://www.npl.co.uk/acoustics/techguides/soundpurewater/ Technical Guides - Speed of Sound in Pure Water].
+===Absorption===
+Many measurements have been made of sound absorption in lakes and the ocean
+<ref name="FrancoisGarrison1982b"/> <ref name="FrancoisGarrison1982a.<ref name = "FrancoisGarrison1982a">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).</ref>  (see [http://www.npl.co.uk/acoustics/techguides/seaabsorption/ 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.<ref>G. M. Wenz, Acoustic ambient noise in the ocean: spectra and sources, J. Acoust. Soc. Am. '''34''', 1936-1956 (1962).</ref> At the lowest frequencies, from about 0.1&nbsp;Hz to 10&nbsp;Hz, ocean [[turbulence]] and [[seismology|microseisms]] are the primary contributors to the noise background.<ref>S. C. Webb, The equilibrium oceanic microseism spectrum, J. Acoust. Soc. Am. '''92''', 2141-2158  (1992).</ref> Typical noise spectrum levels decrease with increasing frequency from about 140 dB re 1 μPa²/Hz at 1&nbsp;Hz to about 30 dB re 1 μPa²/Hz at 100&nbsp;kHz. Distant ship traffic is one of the dominant noise sources in most areas for frequencies of around 100&nbsp;Hz, while wind-induced surface noise is the main source between 1&nbsp;kHz and 30&nbsp;kHz. At very high frequencies, above 100&nbsp;kHz, [[thermal noise]] of water molecules begins to dominate. The thermal noise spectral level at 100&nbsp;kHz is 25 dB re 1 μPa²/Hz.  The spectral density of thermal noise increases by 20 dB per [[Decade (log scale)|decade]] (approximately 6 dB per [[octave]]).<ref>R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. '''24''', 478-480 (1952).</ref>
+Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes,<ref>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).</ref> rainfall on the surface, and biological activity. Biological sources include [[cetacea]]ns (especially [[blue whale|blue]], [[fin whale|fin]] and [[sperm whale|sperm]] whales),<ref>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).</ref><ref>Ocean Noise and Marine Mammals, National Research Council of the National Academies (The National Academies Press, Washington DC, 2003).</ref> certain types of fish, and [[alpheidae|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&nbsp;kHz is that by Chapman and Harris.<ref>R  Chapman and J Harris, Surface backscattering Strengths Measured with Explosive Sound Sources.  J. Acoust. Soc. Am. '''34''', 547 (1962)</ref> 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.<ref>K Mackenzie, Bottom Reverberation for 530 and 1030 cps Sound in Deep Water. J. Acoust. Soc. Am. '''36''', 1596 (1964)</ref>  Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman.<ref>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)</ref>  The under-surface of ice can produce strong reverberation when it is rough, see for example [[Milne]].<ref>A. Milne, Underwater Backscattering Strengths of Arctic Pack Ice. J. Acoust. Soc. Am. '''36''', 1551 (1964)</ref>
+===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.<ref>MGS Station Data Listing and Report Catalog, Nav Oceanog Office Special Publication 142, 1974</ref>  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:<ref>D.M.F. Chapman, D.D. Ellis, The elusive decibel - thoughts on sonars and marine mammals, Can. Acoust. '''26'''(2), 29–31 (1996)</ref>
+* difference in reference pressure: 1 [[Pascal (unit)|μPa]]  (one micropascal, or one [[millionth]] of a [[Pascal (unit)|pascal]]) instead of 20 μPa.<ref name="Morfey">C. L. Morfey, Dictionary of Acoustics (Academic Press, San Diego, 2001).</ref>
+* difference in interpretation: there are two schools of thought, one maintaining that pressures should be compared directly, and that the other that one should first convert to the [[intensity (physics)|intensity]] of an equivalent plane wave.
+* difference in [[audiogram|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.<ref name = "Richardson1995">W. J. Richardson, C. R. Greene, C. I. Malme and D. H. Thomson, Marine Mammals and Noise (Academic Press, San Diego, 1995).</ref>
