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Matched Field Processing (MFP) is an inversion technique often employed in source localization applications. Conventional MFP approaches are incapable of producing precise results in the presence of extremely impulsive noises, which are typically present in actual applications such as underwater acoustics. This is because the covariance matrix for this category of noises does not converge. Moreover, impulsive noise suppression algorithms fail to provide accurate results. Particularly, fractional lower order moment (FLOM)-based approaches have an unbounded output, and data trimming methods introduce uncertainty into the estimation covariance matrix. In this study, a novel MFP method employing the empirical characteristic function (ECF) is developed. The desirable properties of the characteristic function (CF) result in a robust localization method that is ideally suited for extremely strong tailed noise environments. Using the CF array output, a new covariance-like matrix that can be used in MFP methods has been constructed. To demonstrate the efficiency of the ECF-MFP technique, experiments are conducted in a water tank. Experimental results reveal that this method is very robust in the presence of very heavy tailed noise, a low signal-to-noise ratio, and a tiny sample size. Additionally, it outperforms previous approaches in terms of resolution probability. [ABSTRACT FROM AUTHOR]

Acoustic coatings with periodically arranged internal cavities have been widely applied to underwater vessels to reduce the underwater sound scattering. In this study, the simulation results from the finite element method (FEM) have been compared with the theoretical solutions based on the transfer matrix theory (TMT), and the reliability of the FEM has been verified. The Nelder-Mead algorithm has been employed to optimize the structure of the coatings and the material parameters with the sound absorption coefficient as the primary optimization objective. A function that characterizes the shape of a two-dimensional axisymmetric cavity has been proposed, and the peak value of the absorption coefficient can be successfully moved to the target frequency by changing the weighting strategy. The results show that the sound absorption coefficient of the optimized coating increases and the peak shape widens in the middle and low frequency band. The optimized axisymmetric cavity significantly improves the sound absorption performance of the anechoic coatings. The optimization algorithm of the cavity structure and material parameters proposed in this study provide an effective pathway for the optimal design of the anechoic coatings. [ABSTRACT FROM AUTHOR]

The underwater glider is a new type of unpowered, unmanned, moving observational platform with advantages of low-noise level, long operation time, far sustainable range, and high cost-effectiveness. In the paper, based on the underwater glider platform integrated with single vector acoustic sensor, an underwater acoustic glider platform is developed with the ability to detect target direction and observe the ambient noise. The acoustic measurement system and the self-noise of the glider platform under each working condition are tested to analyze the self-noise levels of the acoustic system and the primary noise sources of the platform, and conduct the vibration and noise reduction processing of the platform and optimize the working mode of the acoustic system. The test result shows that the underwater acoustic glider with the optimization has the ability to observe the ambient noise only on the pressure hydrophone channel. With the data sampled from one of the underwater gliders of the sea trial organized in a certain area of the South China Sea in August 2019, authors analyze the variation of spectrum levels of the ambient noise with the depth and the time at the center of seven frequency points (63 Hz, 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1.6 kHz and 3.15 kHz) and discuss the influence of the sailing vessel close to the glider on it. The experimental result shows that the underwater acoustic glider, as an unscrewed moving platform, can be well used to monitor the ambient noise properties over a long term. [ABSTRACT FROM AUTHOR]

The parabolic approximation has been used extensively for underwater acoustic propagation and is attractive because it is computationally efficient. Widely used parabolic equation (PE) model programs such as the range-dependent acoustic model (RAM) are discretized by the finite difference method. Based on the idea of the Pad e ´ series expansion of the depth operator, a new discrete PE model using the Chebyshev collocation method (CCM) is derived, and the code (CCMPE) is developed. Taking the problems of four ideal fluid waveguides as experiments, the correctness of the discrete PE model using the CCM to solve a simple underwater acoustic propagation problem is verified. The test results show that the CCMPE developed in this article achieves higher accuracy in the calculation of underwater acoustic propagation in a simple marine environment and requires fewer discrete grid points than the finite difference discrete PE model. Furthermore, although the running time of the proposed method is longer than that of the finite difference discrete PE program (RAM), it is shorter than that of the Chebyshev–Tau spectral method. [ABSTRACT FROM AUTHOR]

The article offers acoustics news briefs. Topics discussed include Australian Acoustical Society conference on topics such as environmental acoustics and planning, transportation noise, building acoustics and amenity; analysis of sources of underwater sound as determined by Passive acoustic recordings made in Fowlers Bay, South Australia, during the austral winter of 2013–2017; and analysis of sound directivity from sound propagation through turbulent exhaust jets in cross-flow.

