Dissertations / Theses on the topic 'Underwater sea ambient noise'

2012/2013
Sotto la superficie del mare il suono svolge un ruolo fondamentale nella vita di molti organismi marini, in quanto fornisce una visuale in tre dimensioni dello spazio circostante il singolo individuo, che si estende spesso ben oltre quello fornito dagli altri sensi. L’introduzione da parte dell’uomo di diverse tipologie di rumori in questo ambiente, quindi, desta sempre maggiori preoccupazioni, poiché qualsiasi cosa alteri la capacità di individuare e analizzare il panorama acustico circostante può interferire negativamente con la comunicazione, il comportamento, la fitness e, in termini generali, con la sopravvivenza delle specie. La posizione strategica occupata dal golfo di Trieste, un bacino di acque relativamente poco profonde situato nel Nord Adriatico, unitamente alle caratteristiche geomorfologiche delle sue coste, fanno sì che qui possano svilupparsi molteplici attività che dipendono fortemente dal mare, come quella mercantile, alieutica e diportistica. Considerata la facilità di propagazione dell’onda sonora nell’acqua e tenendo conto che il rumore non conosce “barriere” giurisdizionali, le specie che vivono in esso saranno inevitabilmente sottoposte a pressioni di diversa portata, sia di tipo diffuso che puntuale. Nonostante la Comunità Europea, grazie alla Direttiva 2008/56/CE (Direttiva Quadro per l’ambiente marino, Marine Strategy Framework Directive, MSFD)cerchi di fornire gli strumenti per far fronte a questa preoccupante problematica che insiste sulle risorse marine, si sa ancora molto poco sulla distribuzione spaziale e temporale del rumore antropico subacqueo, sia nel golfo di Trieste che in Italia. Il presente lavoro di ricerca, svolto in collaborazione con l’Agenzia Regionale per la Protezione dell’Ambiente del Friuli Venezia Giulia (ARPA FVG),si è posto il fine di colmare le lacune conoscitive in tale ambito ed ha voluto dare 1) un quadro dettagliato della distribuzione annuale del rumore antropico subacqueo in tutto il golfo di Trieste, 2) individuare, grazie ad esso, in termini spazio-temporali, eventuali aree di “sofferenza acustica” per la fauna marina normalmente presente nell’area e, infine, 3) valutare, tramite l’utilizzo di un modello di propagazione del rumore, le modalità sito-specifiche di propagazione del rumore, simulando scenari a diverse frequenze e in diverse stagioni dell’anno. A tal fine il rumore ambientale subacqueo è stato registrato mensilmente da gennaio a dicembre 2012 in 12 stazioni collocate in posizioni strategiche nel golfo di Trieste, valutando contemporaneamente anche il numero di navi, imbarcazioni e natanti presenti al momento della registrazione. La perdita in trasmissione del suono e stata calcolata utilizzando la Parabolic Equation, risolta col modello di propagazione acustica Miami Monterey Parabolic Equation(MMPE). I risultati evidenziano un’assenza di variabilità tra il clima acustico estivo e quello invernale, con un’intensità media è pari a 125 dB re 1 µPa e con picchi di massima intensità in prossimità del porto di Trieste e della zona al largo di Lignano; le intensità medie delle bande di 1/3 di ottava centrate sui 63 e 125 Hz, invece, sono sempre inferiori ai 100 dB re 1 µPa. A livello spaziale la zona caratterizzata dai va-lori di minore intensità è posizionata nella parte occidentale del golfo. La frequentazione antropica è in gran parte a carico del naviglio mercantile e dei natanti da diporto di piccole dimensioni. Esaminando l’andamento nella stagione estiva e in quella invernale, non è possibile rilevare differenze significative nelle diverse tipologie considerate, fatto che sembra giustificare l’assenza di variazione stagionale del clima acustico. A livello spaziale, nelle tre zone considerate, sia annualmente che d’inverno, si notano differenze significative solo nel numero delle imbarcazioni da pesca. In generale, le grandi navi sono quelle che danno il maggior apporto al rumore ambientale locale. I Gadidae, Clupeiformes e Sciaenidae, nelle zone orientali e centrali del golfo di Trieste, sono gli organismi sottoposti al maggior superamento, da parte del rumore di fondo, della rispettiva soglia acustica. Le differenze maggiori si riscontrano per lo più tra i 200 ed i 300 Hz circa, dove si colloca la maggior sensibilità uditiva di molte specie. Proprio in questo range di frequenze il modello MMPE indica la minima perdita in propagazione dell’onda sonora, che può raggiungere anche i 20 km di distanza dalla sorgente. Il modello ha permesso di evidenziare, quindi, che nelle vicinanze di forti sorgenti di rumore potrebbero aver luogo reazioni di tipo comportamentale e, che, per avere quadro più esaustivo, sarebbe consigliato monitorare altre frequenze oltre alle 63 e 125 Hz attualmente proposte. I risultati di questa ricerca, prima in Adriatico su scala spazio-temporale così ampia, hanno fornito una dettagliata analisi delle pressioni, dei potenziali impatti predominanti nell’area e delle condizioni di clima acustico in cui versa il golfo di Trieste. Per rispondere alle richieste della MSFD, i valori di intensità rilevati non possono escludere che siano a livelli tali da non avere effetti negativi sull’ambiente marino: possono verificarsi, infatti, effetti di tipo fisiologico-stressorio a livello del singolo organismo, e di interferenza nella comunicazione nelle specie che utilizzano il suono come strumento di trasferimento di informazione intra e interspecifico. Si ritiene che i valori di riferimento proposti in questo lavoro, in un’ottica precauzionale, siano un valido contributo iniziale per la determinazione dello stato ecologico dell’area. L’attuale prosecuzione dell’attività di monitoraggio del rumore sottomarino condotta da ARPA FVG, da affiancare in futuro a sistemi di acquisizione in continuo ed all’analisi di altre componenti del fenomeno acustico, quali il movimento delle particelle, permetterà sicuramente di ampliare, unitamente ad un confronto con le realtà transfrontaliere, le conoscenze sul rumore antropico. Ciò permetterà di regolamentare, anche da un punto di vista giuridico, l’introduzione del suono sotto la superficie del mare e di raggiungere gli obiettivi della MSFD previsti entro il 2020.