+===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&nbsp;kHz.<ref>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).</ref>  [[Dolphins]] and other [[toothed whales]] are renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50&nbsp;kHz.<ref name="Richardson1995"/><ref>W. W. L. Au, The Sonar of Dolphins (Springer, NY, 1993).</ref>  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 [[Root mean square|RMS]] [[acoustic pressure]] of 0.02 mPa (and frequency 15&nbsp;kHz), corresponding to an SPL threshold of 26 dB re 1 μPa.<ref name="SimmondsMacLennan2005">D. Simmonds & J. MacLennan, Fisheries Acoustics: Theory and Practice, 2nd edition (Blackwell, Oxford, 2005)</ref>  By comparison the most sensitive fish is the [[Soldier (fish)|soldier fish]], whose threshold is 0.32 mPa (50 dB re 1 μPa) at 1.3&nbsp;kHz, whereas the [[lobster]] has a hearing threshold of 1.3 Pa at 70&nbsp;Hz (122 dB re 1 μPa).<ref name="SimmondsMacLennan2005"/>
+===Safety thresholds===
+High levels of underwater sound create a potential hazard to marine and amphibious animals as well as to human divers.<ref name="Richardson1995"/><ref>{{cite journal |author=Steevens CC, Russell KL, Knafelc ME, Smith PF, Hopkins EW, Clark JB |title=Noise-induced neurologic disturbances in divers exposed to intense water-borne sound: two case reports |journal=Undersea Hyperb Med |volume=26 |issue=4 |pages=261–5 |year=1999 |pmid=10642074 |doi= |url=http://archive.rubicon-foundation.org/2333 |accessdate=2009-03-31}}</ref>  Guidelines for exposure of human divers and marine mammals to underwater sound are reported by the SOLMAR project of the [[NATO Undersea Research Centre]].<ref>[http://solmar.nurc.nato.int/solmar/PDF/NURC-SP-2006-008.pdf NATO Undersea Research Centre Human Diver and Marine Mammal Risk Mitigation Rules and Procedures], NURC Special Publication NURC-SP-2006-008, September 2006</ref>  Human divers exposed to SPL above 154 dB re 1 μPa in the frequency range 0.6 to 2.5&nbsp;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]].<ref>{{cite journal |author=Fothergill DM, Sims JR, Curley MD |title=Recreational scuba divers' aversion to low-frequency underwater sound |journal=Undersea Hyperb Med |volume=28 |issue=1 |pages=9–18 |year=2001 |pmid=11732884 |doi= |url=http://archive.rubicon-foundation.org/2368 |accessdate=2009-03-31}}</ref>
+==Applications of underwater acoustics==
+===Sonar===
+{{Main|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===
+{{Main|Underwater acoustic communication}}
+The need for underwater acoustic [[telemetry]] exists in applications such as data harvesting for environmental monitoring, communication with and between manned and [[Autonomous Underwater Vehicle|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 Release|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
+<ref>D. B. Kilfoyle and A. B. Baggeroer, "The state of the art in underwater acoustic telemetry," IEEE J. Oceanic Eng. 25, 4-27 (2000).</ref><ref>M.Stojanovic, "Acoustic (Underwater) Communications," entry in Encyclopedia of Telecommunications, John G. Proakis, Ed., John Wiley & Sons, 2003.</ref> 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 effect|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 scheme|diversity]] combining.
+===Underwater Navigation and Tracking===
+{{Main|Underwater Acoustic Positioning System}}
+Underwater navigation and tracking is a common requirement for exploration and work by divers, [[Remotely operated underwater vehicle|ROV]], [[Autonomous underwater vehicle|autonomous underwater vehicles (AUV)]], [[Submersible|manned submersibles]] and [[submarine]]s alike. Unlike most radio signals which are quickly absorbed, sound propagates far underwater and at a rate that can be precisely measured or estimated.<ref>Underwater Acoustic Positioning Systems, P.H. Milne 1983, ISBN 0-87201-012-0</ref> 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 System|underwater acoustic positioning systems]] which are now widely used.
+===Seismic exploration===
+{{Main|Reflection seismology}}
+Seismic exploration involves the use of low frequency sound (< 100&nbsp;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 [[Air gun (seismic)#Air gun|airguns]], [[vibroseis]] and [[explosives]].
+===Weather and climate observation===