Passive acoustic recordings made in Fowlers Bay, South Australia, during the austral winter of 2013–2017 revealed the presence of several sources of underwater sound. Sound sources of biological origin include baleen and toothed whales, fish and shrimp. Physical sources of underwater sound include wind- and rain-driven noises, and underwater sounds of anthropogenic origin were primarily from boats and occasionally from an aircraft. Biological sound sources were commonly recorded within the frequency range of around 25 Hz to nearly 17 kHz, with baleen whales within the range of ~ 25 Hz to 6 kHz, and dolphins at higher frequencies of approximately 2.5–17 kHz. Broadband sounds from physical and anthropogenic sound sources were noticeable at frequencies above ~ 50 Hz. The ambient noise level in Fowlers Bay at frequencies below 100 Hz was relatively low (around 75 dB re 1 µPa2/Hz for the 95% percentile) due to an insignificant contribution of noise from distant shipping. At higher frequencies, the noise level was governed primarily by noise from wind and varied by nearly 30 dB re 1 µPa2/Hz depending on weather conditions, up to around 80 dB re 1 µPa2/Hz for the 95% percentile during periods of strong winds and intense rainfall. [ABSTRACT FROM AUTHOR]

The ocean mesoscale vortex can make uneven distribution of sound energy, which has a significant impact on underwater acoustic equipment, weapons and submarine warfare. Based on the observed data of 2014 West Pacific sea oceanographic survey, the influence of the properties, strength, position and depth of the mesoscale vortex on the acoustic propagation characteristics is simulated by the parabolic equation. Our observed vortex is a typical cyclone-type cold vortex in the Kuroshio extension body, and the cold water turns on intensely at the center of vortex. The effect depth of vortex is 500 m, and then, the parabolic equation of acoustic model is used to analyze and simulate the impact of mesoscale cold-core vortex on acoustic transmission loss. Results show that the horizontal perturbation of the surface velocity caused by mesoscale vortex has a great influence on the spatial distribution of water sound field. The cold vortex makes the position of the convergence zone move forward, with the width reduced, and the gain efficiency enhanced. When sound propagates outside of the cold-core vortex, compared with inside of the cold-core vortex, the assembled areas are backward, with the width increased, and gain effect weakened. The non-uniform hydrological environment caused by the ocean mesoscale vortex is a key modulating factor of the abnormal variation of acoustic energy in spatial distribution. [ABSTRACT FROM AUTHOR]

Matecho is an automated processing method to extract information and perform echo-integration and fish shoal extraction from various scientific echo-sounder sources providing digital acoustic data on fisheries and aquatic ecosystem. The open-source initiative helps foster collaboration and technological transfer. Matecho supports various formats, such as the international standard format for the exchange of fisheries acoustics raw and edited data. The procedure allows the semiautomatic cleaning of echogram data and the application of automatic data filters, i.e., transient noise, attenuated signal and impulsive noise removal and background noise reduction. Echo-integration processing is executed for each depth layer and integrates their characteristics per elementary sampling unit. Sound scattered layers are automatically detected by segmentation from the echo-integrated echogram, and shoals are extracted according to an iterative process of aggregation of filtered echogram echoes that allows, in both cases, the calculation of the ad hoc parameters describing morphological, spatial location and acoustic characteristics of sound scattered layers and shoals. Matecho is open-source software for researchers and provides end users with a user-friendly, free executable program. [ABSTRACT FROM AUTHOR]

The underwater object localization is important in defense, underwater biological and environmental applications. Localization using a passive sonar system is a challenging task. It is more challenging when the source and receivers are in the reverberant environment. Time delay estimation (TDE)-based localization is a well-known technique to localize source for last few decades. In this work, empirical mode decomposition maximum likelihood (EMD ML TDE) method is used to estimate the time delay in a reverberant environment. The sound source location is estimated by intersecting spherical surfaces from the time delay. The experimental results prove that EMD ML time delay estimation method is effective to localize a sound source in a reverberant environment. [ABSTRACT FROM AUTHOR]

