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57-480: (Redirected from RE ) [REDACTED] Look up re  or re- in Wiktionary, the free dictionary. Re or RE may refer to: Arts, media and entertainment [ edit ] ...Re (film) , a 2016 Indian Kannada-language film Realencyclopädie der classischen Altertumswissenschaft , a German encyclopedia of classical scholarship Resident Evil ,

114-472: A chemical element Reference pressure (Re), as sound pressure level Reference pressure (Re), in underwater acoustics Computing and mathematics [ edit ] RE (complexity) (recursively enumerable), a complexity class of decision problems Recursively enumerable (r.e.), in computability theory Regular expression , a sequence of characters to match text against a specified pattern Re function, in mathematics, where Re( z ) denotes

171-411: A factor of 4.4 and the density ratio is about 820. Absorption of low frequency sound is weak. (see Technical Guides – Calculation of absorption of sound in seawater for an on-line calculator). The main cause of sound attenuation in fresh water, and at high frequency in sea water (above 100 kHz) is viscosity . Important additional contributions at lower frequency in seawater are associated with

228-635: A former Irish regional airline Places [ edit ] Re, Norway , a former municipality in Vestfold county Re, Vestland , a village in Gloppen municipality, Vestland county, Norway Re, Piedmont , an Italian municipality Île de Ré , an island off the west coast of France Le Bois-Plage-en-Ré , a commune on that island Re di Anfo , a torrent (seasonal stream) in Italy Re di Gianico, Re di Niardo, Re di Sellero, and Re di Tredenus, torrents in

285-529: A function of grazing angle for many frequencies in various locations, for example those by the US Marine Geophysical Survey. The loss depends on the sound speed in the bottom (which is affected by gradients and layering) and by roughness. Graphs have been produced for the loss to be expected in particular circumstances. In shallow water bottom loss often has the dominant impact on long range propagation. At low frequencies sound can propagate through

342-694: A horror game franchise Music [ edit ] Re, the second syllable of the scale in solfège D (musical note) or Re, the second note of the musical scale in fixed do solfège Re: (band) , a musical duo based in Canada and the US Albums [ edit ] Re (Café Tacuba album) Re (Les Rita Mitsouko album) Re. , by Aya Ueto Re: (EP) , by Kard Language [ edit ] re (interjection) , in Greek Re (kana) (れ and レ), Japanese syllables In re , Latin for 'in

399-413: A receiver, such as the human ear or a hydrophone , as changes in pressure . These waves may be man-made or naturally generated. The speed of sound c {\displaystyle c\,} (i.e., the longitudinal motion of wavefronts) is related to frequency f {\displaystyle f\,} and wavelength λ {\displaystyle \lambda \,} of

456-455: A sinusoidal waveform is spread in frequency due to the surface motion. For bottom reverberation a Lambert's Law is found often to apply approximately, for example see Mackenzie. Volume reverberation is usually found to occur mainly in layers, which change depth with the time of day, e.g., see Marshall and Chapman. The under-surface of ice can produce strong reverberation when it is rough, see for example Milne. Bottom loss has been measured as

513-658: A sound intensity 5.4 dB, or 3.5 times, higher than the threshold in air (see Measurements above). High levels of underwater sound create a potential hazard to human divers. Guidelines for exposure of human divers to underwater sound are reported by the SOLMAR project of the NATO Undersea Research Centre . Human divers exposed to SPL above 154 dB re 1 μPa in the frequency range 0.6 to 2.5 kHz are reported to experience changes in their heart rate or breathing frequency. Diver aversion to low frequency sound

570-485: A source and receiver, small phase changes in the interference pattern between these paths can lead to large fluctuations in sound intensity. In water, especially with air bubbles, the change in density due to a change in pressure is not exactly linearly proportional. As a consequence for a sinusoidal wave input additional harmonic and subharmonic frequencies are generated. When two sinusoidal waves are input, sum and difference frequencies are generated. The conversion process

627-774: A wave by c = f ⋅ λ {\displaystyle c=f\cdot \lambda } . This is different from the particle velocity u {\displaystyle u\,} , which refers to the motion of molecules in the medium due to the sound, and relates to the plane wave pressure p {\displaystyle p\,} to the fluid density ρ {\displaystyle \rho \,} and sound speed c {\displaystyle c\,} by p = c ⋅ u ⋅ ρ {\displaystyle p=c\cdot u\cdot \rho } . The product of c {\displaystyle c} and ρ {\displaystyle \rho \,} from

