Search

Target identification of ship-radiated noise is a crucial area in underwater target recognition. However, there is currently a lack of multi-target ship datasets that accurately represent real-world underwater acoustic conditions. To tackle this issue, we conducted experimental data acquisition, resulting in the release of QiandaoEar22—a comprehensive underwater acoustic multi-target dataset. This dataset encompasses 9 h and 28 min of real-world ship-radiated noise data and 21 h and 58 min of background noise data. To demonstrate the availability of QiandaoEar22, we executed two experimental tasks. The first task focuses on assessing the presence of ship-radiated noise, while the second task involves identifying specific ships within the recognized targets in the multi-ship mixed data. In the latter task, we extracted eight features from the data and employed six deep learning networks for classification, aiming to evaluate and compare the performance of various features and networks. The experimental results reveal that ship-radiated noise can be identified from background noise in over 99% of cases. For the specific identification of individual ships, the optimal recognition accuracy achieves 99.56%. Finally, we found using spectrum and MFCC as feature inputs and DenseNet as classifier can achieve excellent recognition performance. Considering the computational efficiency, CRNN and ECAPA-TDNN are also good choices. Our work not only establishes a benchmark for algorithm evaluation but also inspires the development of innovative methods to enhance underwater acoustic target detection (UATD) and underwater acoustic target recognition (UATR). [ABSTRACT FROM AUTHOR]

Underwater wireless communication stands as a captivating and swiftly progressing domain in recent. To support higher data rate and higher bandwidth applications, there is significant attention towards underwater acoustics along with Visible Light Communication (VLC) technology. Underwater sensor networking serves as the backbone of oceanic exploration efforts. Based on the traffic load and underwater environmental conditions, an appropriate communication link can be selected to achieve higher data rate. Acoustics supports communication over longer distances with lower data rate. Despite offering higher data rates, Visible Light Communication (VLC) faces limitations in terms of communication range, proving to be relatively short compared to other methods. A seamless transition is anticipated during data transmission and reception by employing Light Emitting Diodes (LEDs) and photo detectors on the optical-acoustic channel, bridging the gap between acoustic and optical communication methods. Utilizing MIMO-based Enhanced Reverse Polarity Orthogonal Frequency Division Multiplexing (MIMO-ERPO-OFDM) in Underwater Optical Communication (UWOC) proves to be a superior approach, achieving a high throughput of 19.9 Mbps and a lower Bit Error Rate (BER) of 10− 1. [ABSTRACT FROM AUTHOR]

Chiral metamaterials have been proven to possess many appealing mechanical phenomena, such as negative Poisson's ratio, high-impact resistance, and energy absorption. This work extends the applications of chiral metamaterials to underwater sound insulation. Various chiral metamaterials with low acoustic impedance and proper stiffness are inversely designed using the topology optimization scheme. Low acoustic impedance enables the metamaterials to have a high and broadband sound transmission loss (STL), while proper stiffness guarantees its robust acoustic performance under a hydrostatic pressure. As proof-of-concept demonstrations, two specimens are fabricated and tested in a water-filled impedance tube. Experimental results show that, on average, over 95% incident sound energy can be isolated by the specimens in a broad frequency range from 1 kHz to 5 kHz, while the sound insulation performance keeps stable under a certain hydrostatic pressure. This work may provide new insights for chiral metamaterials into the underwater applications with sound insulation. [ABSTRACT FROM AUTHOR]

Applying numerical modelling approach the accuracy in determining the distance between underwater sound sources and receivers is assessed at a range of several kilometers from each other in the Kara Sea in autumn. It is suggested that the main source of errors in determining the distance is the lack of accurate data on the vertical sound speed profile along the acoustic signal propagation path. Data from September and November were analyzed, in the interval between which significant changes in the profile take place, when the vertical sound speed gradient changes from negative to positive. Characteristic values of sound speed variations were obtained by statistical processing of hydrological data taken from the World Ocean Database. The results are important for analyzing the capabilities of underwater acoustic navigation. [ABSTRACT FROM AUTHOR]

The article presents the results of a high-frequency experiment to detect and determine the bearing of a small-sized underwater noise sound source, carried out in shallow waters of the Black Sea coast. Noise emission from the source was received by three single vector–scalar receivers located at the bottom. Holographic processing was used to detect and determine the bearing of a moving underwater source against a background of heavy shipping in the water area of the experiment. Estimates of the input signal/noise ratio are given. [ABSTRACT FROM AUTHOR]