An offshore seismic survey was conducted over the western edge of the continental shelf in Bass Strait in 2006. Underwater noise from this survey was recorded on an autonomous sound recorder deployed in the Southern Ocean on the Antarctic continental slope. Sound emission and propagation models were verified by experimental measurements using parameters and position of the airgun array and characteristics of the underwater sound channel. A parabolic equation approximation method was used to calculate the sound field over the continental slope of Australia, and then, a normal mode model was employed to account for the transmission loss due to sound scattering by surface waves south of the polar front. The numerical predictions are consistent with the measurement results within a few dBs for the sound exposure and energy spectral density levels. It is also demonstrated by measurements and modelling that the best coupling of a near-surface sound source with the deep underwater sound channel takes place when the source is located over the continental slope at a sea depth of about half of the channel’s axis depth. The model can be used to predict masking effects of man-made underwater noise on the communication environment of marine mammals in Antarctica. [ABSTRACT FROM AUTHOR]

The parameters of deep ocean sediments are relevant for accurately predicting the sound field; however, it is difficult to measure the parameters in situ. Most inversion methods used in shallow water are inapplicable in the deep ocean because of the considerable differences in propagation characteristics. At present, no method for simultaneously obtaining sound speed, density, and attenuation that considers the sensitivity of sediment parameters is yet available. This study proposes a two-step inversion of geoacoustic parameters in the deep ocean. On the basis of the half-space model, the decline tendency of bottom reverberation level with travel time is used for the inversion of sound speed and density, whereas transmission loss is used for inversion of attenuation. Inversion results can be practical for acoustic applications when this method is used. Experimental data from the South China Sea in the summer of 2014 are processed during the inversion process. The sediment parameters obtained from the inversion process are close to the laboratory-measured sampling values and may be used to predict the sound field in various applications, such as in transmission loss in the deep ocean. [ABSTRACT FROM AUTHOR]

In this paper, a Petrel II autonomous underwater glider (AUG) was used to analyze underwater noise spectrogram in the South China Sea. For ocean observations, AUG has an excellent performance such as being efficient in all weather conditions, lasting for long time periods, being able to be remote controlled, which creates favorable conditions for analyzing underwater noise characteristics. In order to measure the underwater noise, a newly developed and customized underwater passive acoustic monitoring system was firstly installed on the Petrel II glider and tested in the South China Sea trial in August 2016. The observation data at a depth of 1000 meters range were taken as the research object and were used to analyze the noise levels over a long period through the method of the short-time Fourier transform. At the same time, the noise data have been compared in time to the actions of the glider during the sea trial. However, AUGs also have limitations such as undesired instrument and flow noise. By adopting the step-by-step method, these self-noise data of Petrel II glider were acquired and analyzed quantitatively, and the basic characteristics of self-noise under different working conditions are depicted. Removing the corresponding self-noise, the noise data at different depths measured by the Petrel II glider could be used to estimate underwater ambient noise levels across depth. [ABSTRACT FROM AUTHOR]

Steel and lead piles immersed in seawater and their toes made non-reflective (for clarity of the results) are driven by an axisymmetric harmonic force. Axial and radial vibrations travel from head to toe. Radial vibration, which radiates sound into the surrounding medium, is computed using Membrane thin-shell vibration theory. At 1 kHz, a typical construction steel pile (radius 38 cm, wall thickness 2.5 cm) has a phase velocity of 4943 m/s and in cold seawater (sound speed 1485 m/s), the Mach wave radiates at $$a\hbox {cos}(1485 /4943) = 73^{\circ }$$ from the pile axis. According to ray theory, a Mach wave is received at any position providing a $$73^{\circ }$$ line from that position intersects the pile (at what was the emitting point). The vibration energy at this emitting point has travelled there from the pile head. For a given slant range ( R) to a receiver, ray theory predicts that a Mach wave is not received below a minimum colatitude (COMIN). For a typical steel pile, it is found that as R increases beyond the pile length ( L), COMIN increases rapidly from $$0^{\circ }$$ , passes through $$68^{\circ }$$ at $$10 \times L$$ , and asymptotes to $$73^{\circ }$$ as R increases further. For the same pile made of lead, the phase velocity at 1 kHz is 1332 m/s, and no Mach wave is radiated. For both steel and lead piles, radiated SPLs were calculated using both exact and far-field approximate radiation theories as functions of colatitude, at slant ranges from 10 to 1000 m. For the steel piles, the far-field approximate model (which omits the Mach wave) underestimates SPL by up to 20 dB if the receiver's colatitude exceeds COMIN. For the lead piles however, the far-field approximate model is accurate to within 1 dB. [ABSTRACT FROM AUTHOR]