Under the sea surface sound plays a vital role for many marine organisms, as it provides a visual three-dimensional space surrounding the individual, which is often extends beyond that provided by other senses. Introduction by humans of different types of noise in this environment, therefore, affects the ability to identify and analyze the landscape surrounding noise may cause harmful interference with communication, behavior, fitness and, in general terms, with the species’ survival. The strategic position of Trieste Gulf, a shallow water coastal zone located inthe Northern Adriatic Sea, together with the geomorphological characteristics of its coasts, can develop a variety of activities that are highly dependent on the sea, like the merchant , fishing and pleasure boating. Given the ease of propagation of the sound wave in the water and taking into account that the noise does not know jurisdictionalbarriers, the species that live in it will inevitably be subjected to pressures of different scales ,both of which diffuse on time. Despite the European Union, thanks to 2008/56/EC Marine Strategy Framework Directive (MSFD ) seeks to provide the tools to cope with this troubling issue that insists on marine resources , is not yet known very little about the spatial and temporal distribution of anthropogenic underwater noise , both in the Gulf of Trieste in Italy. This research work was performed in collaboration with the Regional Agency for Environmental Protection of Friuli Venezia Giulia (ARPA FVG), place the order to fill gaps in knowledge in this area and wanted to give 1 ) a framework detailed annual distribution of background underwater noise in the Gulf of Trieste , 2 ) to identify, thanks to it, in terms of space and time, any areas of suffering acoustic for marine life normally present in the area and, finally,3 ) to assess, through the use of a model of noise propagation, the site-specific mode of propagation of noise, simulating scenarios at different frequencies and in different seasons of the year. Underwater ambient noise was recorded monthly from January to December 2012 at 12 stations placed at strategic locations in the Gulf of Trieste; at the same time total amount of ships, boats and vessels present at the time of registration were counted. Transmission loss was calculated using the Parabolic Equation, solved with the model of acoustic propagation Monterey Miami Parabolic Equation (MMPE). Results show an absence of the noise climate variability between summer and winter, with an average intensity level equals to 125 dB re 1 Pa and a maximum in the vicinity of the port of Trieste and the area off the coast of Lignano; the average intensities of the bands in 1/3 octave band centered on 63 and 125 Hz, however, are always less than 100 dB re 1 Pa. A spatially area characterized by the values of lower intensity is located in the western part of the Gulf. The attendance is largely anthropogenic load of merchant ships and small recreational boat. Looking at the summer and winter trend, it is not possible to detect significant differences in the various types considered, which seems to justify the absence of seasonal variation of the noise climate. In terms of space, in the three areas considered, both annual and winter, significant differences are noted only in the number of fishing vessels. In general, large ships are the ones that make the greatest contribution to local environmental noise. The Gadidae, Clupeiformes and Sciaenidae, in the eastern and central parts of the Gulf of Trieste, are the organisms subjected to the most overrun by the background noise of the respective acoustic threshold. The largest differences are found mostly between about 200 and 300 Hz, where does the greater auditory sensitivity of many species. In this frequency range MMPE model indicates minimal loss in sound propagation, which can reach up to 20 km away from the source. The model has allowed to show, therefore, that in the vicinity of strong noise sources could take place, and behavioral reactions, which, in order to have more complete picture, it would be advisable to monitor other frequencies in addition to the 63 and 125 Hz currently proposed. The results of this research, first in the Adriatic Sea onspatio-temporal scale so large, they have provided a detailed analysis of the pressures, the potential impacts of the conditions prevailing in the area and of the acoustic climate prevailing in the Gulf of Trieste. To meet the requirements of the MSFD, the intensity values measured cannot rule out that they are at levels that do not have adverse effects on the marine environment can occur, in fact, the effects of physiological stressorio - level of the individual organism, and interference in communication in species that use sound as a tool for intra-and interspecies transfer of information. It is believed that the reference values proposed in this work, from a precautionary measure, are a valuable contribution to the initial determination of the ecological status of the area. The current continuation of the monitoring of the underwater noise conducted by ARPA FVG, alongside in future systems of continuous acquisition and analysis of other components of the acoustic phenomenon, such as the movement of particles, will certainly broaden , together with a comparison with the realities of cross border knowledge about man-made noise. This will allow you to regulate, even from a legal point of view, the introduction of sound in the sea surface and to achieve the objectives of the MSFD expected by 2020.
XXVI Ciclo
1978