A small set of pilots in underwater acoustics OFDM (UWA-OFDM) systems tends to be insufficient for recovering the channel impulse response (CIR). This may result from the fast changes in the propagation environment and the requirement for high transmission data rates. A previous work, namely PE, considered the received subcarriers whose distances to their closest constellation points below the predetermined threshold T as potential pilots. However, extracting these pilots at the same time gives no chance to use some of them to facilitate searching for others. Fixing threshold T is also another limitation for searching pilot candidates. This paper proposes a reliable pilot search (RPS) method that consists of a multi-iterations pilot searching process with an adaptive threshold T a to improve the estimator in UWA-OFDM. The pilot search process gradually extracts reliable pilots at each iteration based on the threshold T a and evaluates them by performing channel estimation. Our method is compared to the MMSE and PE estimators on a wide range of settings, including the number of channel taps, pilot spacing, and various modulation schemes (i.e., MPSK and MQAM). The experimental results show that the RPS method often outperforms the MMSE and PE methods in terms of bit error rate (BER). [ABSTRACT FROM AUTHOR]

The article performs a comparative analysis of the underwater sounds of two sympatric dolphin species living in the Black Sea: bottlenose and common dolphins. During sound processing and analysis, the special program nanoCAD 22 was used. Both the similarity of the physical parameters of the sounds of the two species and a number of specific features of whistles characteristic of each were demonstrated. Common dolphin production of some types of whistles with significant similarities (sometimes almost identical) to the signature whistles of bottlenose dolphins was detected and analyzed. [ABSTRACT FROM AUTHOR]

Directional sensing of underwater acoustic signals is vital in many areas, such as waterborne navigation systems, abyssal signal communication, and underwater sound source localization. Here, we propose an easy-to-make acoustic sensing metamaterial device, which is made of a bubble architecture with a line defect. Both numerical and experimental demonstrations show that an input-direction-sensitive response with an acoustic pressure enhancement factor ~ 13.9 was achieved approximately at the resonant frequency. Furthermore, enhanced waterborne acoustic sensing at different frequencies was realized by adjusting the volume of the defect cavity. This strategy provides an effective and feasible method for underwater weak signal detection and localization. [ABSTRACT FROM AUTHOR]

An algorithm has been substantiated for passive determining the distance to sound sources in convergence zones (CZ), which are observed in most deep-sea areas of the World Ocean. The algorithm is based on the well-known pattern of formation of the ray structure of the acoustic field of the source signal at the input of the receiving array in CZ. The pattern is as follows: when a source enters a CZ by crossing its near boundary, the maximum energy of its signal arrives at the array along rays with negative grazing angles, and when the source enters a CZ by crossing its far boundary, along rays with positive grazing angles. The accuracy in determining distance using the proposed algorithm for detecting surface and underwater sources is assessed. [ABSTRACT FROM AUTHOR]

On the example of an underwater sound channel typical of the Philippine Sea [1–3], it was established by numerical simulation using mode theory that during experimental studies of the propagation of explosive signals by R.A. Vadov [1–3], manifestation of a weakly divergent beam in the spatiotemporal structure of the acoustic field was observed for the first time: at certain locations of the corresponding points in the oceanic waveguide, in addition to classical quadruples of pulses, additional acoustic signals with small time delays with respect to them. [ABSTRACT FROM AUTHOR]

Degradation of polymer composites is a significant problem in many engineering aspects. Due to the interaction of various degradation factors during the exploitation of composites, a synergistic effect of destruction is observed. The article describes the phenomena occurring in glass fiber reinforced polyester laminates under the influence of ultraviolet radiation (UV) in an aquatic environment. The laminates were exposed to UV-A, UV-B and UV-C radiation for 1000 h in free-air and underwater conditions. During the test, the materials were immersed at stable depth of 1 mm and 10 mm, respectively. The three-point bending tests performed on the samples after being exposed to UV showed an increase in the flexural strength of the composites. Simultaneously, degradation of the outer surface layer was observed. The degradation removed the thin resin film from the surface which resulted in a direct exposure of the reinforcing fibers to the environment. The transformations taking place in the deeper layers of the composite increased the mechanical strength due to the additional cross-linking reactions excited by the energy arising from the radiation. Moreover, the formation of polymer structures from free styrene remaining after the technological process and the occurrence of free radical reactions as a result of the cage effect was also observed. [ABSTRACT FROM AUTHOR]

To facilitate wireless communication, ocean exploration, and other military and scientific uses, underwater acoustic sensor networks have been developed. This network's location in a complex and demanding underwater environment increases failure rates and lowers the network's efficiency in terms of hardware and software. As a result, defining the quality of service in wireless sensor networks operating underwater is a developing study field. Supporting QoS is a difficult problem because of resource limits in underwater sensor networks, such as computing power, memory, bandwidth, and power source. The hardware architecture, elements, and structure of underwater sound sensor networks have been described in this study, along with a detailed explanation of the network levels and the difficulties that each layer faces. A summary of different QoS-aware routing methods for underwater sensor networks is also provided, along with a discussion of the QoS requirements for each layer. Finally, based on the crucial quality standards for the routing services in the underwater sensor network, each of the aforementioned protocols has been assessed and contrasted. In the conclusion, the service quality in underwater sensor network routing has been enhanced by the presentation of a routing solution based on multi-cast routing, hybrid clustering, and fuzzy logic. [ABSTRACT FROM AUTHOR]

