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Urick Principles Of Underwater Sound Pdf 19 ##VERIFIED##

Sonar (sound navigation and ranging or sonic navigation and ranging)[2] is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances (ranging), communicate with or detect objects on or under the surface of the water, such as other vessels.[3]

urick principles of underwater sound pdf 19

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"Sonar" can refer to one of two types of technology: passive sonar means listening for the sound made by vessels; active sonar means emitting pulses of sounds and listening for echoes. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may also be used for robot navigation,[4] and SODAR (an upward-looking in-air sonar) is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). The study of underwater sound is known as underwater acoustics or hydroacoustics.

The use of sound to "echo-locate" underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912.[7] The world's first patent for an underwater echo-ranging device was filed at the British Patent Office by English meteorologist Lewis Fry Richardson a month after the sinking of Titanic,[8] and a German physicist Alexander Behm obtained a patent for an echo sounder in 1913.[9]

During World War I the need to detect submarines prompted more research into the use of sound. The British made early use of underwater listening devices called hydrophones, while the French physicist Paul Langevin, working with a Russian immigrant electrical engineer Constantin Chilowsky, worked on the development of active sound devices for detecting submarines in 1915. Although piezoelectric and magnetostrictive transducers later superseded the electrostatic transducers they used, this work influenced future designs. Lightweight sound-sensitive plastic film and fibre optics have been used for hydrophones, while Terfenol-D and lead magnesium niobate (PMN) have been developed for projectors.

Early in World War II (September 1940), British ASDIC technology was transferred for free to the United States. Research on ASDIC and underwater sound was expanded in the UK and in the US. Many new types of military sound detection were developed. These included sonobuoys, first developed by the British in 1944 under the codename High Tea, dipping/dunking sonar and mine-detection sonar. This work formed the basis for post-war developments related to countering the nuclear submarine.

During the 1930s American engineers developed their own underwater sound-detection technology, and important discoveries were made, such as the existence of thermoclines and their effects on sound waves.[17] Americans began to use the term SONAR for their systems, coined by Frederick Hunt to be the equivalent of RADAR.[18]

In 1917, the US Navy acquired J. Warren Horton's services for the first time. On leave from Bell Labs, he served the government as a technical expert, first at the experimental station at Nahant, Massachusetts, and later at US Naval Headquarters, in London, England. At Nahant he applied the newly developed vacuum tube, then associated with the formative stages of the field of applied science now known as electronics, to the detection of underwater signals. As a result, the carbon button microphone, which had been used in earlier detection equipment, was replaced by the precursor of the modern hydrophone. Also during this period, he experimented with methods for towing detection. This was due to the increased sensitivity of his device. The principles are still used in modern towed sonar systems.

Other types of transducers include variable-reluctance (or moving-armature, or electromagnetic) transducers, where magnetic force acts on the surfaces of gaps, and moving coil (or electrodynamic) transducers, similar to conventional speakers; the latter are used in underwater sound calibration, due to their very low resonance frequencies and flat broadband characteristics above them.[23]

Active sonar is also used to measure distance through water between two sonar transducers or a combination of a hydrophone (underwater acoustic microphone) and projector (underwater acoustic speaker). When a hydrophone/transducer receives a specific interrogation signal it responds by transmitting a specific reply signal. To measure distance, one transducer/projector transmits an interrogation signal and measures the time between this transmission and the receipt of the other transducer/hydrophone reply. The time difference, scaled by the speed of sound through water and divided by two, is the distance between the two platforms. This technique, when used with multiple transducers/hydrophones/projectors, can calculate the relative positions of static and moving objects in water.

Sound waves travel differently through fish than through water because a fish's air-filled swim bladder has a different density than seawater. This density difference allows the detection of schools of fish by using reflected sound. Acoustic technology is especially well suited for underwater applications since sound travels farther and faster underwater than in air. Today, commercial fishing vessels rely almost completely on acoustic sonar and sounders to detect fish. Fishermen also use active sonar and echo sounder technology to determine water depth, bottom contour, and bottom composition.

The value of underwater acoustics to the fishing industry has led to the development of other acoustic instruments that operate in a similar fashion to echo-sounders but, because their function is slightly different from the initial model of the echo-sounder, have been given different terms.