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684-544: Is 25 dB re 1 μPa /Hz. The spectral density of thermal noise increases by 20 dB per decade (approximately 6 dB per octave ). Transient sound sources also contribute to ambient noise. These can include intermittent geological activity, such as earthquakes and underwater volcanoes, rainfall on the surface, and biological activity. Biological sources include cetaceans (especially blue , fin and sperm whales), certain types of fish, and snapping shrimp . Rain can produce high levels of ambient noise. However

741-420: Is attained at about 74 °C; sound travels slower in hotter water after that point; the maximum increases with pressure. Many measurements have been made of sound absorption in lakes and the ocean (see Technical Guides – Calculation of absorption of sound in seawater for an on-line calculator). Measurement of acoustic signals are possible if their amplitude exceeds a minimum threshold, determined partly by

798-430: Is described by the wave equation, with appropriate boundary conditions. A number of models have been developed to simplify propagation calculations. These models include ray theory, normal mode solutions, and parabolic equation simplifications of the wave equation. Each set of solutions is generally valid and computationally efficient in a limited frequency and range regime, and may involve other limits as well. Ray theory

855-480: Is different from Wikidata All article disambiguation pages All disambiguation pages re">re The requested page title contains unsupported characters : ">". Return to Main Page . Underwater acoustics#Underwater hearing Hydroacoustics, using sonar technology, is most commonly used for monitoring of underwater physical and biological characteristics. Hydroacoustics can be used to detect

912-436: Is greater at high source levels than small ones. Because of the non-linearity there is a dependence of sound speed on the pressure amplitude so that large changes travel faster than small ones. Thus a sinusoidal waveform gradually becomes a sawtooth one with a steep rise and a gradual tail. Use is made of this phenomenon in parametric sonar and theories have been developed to account for this, e.g. by Westerfield. Sound in water

969-419: 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 two forms: The scale for acoustic pressure in water differs from that used for sound in air. In air

1026-426: Is more appropriate at short range and high frequency, while the other solutions function better at long range and low frequency. Various empirical and analytical formulae have also been derived from measurements that are useful approximations. Transient sounds result in a decaying background that can be of much larger duration than the original transient signal. The cause of this background, known as reverberation,

1083-460: 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

1140-409: Is partly due to scattering from rough boundaries and partly due to scattering from fish and other biota . For an acoustic signal to be detected easily, it must exceed the reverberation level as well as the background noise level . If an underwater object is moving relative to an underwater receiver, the frequency of the received sound is different from that of the sound radiated (or reflected) by

1197-518: Is possible in terms of pressure instead of intensity, giving P L = 20 log ⁡ ( p s / p r ) {\displaystyle {\mathit {PL}}=20\log(p_{s}/p_{r})} , where p s {\displaystyle 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 {\displaystyle p_{r}}

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1254-402: Is the rms wave height. A further complication is the presence of wind-generated bubbles or fish close to the sea surface. The bubbles can also form plumes that absorb some of the incident and scattered sound, and scatter some of the sound themselves. The acoustic impedance mismatch between water and the bottom is generally much less than at the surface and is more complex. It depends on

1311-407: Is the root mean square acoustic pressure. 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

1368-440: Is the RMS pressure at the receiver position. These two definitions are not exactly equivalent because the characteristic impedance at the receiver may be different from that at the source. Because of this, the use of the intensity definition leads to a different sonar equation to the definition based on a pressure ratio. If the source and receiver are both in water, the difference is small. The propagation of sound through water

1425-404: 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

1482-554: The Val Camonica , Lombardy, Italy Réunion (ISO 3166-1 code), a French overseas department and island in the Indian Ocean Science and technology [ edit ] Effective reproduction number , in epidemiology Relative effectiveness or RE factor, of an explosive's demolition power Reynolds number ( Re ), a dimensionless quantity in fluid mechanics used to help predict flow patterns Rhenium (symbol Re),

1539-436: The 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 meters. The presence of this minimum creates a special channel known as deep sound channel, or SOFAR (sound fixing and ranging) channel, permitting guided propagation of underwater sound for thousands of kilometers without interaction with