A working group from the Global Library of Underwater Biological Sounds effort collaborated with the World Register of Marine Species (WoRMS) to create an inventory of species confirmed or expected to produce sound underwater. We used several existing inventories and additional literature searches to compile a dataset categorizing scientific knowledge of sonifery for 33,462 species and subspecies across marine mammals, other tetrapods, fishes, and invertebrates. We found 729 species documented as producing active and/or passive sounds under natural conditions, with another 21,911 species deemed likely to produce sounds based on evaluated taxonomic relationships. The dataset is available on both figshare and WoRMS where it can be regularly updated as new information becomes available. The data can also be integrated with other databases (e.g., SeaLifeBase, Global Biodiversity Information Facility) to advance future research on the distribution, evolution, ecology, management, and conservation of underwater soniferous species worldwide. [ABSTRACT FROM AUTHOR]

Underwater acoustic sensor signal processing relies on sound absorption of underwater acoustic energy. Echo absorption of underwater acoustic waves is a challenging area because of the complexity of a marine ecosystem and the uniqueness of an underwater acoustic route. We developed Improved Weighed Quantum Particle Swarm Optimization-based Ensembles Empirical Mode Decomposition (IWQPSO-EEMD), Mean Square Variance (MSV), & Least Mean Squares Algorithm (LMSA) driven echoes absorbing underwater acoustic waves to address the problem. Initially, the original data was divided into Intrinsic Mode Functions (IMF) separated into noise IMFs and actual IMFs. Next, noise IMFs detected the echo elements and removed them using MSV and LMSA. The final denoised sensor signal was received after reconstructing both genuine and removed noise IMFs. Lastly, we employ a fusion approach that surpasses the Blind Source Separator & Categorization method. Examine simulation sounds with actual underwater sound waves compared to many other echo absorption methods. We demonstrate the validity of the IWQPSO-EEMD-MSV-LMSA with superior echo absorption efficiency and real application potential. [ABSTRACT FROM AUTHOR]

2-1-3 piezoelectric composite is advantageous for its high piezoelectricity, mechanical stability and ease of preparation. Applying piezoelectric shunt damping technology in underwater semi-active sound absorption increases the adjustability of sound absorption performance. This paper analyzes the underwater semi-active sound absorption performance of 2-1-3 composite. The theoretical model and finite element model of 2-1-3 composite are established and the sound absorption coefficient of the composite coating is deduced based on transfer matrix and Mason equivalent circuit theory. The effects of the volume fraction of PZT-5H phase μ, the aspect ratio α and the shunt circuit on the sound absorption coefficient are studied and the root causes are analyzed. The results show that μ and α can affect the peak frequency, bandwidth and the range between open-circuit and short-circuit peak frequencies by changing the longitudinal wave velocity, mechanical loss factor and electromechanical conversion coefficient of the composite. Shunt circuits can adjust peak frequency and bandwidth by changing the elastic constant component c33 and mechanical loss factor of the composite. The conclusions of this study have value as reference for the design of underwater semi-active sound-absorbing material in various application scenarios. [ABSTRACT FROM AUTHOR]

Transmission and reception of images and videos are critical for remote real-time underwater monitoring in internet of underwater things (IoUT) applications. Underwater communication is particularly challenging since it relies on highly lossy underwater acoustics (UWA) channel. High- speed mobility is difficult in UWA systems as UWA includes fast-varying channels. For channel estimation and image transmission in UWA, the performance of the conventionally used orthogonal frequency-division multiplexing (OFDM) might deteriorate dramatically if precise Doppler correction is not used. To alleviate this issue, the orthogonal time frequency space (OTFS) modulation is effective. The advantages of OTFS are two-fold, i.e., the orthogonality in time–frequency domain and clustered sparsity of channel information in the delay-Doppler domain. For efficient image transmission in the UWA channel, the integration of compressed sensing (CS) has also been proposed in this work. The two-dimensional structured Robbins Monro (RM) and the greedy orthogonal matching pursuit (OMP) CS algorithms are considered and the better between the two have been established. The proposed approach converts a time–frequency doubly fading UWA channel to a time invariant channel. Here the high-resolution picture that is to be sent is separable and orthogonal in character. All of the delay- Doppler diversity branches are grouped together. OTFS is able to achieve channel capacity with optimum performance-complexity because of the high-resolution delay-Doppler separation. Moreover, OTFS also provides robustness against Doppler and multipath channel conditions. Simulation results prove that the suggested 2D-OMP and 2D-RM algorithms offer higher MSSIM and PSNR values than the OTFS system alone. [ABSTRACT FROM AUTHOR]