+Acoustic sensors can be used to monitor the sound made by [[wind]] and [[Precipitation (meteorology)|precipitation]].  For example, an acoustic [[rain gauge]] is described by Nystuen.<ref>J. A. Nystuen, Listening to raindrops from underwater: An acoustic disdrometer, J Atmospheric and Oceanic Technology, '''18'''(10), 1640-1657 (2001).</ref> Lightning strikes can also be detected.<ref>R. D. Hill, Investigation of lightning strikes to water surfaces, J. Acoust. Soc. Am. '''78''', 2096-2099 (1985).</ref>[[Acoustic Thermometry of Ocean Climate|Acoustic thermometry of ocean climate]] (ATOC) uses low frequency sound to measure the global ocean temperature.
+===Oceanography===
+{{Expand section|date=May 2008}}
+{{Main|Acoustical oceanography}}
+Large scale ocean features can be detected by [[ocean acoustic tomography|acoustic tomography]].  Bottom characteristics can be measured by [[side-scan sonar]] and [[Sonar#Sub-bottom profiling|sub-bottom profiling]].
+===Marine biology===
+{{Main|Bioacoustics}}
+Due to its excellent propagation properties, underwater sound is used as a tool to aid the study of marine life, from [[Plankton|microplankton]] to the [[blue whale]].  [[Echo sounding|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 fishes 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.<ref>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.</ref>
+[[Alpheidae|Pistol shrimp]] create [[sonoluminescence|sonoluminescent]] cavitation bubbles that reach up to {{convert|5000|K|C|-2}} <ref>{{cite journal |author=D. Lohse, B. Schmitz & M. Versluis |title=Snapping shrimp make flashing bubbles |journal=[[Nature (journal)|Nature]] |volume=413 |issue=6855 |year=2001 |pages=477–478 |doi=10.1038/35097152 |url=http://www.nature.com/nature/journal/v413/n6855/abs/413477a0.html |pmid=11586346}}</ref>
+===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.<ref>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</ref>
+==See also==
+* [[Acoustical Oceanography]]
+* [[Acoustic tag|Acoustic Tags (Acoustic Telemetry)]]
+* [[Bioacoustics]]
+* [[Hydroacoustics]]
+* [[Ocean Tracking Network]]
+* [[Sonar]]
+* [[Underwater Acoustic Positioning System]]
+* [[SOFAR channel]]
+* [[Underwater acoustic wireless communication system]]
+==References==
+{{Reflist|2}}
+==External links==
+* [http://oalib.hlsresearch.com/ Ocean Acoustics Library]
+* [http://www.isvr.soton.ac.uk/FDAG/uaua.HTM Ultrasonics and Underwater Acoustics]
+* [http://www.pmel.noaa.gov/vents/acoustics.html Monitoring the global ocean through underwater acoustics]
+* [http://sonar-fs.lboro.ac.uk/newsite/index.php Underwater Acoustics Research]
+* [http://www.apl.washington.edu/projects/ASA-UATC/index.php ASA Underwater Acoustics Technical Committee]
+* [http://oceanlink.island.net/oinfo/acoustics/acoustics.html An Ocean of Sound]
+* [http://www.arl.nus.edu.sg/web/research/acomms Underwater Acoustic Communications]
+* [http://acomms.whoi.edu/ Acoustic Communications Group] at the [[Woods Hole Oceanographic Institution]]
+* [http://www.oceanexplorer.noaa.gov/explorations/sound01/sound01.html Sound in the Sea]
+* [http://www.physics.sfsu.edu/~bland/research/uarg.html SFSU Underwater Acoustics Research Group]
+* [http://www.dosits.org/ Discovery of Sound in the Sea]
+* [http://www.pambuoy.co.uk/ PAMBuoy Passive Acoustic Monitoring]
+{{hydroacoustics|state=expanded}}
+{{physical oceanography|expanded=none}}
+{{DEFAULTSORT:Underwater Acoustics}}
+[[Category:Acoustics]]

Parabolic Lie algebra: Difference between revisions

Latest revision as of 10:49, 12 July 2013

History

Theory

Sound waves in water

Speed of sound, density and impedance

Absorption of sound

Sound Reflection and Scattering

Boundary interactions

Surface

Seabed

At Target

Propagation of sound

Propagation modelling

Reverberation

Doppler Shift

Intensity Fluctuations

Non-linearity

Measurements

Sound speed

Absorption

Ambient noise

Reverberation

Bottom Loss

Underwater hearing

Comparison with airborne sound levels

Hearing sensitivity

Safety thresholds

Applications of underwater acoustics

Sonar

Underwater communication

Underwater Navigation and Tracking

Seismic exploration

Weather and climate observation

Oceanography

Marine biology

Particle physics

See also

References

External links

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