Long-Range and high ambient noise underwater communication reliability improvement via sea trail performance validation of whole-of-system algorithm and engineering optimisations. The resulting 10 dB performance improvement can be used to increase the transmit source level by up to 10 dB, extend communication range by 50%, operate in environments with up to 10 dB louder ambient noise, reduce hydro-acoustic noise pollution or lowering battery power consumption with the potential of ocean powered networked communication.

Thesis (M.S.)--George Mason University, 2008.
Vita: p. 69. Thesis director: Kathleen E. Wage. Submitted in partial fulfillment of the requirements for the degree of Master of Science in Electrical Engineering. Title from PDF t.p. (viewed Aug. 27, 2008). Includes bibliographical references (p. 67-68). Also issued in print.

Recent theoretical and experimental studies have demonstrated that an estimate of the Green's function between two hydrophones can be extracted passively from the cross‐correlation of ambient noise recorded at these two points. Hence monitoring the temporal evolution of these estimated Green's functions can provide a means for noise‐based acoustic tomography using a distributed sensor network. However, obtaining unbiased Green's function estimate requires a sufficiently spatially and temporally diffuse ambient noise field. Broadband ambient noise ([200 Hz-20 kHz]) was recorded continuously for 2 days during the SWAMSI09 experiment (next to Panama City, FL) using two moored vertical line arrays (VLAs) spanning 7.5m of the 20‐m water column and separated by 150 m. The feasibility of noise‐based acoustic tomography ([300-1000 Hz]) was assessed in this dynamic coastal environment over the whole recording period. Furthermore, coherent array processing of the computed ocean noise cross‐correlations between all pairwise combinations of hydrophones was used to separate acoustic variations between the VLAs caused by genuine environmental fluctuations-such as internal waves-from the apparent variations in the same coherent arrivals caused when the ambient noise field becomes strongly directional, e.g., due to an isolated ship passing in the vicinity of the VLAs.