Due to the unavoidable movement of nodes within oceanic environments and the continual changing of underwater sound speed, it is difficult to synchronize clocks between nodes and locate them. Therefore, this paper proposes a joint time synchronization and localization mechanism (JTSL). Firstly, a time synchronization method is proposed based on three autonomous underwater vehicles (AUVs). In this method, AUVs send synchronization request signals to the moving unknown node three times. The clock skew and offset of each AUV compared to the clock of the unknown node are obtained based on the information transfer delay between nodes as sound speed changes. After time synchronization, the unknown node receives location information from AUVs and obtains the sound speed change function. Based on this function and kinematic equations, a distance calculation method is developed to calculate the distance between the unknown node and AUVs. In addition, a multi-layer fast meshing localization method is designed to improve the accuracy and speed of node localization. The simulation experiments show that JTSL can overcome difficulties related to time synchronization and localization of mobile nodes, and it demonstrates excellent performance in terms of time synchronization accuracy, energy efficiency, localization accuracy, and localization time. [ABSTRACT FROM AUTHOR]

Abstract—The results of studies of the features of the formation of pulse characteristics on the axis of an underwater sound channel in waveguides with different hydrological and bathymetric conditions of the Sea of Japan and the Sea of Okhotsk are discussed. The results of model calculations and experiments characterizing the regularities of propagation of low-frequency pulse signals in complex waveguides including shelf and deep sea for hundreds of kilometers are presented. It is shown that one of the main effects that determines long-range sound propagation in complex waveguides including the shelf and deep sea is the acoustic "landslide" effect. It is also shown that numerical modeling of the process of signal propagation from the shelf to the deep sea on acoustic traces in the Sea of Japan and the Sea of Okhotsk using the RAY program provides good convergence of the calculated and experimentally obtained impulse characteristics. [ABSTRACT FROM AUTHOR]

Abstract—We discuss the results obtained when performing a test acoustic-hydrological experiment in August 2022 at a marine test site from the coast of Sakhalin Island to Kita-Yamato Bank in the Sea of Japan. The methodology of preliminary studies in the water area intended for studying climatic variability of temperature regimes of the aquatic environment based on numerical modeling using the RAY computational program and the NEMO ocean hydrodynamic circulation model is presented. One of the main results is the average temperature of the marine environment calculated with high accuracy on the axis of the underwater sound channel in the Sea of Japan on the 1000-kilometer acoustic trace at the crossing of the vortex system. The shape of the measuring system and the technical and computational means and methods described in the article can be used as a basis for the organization of high-precision operational monitoring of thermodynamic processes in extended sea areas. [ABSTRACT FROM AUTHOR]

Abstract—Numerical modeling with the use of mode theory was used to investigate the regularities of the spatial (depth and horizontal distance) distribution of the acoustic field intensity formed by multiple interaction of a caustic beam with a layered bottom in a shallow oceanic waveguide with an underwater sound channel open to the bottom. It has been established that at the values of the sound velocity at the upper boundary in the sedimentary layer, less than the sound velocity at the bottom in the water layer, the formation of a multi-beam structure of the acoustic field is possible. It was found that, starting from certain distances, newly formed beams can play the main role in the spatial distribution of the acoustic field intensity. [ABSTRACT FROM AUTHOR]

Abstract—The transition from the traditional representation of the wave field in the vertical section of an underwater sound channel as a function of depth and time to the distribution of this field in the 3D phase space "depth–angle–time" is considered. For this purpose, the method of coherent states developed in quantum theory is used. The meaning of the proposed transition is that the field intensity distribution in the specified phase space is less sensitive to sound velocity fluctuations than in the original 2D depth–time space. This fact can be used in solving inverse problems. As an example, we consider the reconstruction of the coordinates of a source in a waveguide from measurements of the field intensity distribution of this source in phase space. [ABSTRACT FROM AUTHOR]

Local dry underwater backward median pulse MIG welding (LDU-BMPMIG) was proposed in this study. The effects of different backward median currents on welding process stability, weld forming, microstructure, and mechanical properties of weldments were investigated comprehensively. The results reveal that the backward median current waveform effectively improved the welding stability. By calculating the approximate entropy of the voltage signal during welding, it is proved that the welding stability increases first and then decreases with the increase of the backward median current. The welding process stability was the most satisfied with the backward median current of 200 A. The quality of weld formation was consistent with welding stability. LDU-BMPMIG weldments exhibited fewer macroscopic defects and better overall morphology compared with LDU-PMIG weldment. The weldment microstructure was mainly composed of γ-austenite and δ-ferrite with different morphologies. The satisfied stability of the welding process contributed to improving the microstructure uniformity. The variation trend of tensile strength for LDU-BMPMIG weldments was approximately kept consistent with welding process stability. The weldment prepared by 200 A showed considerable mechanical properties with minimal fluctuation, which reached 97.58% of maximum average tensile strength, which meant the most excellent compared to the others. The experimental results can improve the stability of local dry underwater welding and provide a theoretical basis and technical support for the application of underwater welding. [ABSTRACT FROM AUTHOR]