Powerful low frequency echo-sounders have been developed for providing profiles of the upper layers of the ocean bottom. One of the most recent devices is Innomar's SES-2000 quattro multi-transducer parametric SBP, used for example in the Puck Bay for underwater archaeological purposes[53]

During last decades, anthropogenic underwater sound and its chronic impact on marine species have been recognised as an environmental protection challenge. At the same time, studies on the spatial and temporal variability of ambient sound, and how it is affected by biotic, abiotic and anthropogenic factors are lacking. This paper presents analysis of a large-scale and long-term underwater sound monitoring in the Baltic Sea. Throughout the year 2014, sound was monitored in 36 Baltic Sea locations. Selected locations covered different natural conditions and ship traffic intensities. The 63 Hz, 125 Hz and 2 kHz one-third octave band sound pressure levels were calculated and analysed. The levels varied significantly from one monitoring location to another. The annual median sound pressure level of the quietest and the loudest location differed almost 50 dB in the 63 Hz one-third octave band. Largest difference in the monthly medians was 15 dB in 63 Hz one-third octave band. The same monitoring locations annual estimated probability density functions for two yearly periods show strong similarity. The data variability grows as the averaging time period is reduced. Maritime traffic elevates the ambient sound levels in many areas of the Baltic Sea during extensive time periods.

Setting up sound monitoring programmes is the first step in the assessment of the levels of anthropogenic underwater sound in marine habitats. Initially, they help to establish the baseline levels of sound. Both deep-ocean observatories12,13 and autonomous recording systems14,15,16 have been used for monitoring. Ideally the monitoring of underwater sound can be imagined to be a network of cabled monitoring stations that sufficiently cover a given marine area. However, costs limit this kind of ambition and a realistic monitoring programme will entail only a few monitoring locations. This has the drawback of not being representative of the whole marine area. In order to circumvent this limitation, sound propagation modelling is commonly used in combination with the monitoring. Modelling helps to estimate the spatial extent of sounds from various anthropogenic sources (mainly ships) in a given marine environment17,18.

In order to assess the levels of underwater sound in a big marine area that is the Baltic Sea a joint international cross-bordering effort is needed. First project of this kind was Life+ Baltic Sea Information on the Acoustic Soundscape (BIAS) launched in 201219. The main aim of the BIAS project was the characterisation of the Baltic Sea soundscape. The modeled soundscape was mapped and a planning tool for using the maps developed20. Therefore, the monitored sound data was primarily used to calibrate and ground truth the sound propagation modelling. Although the modelling provides information about spatial sound level distribution, the data gathered during a sound monitoring programme are potentially a valuable source of additional information. When compared to modelling, it possesses higher resolution and is the most accurate representation of the actual sound levels in a location. Therefore, the data from sound monitoring in a marine area are worthy of being analysed separately and more thoroughly. There is a large number of open questions related to long-term monitoring of sound that need to be answered. Consequently, this paper aims to provide a detailed analysis of the extensive BIAS sound monitoring data. Moreover, the recorded data can serve as a baseline of the Baltic Sea underwater ambient sound that can be used for tracking changes in the following years.

The annual underwater SPL values in different locations are concisely presentable by the estimated probability density function (PDF). A good way to compare different probability density functions of the one-third octave band SPL values is to compile them in the form of violin plots. As common for probability density functions, the area of each violin plot equals unity. The abscissa of the plots shows the probability for the occurrence of the SPL value displayed on the vertical axis. Various statistical measures are added to the violin plots for making them visually comparable and readable. The added statistical measures are the geometric mean (GM) and exceedance levels L5, L10, L25, L50, L75, L90, and L95. In the case of sound monitoring, exceedance level L95 is a low SPL value that is exceeded 95% of the time. Therefore, L95 can be related to the infrequent quieter natural sound levels. The exceedance level L5 is the SPL value that is exceeded only 5% of the time. In most cases it is related to occasional louder events when vessels pass close by the monitoring location. The statistical analysis presented hereafter was performed using bespoke software written in the programming language R27. 350c69d7ab


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