In passive sonar systems, knowledge of low-frequency shipping noise is an important factor for target detection performance. However, an accurate model for the shipping noise structure is difficult to obtain, due to the varying distributions of ships and complicated underwater environment. This work characterizes low-frequency distant shipping noise observed in deep water environments as a function of receiver depth and vertical arrival structure for the case of a receiver below the conjugate depth. Surface shipping noise is examined using Monte Carlo simulations using a normal mode propagation model based on random distribution of ships and realistic parameters. The depth dependence of the simulated distant shipping noise is in agreement with published experimental measurements. A Vertical Line Array (VLA) is used to produce vertical beams that isolate the surface interference from nearby targets. Simulation results quantifying the beamformer output as a function of ocean environment, receiver aperture, and frequency are presented for both conventional and adaptive beamformers. The results suggest that conventional beamforming could detect the noisy target from both direct arrival and bottom bounce in the presence of distant shipping interferers and wind noise. However, the beamwidth of conventional beamforming is wider than that of adaptive beamforming. Once the motion effects of nearby ship interferences are considered, the adaptive beamforming using diagonal loading provides better detection performance. Preliminary adaptive beamforming results corresponding to different snapshot times show that motion effects can be minimized by using short observation times.

Accurate modeling of acoustic propagation in the ocean waveguide is important to SONAR-performance prediction, and requires, particularly in shallow water environments, characterizing the bottom reflection loss with a precision that databank-based modeling cannot achieve. Recent advances in the technology of autonomous underwater vehicles (AUV) make it possible to envision a survey system for seabed characterization composed of a short array mounted on a small AUV. The bottom power reflection coefficient (and the related reflection loss) can be estimated passively by beamforming the naturally occurring marine ambient-noise acoustic field recorded by a vertical line array of hydrophones. However, the reduced array lengths required by small AUV deployment can hinder the process, due to the inherently poor angular resolution. In this dissertation, original data-processing techniques are presented which, by introducing into the processing chain knowledge derived from physics, can improve the performance of short arrays in this particular task. Particularly, the analysis of a model of the ambient-noise spatial coherence function leads to the development of a new proof of the result at the basis of the bottom reflection-loss estimation technique. The proof highlights some shortcomings inherent in the beamforming operation so far used in this technique. A different algorithm is then proposed, which removes the problem achieving improved performance. Furthermore, another technique is presented that uses data from higher frequencies to estimate the noise spatial coherence function at a lower frequency, for sensor spacing values beyond the physical length of the array. By "synthesizing" a longer array, the angular resolution of the bottom-loss estimate can be improved, often making use of data at frequencies above the array design frequency, otherwise not utilized for beamforming. The proposed algorithms are demonstrated both in simulation and on real data acquired during several experimental campaigns.

Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km.
Master of Science

The distribution, abundance and behaviour of soniferous organisms influence the spatial variability of underwater noise characteristics or ‘soundscape’. In this way, the soundscape provides useful information on habitat and assemblages that enables navigation in larvae and can be used for environmental assessment and monitoring. Despite the ecological importance of sea urchins and evidence they produce sound of their sound, knowledge gaps remain regarding the role of adult and larval sea urchins in acoustic ecology. In this thesis the sounds produced by 3 temperate and 3 tropical species of sea urchins were characterised. The soundscape of sea urchin habitat from important ecological systems (sea grass, temperate rocky reef, coral reef) in eastern Australia were also characterised. The sea urchins made a ‘crunching’ sound when feeding that was commonly produced around dawn, dusk or midnight. This sound ranged from 2-22 kHz with highest intensity from 2-8 kHz. Centrostephanus rodgersii appeared to contribute to a diurnal chorus between 2-8 kHz in the ‘barrens’ habitat of Jervis Bay, NSW. Analysis of the 5000 Hz 1/3rd octave band revealed a difference between barrens sites of up to 10 dB re 1 µPa2. A similar difference was found between two coral reef habitats recorded at One Tree Island, QLD. Arm angle development reflects swimming ability in sea urchin larvae and so was documented for two-armed and multi-armed larvae forms to characterise their swimming biology. In a behavioural study of early and settlement stage larvae of H. tuberculata and C. rodgersii, early stage larvae exhibited signs of negative phototaxis swimming down during the day, suggesting that they exhibit diurnal migration in the field. However, larvae did not modify this swimming behaviour in response to a reef noise. This research will enable detection of sea urchin sounds in ambient noise recordings and provides insight into the impacts of sea urchin populations on the marine soundscape.