The physical transformation of light will cause the quality of underwater images to decrease, which impacts the precision of object detection, recognition, and segmentation in underwater circumstances. In this study, we suggest an underwater image enhancement combining low-dimensional and global features (UIELG). This model can availably heighten the texture minutiae and global key features of subaquatic images. Moreover, we advise a new loss function, which can effectively boost the structure and texture similarity of underwater images. Finally, we train and test the model on the synthetic subaquatic images. The experimental outcomes declare that this model is preferable to the existing models in both SSIM and PSNR scores. And the experimental outcomes on the real-world subaquatic image dataset present the generalization and robustness of the suggested model. [ABSTRACT FROM AUTHOR]

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]

High-speed video filming of surface currents and synchronized acoustic measurements of the underwater sound signals of falling drops were performed in a laboratory tank. During successive falling, the main structural elements of collision of a single drop with the surface are preserved in distorted form in the flow pattern: cavity, splashes, crown, and splash; shock pulses accompanying each contact are stably repeated in the phonogram. In addition, rare resonance packets are observed. For multiple falling drops, the flow pattern changes dramatically: the main structural elements of the drop impact flow disappear, and the surface is covered with floating bubbles. The phonogram assumes the form of a noise signal, in the spectrum of which separate linear sections stand out. [ABSTRACT FROM AUTHOR]

Underwater light availability and exposure of ultraviolet radiation (UV) in mountain lakes is mainly controlled by dissolved organic matter and ice cover. However, both of these factors are affected by climate warming and other anthropogenic pressures. Still, little is known of the impacts of long-term fluctuations in underwater light conditions onto functional distribution of diatoms, species sensitive to changes in climate and UV penetration in mountain lakes. Two mountain lakes in the Italian Alps were analysed using paleolimnological approaches to investigate impacts of anthropogenic pressures (e.g., climate warming, acidification) on underwater light availability, exposure to UV and diatom functional group distributions, focusing on the post-industrial era. Contemporary diatom communities were collected from the main habitats in the shallow, high irradiance littoral zones of these lakes and geochemical proxies describing the development of lake-water transparency and carbon dynamics were analyzed from the sediment cores covering the time period from ca. 1400 CE to present. The geochemical data indicate that cultural acidification decreased lake-water organic carbon concentrations in both lakes, which suggests increased light availability and UV exposure during the past century. The responses of the studied lakes to anthropogenic pressures varied, as the lake situated at a lower altitude with a larger catchment showed only a few biotic changes indicating higher resilience, whereas the lake situated at higher altitude showed distinct changes in its ecological status. In this alpine lake, almost 30% of the diatom functional groups shifted from benthic to planktic during the post-industrial era. The role of increased light availability and UV exposure as a driver of diatom functional group distributions could not be unambiguously separated, and such diatom assemblage changes have been shown in many regions and most closely linked to a warming climate. However, low guild functional groups, prevalent also in the contemporary samples from the shallow littoral zone, dominated the diatom communities throughout the studied period, suggesting high tolerance to UV radiation. [ABSTRACT FROM AUTHOR]

In this work, an ultrathin acoustic metamaterial formed by space-coiled water channels with a rubber coating is proposed for underwater sound absorption. The proposed metamaterial achieves perfect sound absorption (α > 0.99) at 181 Hz, which has a deep subwavelength thickness ( λ / 162 ). The theoretical prediction is consistent with the numerical simulation, which demonstrate the broadband low-frequency sound absorption performance of the proposed super absorber. The introduction of rubber coating leads to a significant decrease of the effective sound speed in the water channel, resulting in the phenomenon of slow-sound propagation. From the perspective of numerical simulations and acoustic impedance analysis, it is proved that the rubber coating on the channel boundary causes slow-sound propagation with inherent dissipation, which is the key to meet the impedance matching condition and achieve perfect low-frequency sound absorption. Parametric studies are also carried out to investigate the effect of specific structural and material parameters on sound absorption. By tailoring key geometric parameters, an ultra-broadband underwater sound absorber is constructed, with a perfect absorption range of 365–900 Hz and a deep subwavelength thickness of 33 mm. This work paves a new way for designing underwater acoustic metamaterials and controlling underwater acoustic waves. [ABSTRACT FROM AUTHOR]

Submersed aquatic vegetation (SAV) plays an important role in ecosystems. Inventories of SAV spatial distribution and composition are important for monitoring changes in SAV. In this study, we compared three common SAV sampling methods to quantify SAV in western Lake Erie. Aerial imagery of near-shore areas in western Lake Erie was classified using object-based image analysis (OBIA) and evaluated against field-based surveys using single-beam sonar or rake samples. To assess variation among methods, data were assigned either vegetation 'presence' or 'absence' and compared for simple correspondence and agreement (Cohen's Kappa, κ). The two field-based methods had the highest correspondence at 78% (n = 782) and the highest κ = 0.545. Correspondence between OBIA and rake surveys was 69% (n = 245) and κ = 0.36. Correspondence between OBIA and hydroacoustics was the lowest of 54% (n = 30,768) with an agreement of κ = 0.17. Environmental factors such as water turbidity may have played a role in reduced agreement between OBIA and field methods. Determining the optimal method or combination of methods will depend upon research goals, effort, and cost, but each method can provide reliable SAV information for resource management. [ABSTRACT FROM AUTHOR]