Underwater noise environments are increasingly being considered in marine spatial planning and habitat quality assessments. Although the overall aim of regulation is to quieten anthropogenically noise-rich habitats whilst maintaining pristine habitats free of man-made noise, effective management plans require knowledge upon which to base decisions. This is particularly true for managers of acoustically specialised species. This study aimed to compare the acoustic environment of coastal dolphins in two locations within Western Australia by comparing a 'pristine' habitat (Roebuck Bay) with an 'urban' habitat (Fremantle Inner Harbour). Autonomous underwater acoustic recorders collected approximately 940 and 1080 h of data from these two sites, respectively. Additionally, in Roebuck Bay opportunistic in situ recordings with concurrent visual observations were collected in the presence of two dolphin species. Acoustic data were assessed via weekly spectrograms, power spectrum density percentile plots and probability densities, octave band levels, broadband noise levels, and generalised estimating equations. Results indicated that the two sites had highly contrasting acoustic environments. In Roebuck Bay, the local soundscape was dominated by biotic sounds, with only sporadic vessel noise. However, in Fremantle Inner Harbour, anthropogenic noise was prevalent. On average, Roebuck Bay was 20 dB quieter than the Fremantle Inner Harbour over the frequency band 10 Hz-11 kHz. Dolphin communications had a greater potential to be masked in Fremantle Inner Harbour than in Roebuck Bay based on elevated anthropogenic noise levels. If noise levels were to increase in Roebuck Bay, coastal dolphins may show behavioural and/or acoustical responses as observed at other locations. [ABSTRACT FROM AUTHOR]

The present accuracy of ocean and atmospheric models permits the description of water column features, and sea surface wind stress, at a resolution which enables employment in underwater acoustic transmission applications. In an investigation of aspects of linking modelled ocean data with range-dependent acoustic transmission models, a Parabolic Equation (PE) transmission code was used with data generated by the BLUElink suite of ocean and atmospheric models for a deep-water region off the east Australian coast for a particular summer period. A typical presence of warm and cold core eddies was observed to accompany a highly variable acoustic environment. Variations in expected range to particular levels of Transmission Loss were found to be mainly related to changes in the depth of the mixed surface layer, but also due to changes in the sound speed gradient in the thermocline. The study also made a brief consideration of the likely impact of wind speed variation over the region, and the effects of modelled ocean currents on acoustic transmission. [ABSTRACT FROM AUTHOR]

Passive acoustic recording of marine noise has advanced considerably over recent years. For a long time, a lack of widely available technology limited the acquisition of long-term acoustic data sets to a small number of large, cabled installations mostly restricted to military use. For other users, recordings were limited by the available technology to short snapshots of minutes to possibly days of data at a time. As technology has improved, passive acoustic monitoring has shown marine soundscapes are filled with biotic and abiotic sounds that occur on a range of often unpredictable timescales. Thus, snapshot recordings can lead to biased data. In 1999, the Centre for Marine Science and Technology, together with Australia's Defence Science and Technology Organisation, began developing remote underwater sound recorders to increase the duration and quality of recordings. As time passed, the sound recorders were developed significantly, have been deployed over 600 times at a variety of Australian and international locations and have identified a plethora of biological, geophysical and anthropogenic sound sources. This paper presents a brief history of the recorders' development and characteristics, some examples of the information they have provided and future direction for their next generation. [ABSTRACT FROM AUTHOR]

Several soniferous (sound-producing) fish species can generate a number of different call types, providing significant information for intended recipients and observers. Understanding the level of variation they are capable of and whether it is voluntary or not gives an indication of how individually specific their calls might be. Similar to other Sciaenidae, mulloway ( Argyrosomus japonicus) produce calls associated with spawning behaviour by vibrating their swimbladder. Dissection revealed mature mulloway have bi-lateral, highly vascularised 'sonic' muscles, with dorsoventrally orientated muscle fibres along the posterior two-thirds of the swimbladder, similar to French meagre ( A. regius). Repetitive contraction of these muscles likely impinges on the sides of a loosely supported swimbladder, to generate motion. In the Swan River, Western Australia, mulloway produce a category of 'long' calls comprising 11-32 contractions at pulse repetition intervals and resulting carrier frequencies of approximately 16.6 ms and 60 Hz, respectively. However, throughout the $$\approx $$ 0.3 s calls this repetition rate may stay constant, rise, fall or a combination. In 13% of analysed calls, a pre-call pulse of lower amplitude than subsequent pulses was observed. A category of short calls (1-5 pulses) employed carrier frequencies of up to 114 Hz initially, but this decreased with each pulse, suggesting that these calls are at the limit of contraction rates of the sonic muscles. The study shows that even within previously defined call categories mulloway have the ability to produce several variations. Whether these variations are generated voluntarily or not and whether they can be perceived by the intended recipient are unknown and suggested as a topic of further work. [ABSTRACT FROM AUTHOR]