Anthropogenic noise in the sea is now classed as a pollutant alongside chemical pollution and marine litter in accordance with the Marine Strategy Framework Directive. Noise from shipping is a major contributor to the ambient noise levels in the ocean, particularly at low (<300Hz) frequencies. The properties of sound propagation in shallow waters are highly influenced by the marine physical environment. Ocean modelling plays an important role in underwater noise studies since it can provide high resolution water column parameters over large geographic areas. This study investigates the noise patterns and their temporal variations in the Celtic Sea by using a coupled ocean model (POLCOMS) and an acoustic model (HARCAM). A method to predict noise exposure experienced by marine animals is then developed, following an application for diving seals. The ocean model is applied in the Celtic Sea to provide high-resolution 3D hourly temperature and salinity fields for the acoustic model. The model is validated against in-situ and satellite observations, giving high skills to simulate the water column structures. Sensitivity studies of modelled results to different atmospheric forcing are carried out in order to improve the accuracy of the model. The results show that the modelled sea surface temperature, stratification and water column structures are highly sensitive to the choice of surface forcing, especially in the summer time. The increase in resolution of surface forcing does not necessarily lead to more accurate results. The tidally frontal position is, however, insensitive to the forcing. The variability of noise propagation is studied using the coupled model, demonstrating high dependence on oceanographic conditions, geographic location of sound source and its depth. In summer, when the source of sound is on the inshore side of the bottom front, the sound energy is mostly concentrated in the near-bottom layer. In winter, the sound from the same source is distributed more evenly in the vertical. When the source is on the seaward side of the front, the sound level from a shallow source is nearly uniform in the vertical and the transmission loss is significantly greater (~16dB at 40km distance) in summer than in winter. In contrast, sound energy from a deep source is trapped in the bottom cold water, leading to a much lower transmission loss (~20dB) in summer than in winter. Note that ~10dB fluctuation of sound energy is found during the deterioration of the thermocline in late autumn. Shallow sources (e.g. ships) are sensitive to the surface heat flux as it changes significantly the vertical temperature gradient, while tides play an important role in determining the TL variability of deeper sources (e.g. pile driving) since they cause adjustments of positions of subsurface fronts. The seasonal noise patterns radiated by a large cargo ship are modelled by relating the AIS ship track data and the coupled model, showing a clear influence of the seasonal thermocline and associated bottom fronts on shipping noise distribution. The noise propagates much further (tens of kilometres) in winter than in summer. The predicted shipping noise exposure perceived by grey seals shows strong step changes in the sound level during their descent/ascent through the water column. Since grey seals tend to be benthic foragers, a hypothesis that the step change in sound exposure may have negative impacts on their foraging behaviour is proposed for biological specialists.

Long-term omni-directional ambient noise was collected at several sites in the Gulf of Mexico during 2004 and 2005. The Naval Oceanographic Office deployed bottom moored Environmental Acoustic Recording System (EARS) buoys approximately 159 nautical miles south of Panama City, Florida, in water depths of 3200 meters. The hydrophone of each buoy was 265 meters above the bottom. The data duration ranged from 10-14 months. The buoys were located near a major shipping lane, with an estimated 1.5 to 4.5 ships per day passing nearby. The data were sampled at 2500 Hz and have a bandwidth of 10-1000 Hz. Data are processed in eight 1/3-octave frequency bands, centered from 25 to 950 Hz, and monthly values of the following statistical quantities are computed from the resulting eight time series of noise spectral level: mean, median, standard deviation, skewness, kurtosis and coherence time. Four hurricanes were recorded during the summer of 2004 and they have a major impact on all of the noise statistics. Noise levels at higher frequencies (400-950 Hz) peak during extremely windy months (summer hurricanes and winter storms). Standard deviation is least in the region 100-200 Hz but increases at higher frequencies, especially during periods of high wind variability (summer hurricanes). Skewness is positive from 25-400 Hz and negative from 630-950 Hz. Skewness and kurtosis are greatest near 100 Hz. Coherence time is low in shipping bands and high in weather bands, and it peaks during hurricanes. The noise coherence is also analyzed. The 14-month time series in each 1/3- octave band is highly correlated with other 1/3-octave band time series ranging from 2 octaves below to 2 octaves above the band's center frequency. Spatial coherence between hydrophones is also analyzed for hydrophone separations of 2.29, 2.56 and 4.84 km over a 10-month period. The noise field is highly coherent out to the maximum distance studied, 4.84 km. Additionally, fluctuations of each time series are analyzed to determine time scales of greatest variability. The 14-month data show clearly that variability occurs primarily over three time scales: 7-22 hours (shipping-related), 56-282 hours (2-12 days, weather-related) and over an 8-12 month period.