1596-496: 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. Thus it excludes reverberation and towing noise for example. The background noise present in the ocean, or ambient noise, has many different sources and varies with location and frequency. At the lowest frequencies, from about 0.1 Hz to 10 Hz, ocean turbulence and microseisms are

1653-412: The target strength of various simple shapes as a function of angle of sound incidence. More complex shapes may be approximated by combining these simple ones. Underwater acoustic propagation depends on many factors. The direction of sound propagation is determined by the sound speed gradients in the water. These speed gradients transform the sound wave through refraction, reflection, and dispersion. In

1710-395: 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 ) {\displaystyle I=q^{2}/(\rho c)\,} , where q {\displaystyle q\,}

1767-418: The bottom material types and depth of the layers. Theories have been developed for predicting the sound propagation in the bottom in this case, for example by Biot and by Buckingham. The reflection of sound at a target whose dimensions are large compared with the acoustic wavelength depends on its size and shape as well as the impedance of the target relative to that of water. Formulae have been developed for

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1824-592: The change in frequency is 0.69 Hz per knot per kHz and half this for passive systems as propagation is only one way. The shift corresponds to an increase in frequency for an approaching target. Though acoustic propagation modelling generally predicts a constant received sound level, in practice there are both temporal and spatial fluctuations. These may be due to both small and large scale environmental phenomena. These can include sound speed profile fine structure and frontal zones as well as internal waves. Because in general there are multiple propagation paths between

1881-481: The depth of a water body ( bathymetry ), as well as the presence or absence, abundance, distribution, size, and behavior of underwater plants and animals. Hydroacoustic sensing involves " passive acoustics " (listening for sounds) or active acoustics making a sound and listening for the echo, hence the common name for the device, echo sounder or echosounder . There are a number of different causes of noise from shipping. These can be subdivided into those caused by

1938-582: 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

1995-444: The field of underwater acoustics, including the calculation of underwater sound pressure levels. 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/m for the density. The speed of sound in water increases with increasing pressure , temperature and salinity . The maximum speed in pure water under atmospheric pressure

2052-409: The following, 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

2109-422: The ionic relaxation of boric acid (up to c. 10 kHz) and magnesium sulfate (c. 10 kHz-100 kHz). Sound may be absorbed by losses at the fluid boundaries. Near the surface of the sea losses can occur in a bubble layer or in ice, while at the bottom sound can penetrate into the sediment and be absorbed. Both the water surface and bottom are reflecting and scattering boundaries. For many purposes

2166-636: The matter of...' RE: and Re:, a standard email subject line prefix Organisations [ edit ] Renew Europe , a political group in the European Parliament Renovación Española , a former Spanish monarchist political party Royal Engineers , a part of the British Army Royal Society of Painter-Printmakers , whose fellows may use the Post-nominal letters RE Stobart Air (former IATA code: RE),

2223-404: The numerical relationship between rain rate and ambient noise level is difficult to determine because measurement of rain rate is problematic at sea. Many measurements have been made of sea surface, bottom and volume reverberation. Empirical models have sometimes been derived from these. A commonly used expression for the band 0.4 to 6.4 kHz is that by Chapman and Harris. It is found that

2280-437: The object. This change in frequency is known as a Doppler shift . The shift can be easily observed in active sonar systems, particularly narrow-band ones, because the transmitter frequency is known, and the relative motion between sonar and object can be calculated. Sometimes the frequency of the radiated noise (a tonal ) may also be known, in which case the same calculation can be done for passive sonar. For active systems

2337-566: The primary contributors to the noise background. Typical noise spectrum levels decrease with increasing frequency from about 140 dB re 1 μPa /Hz at 1 Hz to about 30 dB re 1 μPa /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

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2394-418: The propeller, those caused by machinery, and those caused by the movement of the hull through the water. The relative importance of these three different categories will depend, amongst other things, on the ship type. One of the main causes of hydro acoustic noise from fully submerged lifting surfaces is the unsteady separated turbulent flow near the surface's trailing edge that produces pressure fluctuations on

2451-631: The real part of a complex number z People [ edit ] Ré (futsal player) , Portuguese futsal player Andrea Re (born 1963), Italian rower Cayetano Ré (1938–2013), Paraguayan footballer Christopher Ré , American computer scientist Davide Re , Italian runner Germán Ré , Argentine footballer Giovanni Battista Re (born 1934), Italian cardinal Maximiliano Ré , Argentine-Italian footballer Paul Ré , American artist and writer Savannah Ré , Canadian singer-songwriter Vincenzo Re , Italian scenic designer Other uses [ edit ] Religious education (RE),