Matched field processing (MFP) has been a method widely applied for shallow underwater target localization, which is a critical issue in underwater acoustic. To enhance the efficiency of conventional MFP methods, different adaptive MFP algorithms have been developed; the white noise constraints (WNC) MFP or diagonal loading (DL) algorithm is such a typical one. The WNC or DL one has been considered to be the most desirable method because it is more robust to environment mismatch in practical in comparison with the minimum-variance distortionless response MFP algorithm, a popular high-resolution method. Although having exceptional ability to localize underwater sources in mismatch scenarios, the DL method has still been not reach high resolution in certain cases. In the paper, we proposed an adaptive method known as improved diagonal loading algorithm to make an increase in the resolution and the peak background rate in the ambiguity surface of source localization results in comparison with DL one. The proposed algorithm works by adding one more parameter that is adjusted in the steering vector of the DL algorithm. The simulation results show that the new algorithm attains better beamforming performance in terms of high resolution than the existing adaptive MFP algorithms in the case of environmental mismatch caused by noise effects and the limitation of the snapshots. [ABSTRACT FROM AUTHOR]

Fisheries acoustics is now a standard tool for monitoring marine organisms. Another use of active-acoustics techniques is the potential to qualitatively describe fish school and seafloor characteristics or the distribution of fish density hotspots. Here, we use a geostatistical approach to describe the distribution of acoustic density hotspots within three fishing regions of the Northern Demersal Scalefish Fishery in Western Australia. This revealed a patchy distribution of hotspots within the three regions, covering almost half of the total areas. Energetic, geometric and bathymetric descriptors of acoustically identified fish schools were clustered using robust sparse k-means clustering with a Clest algorithm to determine the ideal number of clusters. Identified clusters were mainly defined by the energetic component of the school. Seabed descriptors considered were depth, roughness, first bottom length, maximum $$S_{v}$$ , kurtosis, skewness and bottom rise time. The ideal number of bottom clusters (maximisation rule with D-Index, Hubert Score and Weighted Sum of Squares), following the majority rule, was three. Cluster 1 (mainly driven by depth) was the sole type present in Region 1, Cluster 2 (mainly driven by roughness and maximum $$S_{v})$$ dominated Region 3, while Region 2 was split up almost equally between Cluster 2 and 3. Detection of indicator species for the three seabed clusters revealed that the selected clusters could be related to biological information. Goldband snapper and miscellaneous fish were indicators for Cluster 1; Cods, Lethrinids, Red Emperor and other Lutjanids were linked with Cluster 2, while Rankin Cod and Triggerfish were indicators for Cluster 3. [ABSTRACT FROM AUTHOR]

In this paper, two typical methods of designing underwater acoustic (UWA) channel simulators, the geometry based and the measurement based, are investigated. Then, the performance of each method is analyzed by comparing the statistical properties of the simulated channels with those of the real shallow UWA channel measured in Halong bay, Vietnam, in June 2015. The results show that the measurement-based channel simulator provides the simulated channel which matches well with the real UWA channel, but it requires application of complex optimization computation methods to estimate a larger number of channel parameters. The geometry-based simulator has a lower complexity than the measurement-based simulator. However, the statistical properties obtained by this simulator do not fit with those of the real UWA channel. This is our motivation to propose an effective channel simulator, which is not only simple in computation but also in good agreement with the real UWA channel. The parameters of the proposed UWA channel simulator can be directly exploited from the measurement data without applying any optimization computation method. Moreover, the simulation results show that the channel statistical properties obtained by the proposed method match well with those of the real UWA channel. [ABSTRACT FROM AUTHOR]

Multipath arrivals received by a towed array with different arrival angles result in a bearing estimation error (BEE) when conventional beamforming is used in the real ocean waveguide. This paper investigates the errors based on simulated data, focusing on their dependence on the range and bearing of a far-field broadband source. The BEE varies periodically with range, reaching local minima and local maxima at the edges of convergence zones (CZs). The estimated source bearing always lies between the true source bearing and the broadside direction of the towed array. The physical mechanism is analyzed by using normal-mode theory for theoretical interpretation and the ray method for perceptual analysis. Based on conventional beamforming, the approximate analytical solution of the beam-forming output is derived in terms of normal modes and can predict the BEE quantitatively. The ray trace is used to illustrate the range dependence. The BEEs for different ocean depths are also discussed and the results can be used as auxiliary information for source localization. [ABSTRACT FROM AUTHOR]