A receiver system and azimuth estimation method for non-cooperative underwater acoustic frequency hopping (FH) signal are proposed. Using single acoustic vector sensor as receiver, the time–frequency information and spatial information are combined by constructing space–time–frequency distribution matrix. Then the time–frequency ridge line is extracted by neural network, and the energy extracted from the time–frequency ridge line is integrated with time. Finally, the target azimuth is estimated based on the principle of complex acoustic intensity method. Simulation analysis and experimental results show that the proposed method can effectively estimate the azimuth of the non-cooperative underwater acoustic FH signal under the condition of low signal-to-noise ratio (SNR). The root mean square error of azimuth estimation is less than 1° under the condition SNR > –10 dB, which improves the azimuth estimation performance of the traditional complex acoustic intensity method. [ABSTRACT FROM AUTHOR]

The acoustic noise radiated from various ships in the same class is varying due to the changing machinery regimes, the multi-path propagation effect, time-varying underwater channels, and fluctuating ambient noise. The complex underwater propagation environment causes random fluctuations in the frequency, amplitude, and phase of the signal at the receiver point. Hence, in order to overcome the aforementioned problems, this study proposes a novel deep convolutional-recurrent autoencoder, evolving by a compound cepstral lifter. In this approach, the compound autoencoders automatically extract the features without information loss and human intervention, and hybrid cepstral lifters reduce the multi-path distortion and time-varying shallow underwater channel effects. In order to evaluate the performance of the proposed model, three underwater acoustic datasets, including synthetic, ShipsEar, and real experimental datasets, are exploited. For the sake of having a comprehensive comparison, the performance of the designed model is compared with ten recently proposed benchmark models. The results approve that the designed model with an average accuracy of 96.11% and average giga-multiplier–accumulators equal to 0.019 reports the best accuracy and complexity than other benchmark models. Furthermore, the proposed model is less sensitive to SNR level compared to other benchmark models. [ABSTRACT FROM AUTHOR]

10.1007/s00024-020-02508-x 23 Liu B, Kalinowski M, Sun Y, Carrigan CR, Saragiotis C, Wang J, Ertl M, Kijima Y, Schoemaker R, Ku'smierczyk-Michulec J, Tipka A, Kreek S, Antoun T. Characterization of CTBT-relevant radioxenon detections at IMS stations using isotopic activity ratio analysis. Liu et al. ([23]) provides a rigorous mathematical approach to establish the spatial and temporal relationships between a signal created by a nuclear explosion and a radioxenon measurements at an IMS station, including the isotopic activity ratios and their uncertainties. The purpose of the IMS sensor network is to detect signals that are indicative of nuclear explosions, as well as to identify and to locate nuclear explosions underground, underwater or in the atmosphere. The Comprehensive Nuclear-Test-Ban Treaty (CTBT) was negotiated at the Conference on Disarmament in Geneva between 1993 and 1996. [Extracted from the article]

On 9 April 2019, an F-35A fighter jet of the Japan Air Self-Defense Force was lost offshore northern Honshu, Japan. Underwater sound phases deemed to be associated with the crash of the aircraft were recorded by a nearby seafloor observatory and the International Monitoring System hydrophone station at Wake Island. A location and origin time estimate is derived by combining the two datasets and distinctly matches the last known position of the aircraft. [ABSTRACT FROM AUTHOR]

The protection of marine habitats from human-generated underwater noise is an emerging challenge. Baseline information on sound levels, however, is poorly available, especially in the Mediterranean Sea. To bridge this knowledge gap, the SOUNDSCAPE project ran a basin-scale, cross-national, long-term underwater monitoring in the Northern Adriatic Sea. A network of nine monitoring stations, characterized by different natural conditions and anthropogenic pressures, ensured acoustic data collection from March 2020 to June 2021, including the full lockdown period related to the COVID-19 pandemic. Calibrated stationary recorders featured with an omnidirectional Neptune Sonar D60 Hydrophone recorded continuously 24 h a day (48 kHz sampling rate, 16 bit resolution). Data were analysed to Sound Pressure Levels (SPLs) with a specially developed and validated processing app. Here, we release the dataset composed of 20 and 60 seconds averaged SPLs (one-third octave, base 10) output files and a Python script to postprocess them. This dataset represents a benchmark for scientists and policymakers addressing the risk of noise impacts on marine fauna in the Mediterranean Sea and worldwide. [ABSTRACT FROM AUTHOR]