The area of the northern Adriatic Sea has a high productivity rate regarding the fishing activities and it is recognised as one of the most exploited areas of the Mediterranean Sea. In order to make fishing activities sustainable and to guarantee a productive and healthy ecosystem, there is a strong need to develop effective fishery management plans for constant monitoring and for prediction. For this reason, it is of fundamental importance to analyse the data relating to the movements of fishing vessels and their catch. Starting from the AIS (Automatic Identification System) data, we reconstruct and enrich the trajectories by assigning to each segment the activity carried out by the boat (in port, entering/exiting from the port, navigation and fishing). In this way, considering only the portions of the trajectory in which the vessel is fishing, we compute the fishing effort, an essential indicator for monitoring the fishing pressure on an area of interest over time. By enriching our initial dataset with daily environmental factors such as sea temperature, waves height, wind and salinity, a variety of prediction methods are used in order to assess their prediction ability related to the fishing effort. Concerning the Northern Adriatic Sea exploitation, another fundamental aspect is the underwater noise generated by vessels, which has a significant short and long term impact on animal species. Using AIS data and the characteristics of the boats, we build a model for the propagation of underwater noise based not only on the technical characteristics of the fishing boats' engine but also on environmental factors that vary in each season. In this way we get a map of the underwater noise useful for identifying areas where underwater noise can damage the marine environment, even permanently. To accomplish this project we use MobilityDB, an extension of PostgreSQL and PostGIS that allows the storage and analysis of space-time objects. We also use Python and in particular Scikit-learn library to perform the experiments using machine learning models for regression. Furthermore, using the QGIS open source software, we create maps to display, in a simple and clear way, the areas in the Adriatic Sea characterised by the most intense noise pollution and to view the fishing effort forecasts.

碩士
國立臺灣大學
工程科學及海洋工程學研究所
101
Today, wind-generated electricity is one of the success green energy in the world. The potential for developing wind energy in the ocean is huge. Absolutely, Taiwan has good condition in developing wind energy. The west ocean of Taiwan is the sites of offshore wind farm. At the same time, this plant also contains lots marine mammals. Therefore, we are easy to neglect the problem between the environment and construction and operation of the offshore wind farm. This research have two parts. The first part is direct to the underwater noise during the construction of the wind farm. And use the construction report of Alpha Ventus in Germany and the historical data of hydrological and terrain to imitate the spreading range and impact range of underwater noise arising from the construction of offshore wind farm. Second part is measurement of the underwater ambient noise. Laying the hydrophone on the sea-bed and measuring the long term ambient noise. To analysis the seasonal variability and the characteristics of the local underwater ambient noise.

碩士
國立中山大學
海下技術研究所
91
The study is based upon the Vertical Line Array (VLA) of the Asian Seas International Acoustic Experiment (ASIAEX), dated from May 3, 2001 to May 16, 2001, in which the ambient noise was measured and the six frequencies (including 50, 100, 200, 400, 800, 1,200 Hz) were analyzed. The depth dependence of ambient noise levels in shallow water is not significant, and the fluctuations of ambient noise levels can be measured by Fourier analysis. With the similar analysis on static water pressure and temperature variation, the acoustic data has shown obvious in K1 and M2 tidal periods. The frequency dependence of ambient noise was also investigated, the lower frequency components, 50, 100, and 200 Hz, were mainly contributed from distant shipping noise, and the local wind was the dominating factor in higher frequencies such as 400, 800, and 1,200 Hz. This result was further verified by the increased levels at higher frequency due to the typhoon Cimaron during the experiment. Finally, the probability distribution function of ambient noise levels was calculated at each frequency, and was found the lower frequency ambient noise levels were not significantly affected by the typhoon as higher frequency. The threshold frequency of the ambient noise levels affected by the wind wave is about 400 Hz.