Human waterborne activities emit noise into the marine environment. This is of particular concern with regard to the potential impact on marine fauna such as cetaceans due to their acoustic specialisations. The Swan-Canning River system in Western Australia is home to a resident community of Indo-Pacific bottlenose dolphins ( Tursiops aduncus), but is also a site regularly used for various human activities. As underwater noise levels increasingly become considered as an indicator of habitat quality, there is a need to characterise the soundscapes of such areas with regard to their cetacean fauna. This study aimed to provide a description of a site within the river system known as 'The Narrows'. Acoustic data were collected over a six-week period with an autonomous underwater acoustic recorder. These data were analysed using a combination of weekly spectrograms, power spectral density percentile plots, 1/3 octave band levels, and generalised estimating equations to identify prominent soundscape contributors and investigate temporal patterns in their occurrence. The soundscape was found to be strongly influenced by wind, snapping shrimp, and vessel traffic, with the sounds of bridge traffic, waves, fish, machinery, dolphins, and precipitation also contributing to the acoustic environment. Furthermore, three of these sound sources (boats, waves, and fish) were found to vary at a range of temporal scales. These results take a vital step in characterising the acoustic habitat of this river system, highlighting the need to consider temporal patterns when assessing the composition of underwater soundscapes. [ABSTRACT FROM AUTHOR]

Underwater sound generated by pile driving was recorded during the Hong Kong-Zhuhai-Macau Bridge (China) construction in 2014.The important parameters on the source level, the transmission loss, the time-frequency characteristics, and the method of the noise measurement were introduced in this paper. Signal analysis showed that the waveform of pile driving resembles a series of short pulses, composed of many different frequencies with main frequency energy distribution from 100 Hz to 2 kHz; the maximum value occurs at about 200 Hz. The best-fit shallow water sound propagation model gave a root-mean-square source level of 208.2 dB re $$1~\upmu \hbox {Pa}$$ at 1 m, a transmission spreading coefficient of 17.76, and an absorption coefficient of 0.015 dB/m. The value of transmission loss is in good agreement with the results calculated by the Marsh and Schulkin (M&S) semi-empirical expressions above 1 kHz, but is higher than the M&S results under 1 kHz, which may be due to the muddy seabed and turbid sea water in our study area. Although it is difficult to compare our research results with other studies as there is a lack of details on monitoring conditions (including pile size, hammer energy, environmental background, etc.), this paper provides a new method for underwater noise monitoring and analysis for pile driving in shallow water. [ABSTRACT FROM AUTHOR]

The advent of low-cost, high-quality underwater sound recording systems has greatly increased the acquisition of large (multi-GB) acoustic datasets that can span from hours to several months in length. The task of scrutinizing such datasets to detect points of interest can be laborious, thus the ability to view large portions of the dataset in a single screen, or apply a level of automation to find or select individual sounds is required. A toolbox that can be continually revised, the userfriendly "Characterisation Of Recorded Underwater Sound" (CHORUS) Matlab graphic user interface, was designed for processing such datasets, isolating signals, quantifying calibrated received levels and visually teasing out long and shortterm variations in the noise spectrum. A function to automatically detect, count and measure particular signals (e.g. blue whale sounds) is integrated in the toolbox, with the ability to include categorised calls of other marine fauna in the future. Sunrise and sunset times can be displayed in long-term average spectrograms of sea noise to reveal diurnal cycles in the vocal activity of marine fauna. A number of example stud [ABSTRACT FROM AUTHOR]

This paper describes an initial assessment of the role of glider data as an input into a sonar nowcast acoustic detection range prediction model. It includes an analysis of the temporal and spatial variability of the water column data measured by a glider in shallow Australian waters. The area covered by the data includes a region where there is a known persistent frontal feature. The glider data verified that a persistent front was present in the data. [ABSTRACT FROM AUTHOR]