The extraordinary circumstances of the COVID-19 pandemic led to measures to mitigate the spread of the disease, with lockdowns and mobility restrictions at national and international levels. These measures led to sudden and sometimes dramatic reductions in human activity, including significant reductions in ship traffic in the maritime sector. We report on a reduction of deep-ocean acoustic noise in three ocean basins in 2020, based on data acquired by hydroacoustic stations in the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty. The noise levels measured in 2020 are compared with predicted levels obtained from modelling data from previous years using Gaussian Process regression. Comparison of the predictions with measured data for 2020 shows reductions of between 1 and 3 dB in the frequency range from 10 to 100 Hz for all but one of the stations. [ABSTRACT FROM AUTHOR]

Underwater acoustic is one of the hot-topic and complex research areas for advanced signal processing. In this research, our main motivation is to recommend a high accurate underwater sound classification method using a special graph-based feature generator. The most valuable features have been selected using ReliefF iterative neighborhood component analysis (RFINCA) selector. In the classification phase, Decision Tree (DT), k nearest neighbor (kNN), Linear Discriminant (LD), Naïve Bayes (NB), and support vector machine (SVM) classifiers have been used with 10-fold cross-validation. To calculate the performance of the TQWT and antiprism graph pattern-based feature generation and RFINCA selector-based sound classification method, two underwater acoustic datasets have been collected. According to tests, the best accurate classifier is SVM. SVM attained 90.33% and 96.91% accuracies for the collected depth and direction datasets respectively. The calculated results denoted the success of the presented antiprism graph pattern-based method for underwater acoustic classification. [ABSTRACT FROM AUTHOR]

Methodological and technical possibilities of monitoring temperature fields along a 1000-km track in the Sea of Japan using acoustic thermometry are presented. The proposed tomographic method for monitoring the dynamics and structure of waters is based on emission and reception of complex phase-shift keyed signals on a diagnosed path with determination of the propagation time along various ray trajectories with further measurement of the speed of sound and temperature. The physical prerequisites for practical use of thermometric studies at large distances are based on the acoustic "mudslide" effect: the phenomenon of the transition of acoustic energy from the bottom shelf area to an underwater sound signal of the deep ocean. A high-precision system of acoustic thermometry on the basis of tomographic schemes with mobile and stationary hydroacoustic emitters and receiving systems is proposed and tested with the example of the Sea of Japan. [ABSTRACT FROM AUTHOR]

It is important to consider horizontal heterogeneity in an underwater sound speed structure to accurately estimate positions of GNSS-acoustic sites. Although large amounts of moving survey data (a sea-surface platform moves around when acoustic signals are transmitted) are required to accurately detect a sloping sound speed structure, the actual observational data do not necessarily include sufficient moving survey data due to sea conditions or observational time. To treat these insufficient data, it was assumed that a shallow sound speed gradient was continuously present up to a fixed water depth (gradient depth). However, the validity of this assumption has not been investigated, and the gradient depth has not been optimized. In this study, we developed a new GNSS-acoustic array positioning method that optimizes the gradient depth using an MCMC technique. To employ this technique, we also developed an approximate technique for rapidly calculating travel time, because the conventional travel time calculation requires high computational cost for integration into the MCMC technique. We assessed the performance of the approximate travel time calculation technique and demonstrated its sufficient accuracy and precision for estimating array positions. Then, we applied the new GNSS-acoustic array positioning method to the actual observational data collected by the Japan Coast Guard and Tohoku University. Using enough amounts of the moving survey data, our method demonstrated the results comparable with the conventional GNSS-acoustic positioning method estimating a sloping sound speed structure; thus, the assumption of the sound speed gradient with the fixed water depth was valid. Moreover, due to the physical restriction of this assumption, our method provided robust solutions even when the observational data contained small quantities of moving survey data with a simple sea-surface track. Although our method still cannot be used in the scenario, where no moving survey data are available, it can work robustly compared with the conventional methods. [ABSTRACT FROM AUTHOR]

Long-range sound propagation in the deep ocean is considered. Attention is concentrated on the procedure of acoustic wavefield refocusing by means of the wavefront reversal. Temporal variations of oceanic medium in course of propagation from the probe source to the acoustic mirror and back violate the reciprocity principle and thereby lower efficiency of refocusing. It is shown that ray chaos caused by scattering on internal waves enhances sensitivity of the reversed wavefield to medium variations. An underwater sound channel crossing several synoptic eddies is considered as an example. It is shown that refocusing becomes almost impossible if distance between the probe source and the acoustic mirror exceeds 350 km. Nonetheless, one observes an anomalous zone of enhanced refocusing at distance of 300 km from the acoustic mirror. [ABSTRACT FROM AUTHOR]