碩士
國立中山大學
海下技術研究所
96
Sound wave has much better transmission in ocean environment than electromagnetic waves, therefore sonar systems are widely applied in underwater investigations. However, not only the target signal is received by the sonar but also the noise from different directions. The noise will affect the performance of the sonar, so the understanding of ocean ambient is an important issue both in academic study and military applications. The ambient noise data of this research was collected by a passive acoustic recording system deployed in the southwest sea of Taiwan, along with the information of wind velocity in the experimented area. The influence on noise level fluctuations by the variation of the wind velocity was first discussed in light of correlation analysis. The fluctuations were expressed in terms of statistic distribution, mean value, standard deviation in different time series. As results, 500 Hz and 1.5k Hz were saturated by high levels signal from unknown sources in spring and summer, so the average sound levels were higher than in fall and winter, about 10 dB and 5 dB higher for 500 Hz and 1.5k Hz respectively. In seasonal analysis, 2.4k and 3.6k Hz have quite stable the mean levels and their standard deviations were around 3 dB. Especially, the noise level of 3.6 Hz has the least fluctuation throughout the year than any other frequencies analyzed. It was also observed that the noise level was decreased with the increase of frequency. Calculated by linear regression, this research worked out the estimation equation for the ambient noise level at high wind speed. However, the estimated values are higher than the measured data, it is due to the distribution of wind velocity. The wind data in this study was skewed towards the lower velocity, consequently the predicted values were overestimated.

In the light of global warming, large-scale transition to renewable power sources is a worldwide challenge, playing wind power a significant role. Sea wave energy is being increasingly regarded in many countries as a major and promising resource but, like all forms of energy conversion, it will inevitably have an impact on the marine environment. WaveRoller, a Wave Energy Conversion Device, is installed in front of Almagreira beach, on the west coast of Portugal. The purpose of this thesis is to study and quantify the underwater radiated noise from this device using an underwater acoustic model in order to estimate potential effects it may have in the marine environment. The model used to run the data will be MIKE Zero – Underwater Acoustic Simulator by DHI . In the study site only cetacean species are expected to occur. Results showed that behavioural responses might be expected for low and mid-frequency cetaceans if they swim close to the device. Also, the device shouldn’t be installed in an area in which a population of cetaceans exists in a 28m ray. For these individuals, injury can be assumed if SEL (Sound Exposure Level) is higher than 215 dB re 1μPa2.s, for non-pulse sounds. Results showed the calculated maximum SEL of the Waveroller sound is 150 dB re 1μPa2.s and therefore no injury is expected. MIKE Zero – Underwater Acoustic Simulator is a powerful tool to test any device that produces underwater noise and offers the possibility to create Surface Sound maps of results by using MIKEXYZ Converter tool.

碩士
國立中山大學
海下科技暨應用海洋物理研究所
101
The ocean ambient noise is one of the important parameters in sonar equation. The ocean ambient noise includes diverse and complex sources like waves, rain, marine life, ships, and etc. Using different ways to analyze are needed to understand the complicated properties of ambient noise. According to the data of Central Weather Bureau, northeastern Taiwan is the area with most rainfall in Taiwan. Among all kinds of ambient noise, rain-generated noise is wide in frequency range, if we know more about the rain-generated noise at sea, we can predict the effect of rainfall on underwater communication and survey. In this article, the ambient noise data were collected from MACHO（Marine Cable Hosted Observatory）system at northeastern sea of Taiwan in 2011 and 2012. Applying corresponding rainfall intensity data to observed noise level time series, observed ambient noise characteristic of each frquencies. Correlation analysis between rainfall intensity and underwater noise was performed, so the rain-generated noise level can be predicted by linear regression. The ocean ambient noise still has variations due to time and variance of sound sources, so it is important to be investigated. This study compares the statistical properties and distribution of ambient noise level at different frequencies and rainfall intensities, and also seasonal variation were discussed. Speculated the reasons for these differences, this can be the information of ambient noise analysis.