A multi-beam sonar (MBS) has been used to map Australia's continental margin seabed from the marine national facility vessel Southern Surveyor on opportunistic transit and research voyages since 2004 with 0.35 M km² mapped. The MBS data are used to infer key ecological features based on bathymetry (e.g. seamounts, canyons, terraces, banks and deep reefs) and backscatter data for ecological hard (consolidated, e.g. rock for attachment of fauna) and soft (unconsolidated, e.g. mud for burrowing fauna) substrate. Seabed consolidation inference is consistent with a seabed scattering model. To consistently infer ecological significant hard and soft substrate from the backscatter data requires minimisation of errors due to changing absorption (~2 dB) with temperature and depth, calibration drift, changes in pulse length and estimates of area insonified due to seabed slope (<8 dB). Area insonified corrections were required for both across and along-ship slopes. Highest corrections were needed for along-ship slopes in canyon regions and large incidence angles (>60°). A data collection and processing framework is described that works towards a national backscatter mapping program for environmental seabed mapping. Data collected and automated processing for depth, sound absorption and area insonified at level 2 of a possible 5 level data processing hierarchy is available for viewing at http://www.marine.csiro.au/geoserver. [ABSTRACT FROM AUTHOR]

Three methods of classifying large sets of acoustic signals are briefly discussed. The purpose of the discussion is to broadcast the existence and summary details of the methods to a wider audience. Large implies that hundreds of signals to several tens of thousands of signals may be detected. The signals of interest are broadly Gaussian, Rayleigh, or sinusoidal in shape, and of finite duration, such as seabed echoes and beaked whale chirps. The classification methods are (1) feature analysis, (2) direct statistical clustering of signals treated as single-valued curves, and (3) matched filtering with use of normalisations and kurtosis of the cross-correlation function output of the matched filter. Method (1) has been used for many years in several fields of science. It is suitable for many applications, but classifies on proxies, not the actual signals, which may lead to loss or distortion of information. Method (2) is a post processing operation suitable for signals with well defined signal to noise ratio which can be well aligned in time. Although simple in concept, it is a recent innovation, as it was apparently not previously realised that it could be done. A suitable clustering algorithm can classify signals into groups or sets where each set has a different average shape from the other sets, and can also classify signals forming quasicontinuums, such as those which can be viewed as morphing from one shape to another. Method (3) is suitable for detection and classification of stereotypical signals (those with strongly repeating waveform or signal shape), including weak signals in noisy backgrounds. In the usual application of matched filtering, classification is made solely on the un-normalised amplitude of the cross-correlation function. A novel extension of method (3) is to provide a confidence estimate for the classification through the kurtosis of the normalised autocorrelation function. The kurtosis is observed to be related to the degree of signal distortion or malformation relative to the template signal. When incoming signals of the same type vary in energy and degree of distortion or malformation, this scheme greatly outperforms standard matched filtering. [ABSTRACT FROM AUTHOR]

The propagation of underwater acoustic signals in polar regions is dominated by an upward refracting sound speed environment and the presence of a dynamic highly variable ice canopy. This paper provides an overview of the acoustic properties of sea ice and assesses the influence of ice canopy and water column properties on acoustic transmission loss for propagation within 20 km of a sound source at 20 m depth. The influence of the ice canopy is assessed first as a perfectly flat surface, and then as a statistically rough surface. A Monte Carlo method is used for the inclusion of ice deformation and roughness. This involves the creation of sets of synthetic ice profiles based on a given sea ice thickness distribution, followed by statistical methods for combining the output of individually evaluated ice realisations. The experimental situation being considered in the framing of this problem is that of an Autonomous Underwater Vehicle (AUV) operating within 50 m of the surface. This scenario is associated with a frequency band of interest of 9-12 kHz and a horizontal range of interest up to 20 km. The situation has been evaluated for a set of typical ice statistics using Ray and Beam acoustic propagation techniques. The sound speed profile (based on real data) results in a strong defocussing of direct path signals at ranges from 9-20 km and depths shallower than 50 m. This reduction in the signal strength of the direct path creates areas where the influence of surface reflected paths becomes significant. The inclusion of a perfectly flat ice layer reduces the transmission loss between 9-20 km by 15-50 dB. When the ice layer is included as a rough surface layer the results show a boost to signal strength of up to 8 dB in the small areas of maximum defocussing. Sea ice is a strongly time and space varying sea surface and exists in areas where defocussing of the direct path due to the sound speed profile reduces the range of direct path dominated transmission. This work presents methods for including a statistically relevant rough surface through a technique for generation of sets of surfaces based on ice deformation statistics. It outlines methods for including ice in acoustic modelling tools and demonstrates the influence of one set of ice statistics on transmission loss. [ABSTRACT FROM AUTHOR]