2508-651: The reduction in sound intensity between two points, normally the sound source and a distant receiver. If I s {\displaystyle I_{s}} is the far field intensity of the source referred to a point 1 m from its acoustic center and I r {\displaystyle I_{r}} is the intensity at the receiver, then the propagation loss is given by P L = 10 log ⁡ ( I s / I r ) {\displaystyle {\mathit {PL}}=10\log(I_{s}/I_{r})} . In this equation I r {\displaystyle I_{r}}

2565-514: The reference pressure is 20 μPa rather than 1 μPa. For the same numerical value of SPL, the intensity of a plane wave (power per unit area, proportional to mean square sound pressure divided by acoustic impedance) in air is about 20 ×3600 = 1 440 000 times higher than in water. Similarly, the intensity is about the same if the SPL is 61.6 dB higher in the water. The 2017 standard ISO 18405 defines terms and expressions used in

2622-492: The same term [REDACTED] This disambiguation page lists articles associated with the title Re . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Re&oldid=1250808682 " Categories : Disambiguation pages Disambiguation pages with surname-holder lists Place name disambiguation pages Hidden categories: Short description

2679-431: 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

2736-515: 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 sin 2 ⁡ A {\displaystyle R=-e^{-2k^{2}h^{2}\sin ^{2}A}} , where h

2793-561: 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

2850-416: 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

2907-486: The sediment then back into the water. As with airborne sound , sound pressure level underwater is usually reported in units of decibels , but there are some important differences that make it difficult (and often inappropriate) to compare SPL in water with SPL in air. These differences include: The lowest audible SPL for a human diver with normal hearing is about 67 dB re 1 μPa, with greatest sensitivity occurring at frequencies around 1 kHz. This corresponds to

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2964-543: The series Physics of Sound in the Sea , published in 1946. After World War II, the development of sonar systems was driven largely by the Cold War , resulting in advances in the theoretical and practical understanding of underwater acoustics, aided by computer-based techniques. A sound wave propagating underwater consists of alternating compressions and rarefactions of the water. These compressions and rarefactions are detected by

3021-494: The sound of a submerged ship's bell heard using an underwater listening horn. They measured a sound speed of 1435 metres per second over a 17 kilometre (km) distance, providing the first quantitative measurement of sound speed in water. The result they obtained was within about 2% of currently accepted values. In 1877 Lord Rayleigh wrote the Theory of Sound and established modern acoustic theory. The sinking of Titanic in 1912 and

3078-757: The start of World War I provided the impetus for the next wave of progress in underwater acoustics. Systems for detecting icebergs and U-boats were developed. Between 1912 and 1914, a number of echolocation patents were granted in Europe and the U.S., culminating in Reginald A. Fessenden 's echo-ranger in 1914. Pioneering work was carried out during this time in France by Paul Langevin and in Britain by A B Wood and associates. The development of both active ASDIC and passive sonar (SOund Navigation And Ranging) proceeded apace during

3135-563: The study of religion Re (Egyptian religion) , an ancient Egyptian god Regional-Express , a type of regional train in Germany, Luxembourg and Austria See also [ edit ] [REDACTED] Search for "re" on Misplaced Pages. Il re , Italian-language opera Rhee (disambiguation) Ree (disambiguation) All pages with titles beginning with Re All pages with titles containing Re Res (disambiguation) Ra (disambiguation) Topics referred to by

3192-662: The surface and unsteady oscillatory flow in the near wake. The relative motion between the surface and the ocean creates a turbulent boundary layer (TBL) that surrounds the surface. The noise is generated by the fluctuating velocity and pressure fields within this TBL. The field of underwater acoustics is closely related to a number of other fields of acoustic study, including sonar , transduction , signal processing , acoustical oceanography , bioacoustics , and physical acoustics . 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

3249-476: The war, driven by the first large scale deployments of submarines . Other advances in underwater acoustics included the development of acoustic mines . In 1919, the first scientific paper on underwater acoustics was published, theoretically describing the refraction of sound waves produced by temperature and salinity gradients in the ocean. The range predictions of the paper were experimentally validated by propagation loss measurements. The next two decades saw

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