碩士
國立中山大學
海下技術研究所
92
This study is based upon the vertical line array (VLA) in South China Sea experiment of the Asian Seas International Acoustic Experiment (ASIAEX), dated from May 3, 2001 to May 16, 2001, in which the ambient noise in shallow water was measured. In this paper, we use the beamforming method to calculate the vertical directionality of ambient noise in shallow water, for discussing the noise source distribution, and environmental effects on vertical directionality. The results show that distant shipping noise was observed near the horizontal angles, and surface noise occurred at high grazing angles. It is also discussed that the wind speed effect on the vertical directionality in this paper. Because of the typhoon Cimaron passed the experimental area during the experiment, which make wind speed changed obviously, so it can be found out the wind effect of ambient noise. By analyze the noise fluctuation, it is concluded that the threshold frequency of wind wave affected ambient noise levels were about 400 Hz. And after analyzing the vertical directionality, it is verified that the threshold was lower to 200 Hz. On the other hand, we also note the phenomena of “noise notch” appeared at some duration when calculating the vertical directionality. The environmental effects (sound speed profile) on the notch, and the presentation of noise notch at different frequencies were discussed. In the end part of this paper, we determined the time when the notch occurred by using the power difference of vertical directionality, and we expect that the results would be important for similar researches in the future.

Soundscape Ecology, the physical combination of sounds at a particular time and place, is a rapidly growing field. As acoustic technology advances, several possible future uses of passive acoustic monitoring (PAM), such as biodiversity counts and monitoring of habitat health, are being explored. This thesis is divided into two chapters; each is a stand-alone paper. The first chapter provides a review of soundscape ecology, ambient sound, current recording methods and data analysis used in PAM studies, and identifies several major future recommendations for the field. One of these recommendations is to standardize recording methods and indices used during analysis in long-term studies. The second chapter analyzes a 55-minute continuous recording on a coral reef in Tunicate Cove, Belize in 1996 by Professor P. Lobel. This recording was then subsampled with several intermittent recording schedules to explore the amount of acoustic information lost as periods of active and inactive recording vary. The continuous recording consisted of a high frequency band (3-4 kHz), which may correspond to abiotic sounds, and a low frequency band (0.1-0.5 kHz), which generally corresponds to biotic sounds. Two recording schedules, 30 seconds every 4 minutes and 2 minutes every 10 minutes, were significantly correlated with the continuous recording. The statistical significance of the other five recording schedules varied among the three parameters tested in this study (average power (dB), average entropy, and aggregate entropy).

Approved for public release; distribution is unlimited
During the month of July 1987 an acoustical experiment was conducted by the United States Naval Research Laboratory (NRL) in the East Greenland Sea Marginal Ice Zone (MIZ) . Ambient noise "hot spots" or concentrated areas of relatively high noise levels were found along the ice edge using a towed array. Ambient noise levels were obtained on 27 and 28 July using AN/SSQ-57A and AN/SSQ-57XN5 calibrated sonobuoys . The temperature structure of the area was determined using XBT (ship) and AXBT (P3C aircraft) buoys placed inside and outside the ice edge. The ice edge was determined from coincident satellite photos, 90 GHz microwave imagery and P3 radar ice edge maps. Weather data (sea state and wind speed and direction) were recorded on the ship. The data seem to indicate a correlation between the high ambient noise levels of the hot spots and the presence of a large topographically controlled mesoscale eddy located at the southeastern extent of the MIZ.
http://archive.org/details/spatialvariabili00bigg
Lieutenant, United States Navy

Given the interest on underwater axisymmetric cylindrical bodies for the development of high-speed underwater weapons, characterization of the boundary layer flow-induced noise received by a Sound NAvigation and Ranging (SONAR) is very important to improve sonar detection ranges. The debate on generating mechanisms of the flow induced noise received at the stagnation point is still on as there is no experimental evidence conclusively suggesting whether it is a near-field or far-field phenomenon, thereby introducing an element of uncertainty in the prediction models. Further, the models developed thus far were based on low Reynolds numbers involving flows in water tunnels and buoyant vehicles. Therefore, the main focus of the thesis is to measure the flow induced noise using a sonar fitted at the most forward stagnation point of an underwater axisymmetric body as realistically as possible and predict the same theoretically for identifying a suitable flow noise model for future use by designers. In order to meet the stated goal, two exclusive experiments were conducted at sea using an underwater autonomous high-speed axisymmetric vehicle fitted with a planar hydrophone array (8X8) in its nose cone which measured the flow noise signature. Two different sets of existing models are used in characterizing the flow noise received by the array, while the first set comprises of models developed based on the Turbulent Boundary Layer induced noise and other is based on the transition zone radiated noise model. Through this study, it was found that the transition zone radiated noise model is in close agreement with the measured data.