Doppler frequency spreading and arrival delay spreading of underwater acoustic communication signals under the influence of surface waves and transmitter-receiver motion were investigated in a channel probing experiment conducted primarily with binary pseudo-noise (PN) sequences ranging from 21ms to 1.4s duration. Testing was conducted in a 13.5m deep environment at transmission distances ranging from 44m to 1007m. The channel Doppler response was investigated both by time-domain Doppler search of the transmit-receive correlation for successive repeats of a 1.4s probe sequence, and by Fourier analysis of the channel impulse response history from a repeated 21ms probe sequence (i.e. Spreading Function). The bounds of Doppler shift imparted by relative transmitter/receiver motion and surface wave motion to idealised sound-ray transmission paths has been compared with experimental Doppler indicated by the Spreading Function. The coherence of the experimental channel response was also examined for different propagation ranges and at different delayed arrivals of the experimental signals. [ABSTRACT FROM AUTHOR]

The current study presents the numerical prediction of the noise and vibration of a small-scaled submarine under axial excitation from a 5-bladed propeller and excitation from the flow noise induced by the pulsating pressure of the hull. Firstly, the propeller flow and submarine flow were independently validated. The propulsion of the hull-propeller was simulated using computational fluid dynamics (CFD), so as to obtain the transient responses of the propeller axial excitation and the boundary pressure of the hull. Finally, the acoustic response of the submarine under axial excitation was predicted using a finite element/boundary element model in the frequency domain, and the flow noise was predicted using Curle's analogy in the time domain. [ABSTRACT FROM AUTHOR]

Signal transmission in ocean using water as a channel is a challenging process due to the effect of attenuation, spreading, reverberation, absorption etc., apart from the contribution of acoustic signals due to ambient noises. Ambient noises in sea are of two types namely manmade (shipping, aircraft over the sea, motor on boat, etc) and natural (rain, wind, marine fishes, seismic, etc). The ambient noises contribute more effect on reducing the quality of acoustic signal. In this paper we concentrate on denoising the effect due to wind on underwater acoustic signal using the LMS algorithm. The wind speed of the collected data ranges from 2.11 m/s to 6.57 m/s. The analysis is carried out for acoustic frequencies ranging from 100 Hz to 8 kHz. It is found that a linear relationship between noise spectrum and wind speed exists over the entire frequency range. The results of the empirical data are compared with the results obtained with the aid of the noise model developed. An adaptive model exploiting the Least Mean Square (LMS) algorithm to denoise wind driven ambient noise in shallow water has been proposed. The observation shows that the Signal to Noise Ratio (SNR) is enhanced two fold and the Mean Square Error (MSE) decreases exponentially with the aid of the LMS adaptive algorithm. [ABSTRACT FROM AUTHOR]

Multi-path interference is often considered when modelling propagation of underwater sounds from source to receiver. When close to, or on the seabed, a hydrophone receives the direct and bottom reflected signals at nearly identical times. The resulting interference leads to an effective receive beam pattern that depends not only on source and receiver position and water depth, but also on the seabed characteristics, which affect the phase and magnitude of the reflection coefficient. Numerical acoustic propagation models account for this phenomenon automatically, however; it is important that it be taken into account when received signals are used to carry out simple calculations of source levels of nearby sources based on spherical spreading. Australian waters lie above seabeds of greatly differing acoustic properties. Compressional and shear sound speeds and absorption properties for four bottom types (basalt, calcarenite, sand and silt) were used to model the effective receive beam pattern of a hydrophone located on the seafloor. Modelling was carried out for all four seabeds as well as a seabed comprising differing thicknesses of sand over a calcarenite half space. The effects of the resulting receive beam pattern on estimations of source levels and locations are discussed. INSET: BACKGROUND NOISE AFFECTS THE TASTE OF FOODS. [ABSTRACT FROM AUTHOR]

This article presents measurements of underwater pile driving noise recorded during the construction of the duplicate Houghton Highway bridge in western Moreton Bay, Queensland. Moreton Bay is a protected marine park, a World Heritage Site and a Ramsar Wetland, providing habitat for turtles, dugong, sharks, dolphins and whales, some species of which are listed as vulnerable to endangered. Pile driving noise was measured for small and large piles at various locations and ranges. Using an acoustic propagation model, a sound map was computed for Bramble Bay. Sound levels were compared to currently available information on impact thresholds. Ranges greater than those corresponding to impact thresholds were scanned for the absence of dolphins before and during pile driving in line with a monitoring and response plan. [ABSTRACT FROM AUTHOR]