As an Arctic gateway, the Norwegian Sea sustains a rich diversity of seasonal and resident species of soniferous animals, vulnerable to the effects of climate change and anthropogenic activities. We show the occurrence of seasonal patterns of acoustic signals in a small canyon off Northern Norway, and investigate cetacean vocal behavior, human-made noise, and climatic contributions to underwater sound between January and May 2018. Mostly median sound levels ranged between 68.3 and 96.31 dB re 1 μPa2 across 1/3 octave bands (13 Hz–16 kHz), with peaks in February and March. Frequencies under 2 kHz were dominated by sounds from baleen whales with highest rates of occurrence during winter and early spring. During late-spring non-biological sounds were predominant at higher frequencies that were linked mainly to ship traffic. Seismic pulses were also recorded during spring. We observed a significant effect of wind speed and ship sailing time on received sound levels across multiple distance ranges. Our results provide a new assessment of high-latitude continental soundscapes in the East Atlantic Ocean, useful for management strategies in areas where anthropogenic pressure is increasing. Based on the current status of the local soundscape, we propose considerations for acoustic monitoring to be included in future management plans. [ABSTRACT FROM AUTHOR]

Passive localization of underwater targets was a thorny problem in underwater acoustics. For traditional model-driven passive localization methods, the main challenges are the inevitable environmental mismatch and the presence of interference and noise everywhere. In recent years, data-driven machine learning approaches have opened up new possibilities for passive localization of underwater acoustics. However, the acquisition and processing of underwater acoustics data are more restricted than other scenarios, and the lack of data is one of the most enormous difficulties in the application of machine learning to underwater acoustics. To take full advantage of the relatively easy accessed unlabeled data, this paper proposes a framework for underwater acoustic source localization based on a two-step semi-supervised learning classification model. The first step is trained in unsupervised mode with the whole available dataset (labeled and unlabeled dataset), and it consists of a convolutional autoencoder (CAE) for feature extraction and self-attention (RA) mechanism for picking more useful features by applying constraints on the CAE. The second step is trained in supervised mode with the labeled dataset, and it consists of a multilayer perceptron connected to an encoder from the first step and is used to perform the source location task. The proposed framework is validated on uniform vertical line array data of SWellEx-96 event S5. Compared with the supervised model and the model without the RA, the proposed framework maintains good localization performance with the reduced labeled dataset, and the proposed framework is more robust when the training dataset and the test dataset of the second step are distributed differently, which is called "data mismatch." [ABSTRACT FROM AUTHOR]

During the summer of 2015, four 4D seismic surveys were conducted on the northeastern Sakhalin shelf near the feeding grounds of the Korean-Okhotsk (western) gray whale (Eschrichtius robustus) population. In addition to the seismic surveys, onshore pile driving activities and vessel operations occurred. Forty autonomous underwater acoustic recorders provided data in the 2 Hz to15 kHz frequency band. Recordings were analyzed to evaluate the characteristics of impulses propagating from the seismic sources. Acoustic metrics analyzed comprised peak sound pressure level (PK), mean square sound pressure level (SPL), sound exposure level (SEL), T100%, T90% (the time intervals that contain the full and 90% of the energy of the impulse), and kurtosis. The impulses analyzed differed significantly due to the variability and complexity of propagation in the shallow water of the northeast Sakhalin shelf. At larger ranges, a seismic precursor propagated in the seabed ahead of the acoustic impulse, and the impulses often interfered with each other, complicating analyses. Additional processing of recordings allowed evaluation and documentation of relevant metrics for pile driving, vessel sounds, and ambient background levels. The computed metrics were used to calibrate acoustic models, generating time resolved estimates of the acoustic levels from seismic surveys, pile driving, and vessel operations on a gray whale distribution grid and along observed gray whale tracks. This paper describes the development of the metrics and the calibrated acoustic models, both of which will be used in work quantifying gray whale behavioral and distribution responses to underwater sounds and to determine whether these observed responses have the potential to impact important parameters at the population level (e.g., reproductive success). [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 possibility of controlling the acoustic transparency of resonator boundaries by using piezoactive materials is considered. For a piezoactive structure with a special type of boundary conditions, a stationary electroacoustic vibration problem is solved in the plane-wave approximation. Transient radiation mode of a pumped resonator is considered. Possible applications of resonators with controlled boundaries in underwater acoustics and ultrasonic echo sounding are indicated. [ABSTRACT FROM AUTHOR]

Oceanic noise is the background interference in sonar performance prediction and evaluation at high sea states. Statistics of underwater ambient noise during Typhoons Soulik and Nida were analyzed on the basis of experimental measurements conducted in a deep area of the Philippine Sea and the South China Sea. Generated linear regression, frequency correlation matrix (FCM), Burr distribution and Gumbel distribution were described for the analysis of correlation with environmental parameters including wind speed (WS), significant wave height (SWH), and the inter-frequency relationship and probability density function of noise levels (NLs). When the typhoons were quite close to the receivers, the increment of NLs exceeded 10 dB. Whilst ambient noise was completely dominated by wind agitation, NLs were proportional to the cubic and quintic functions of WS and SWH, respectively. The fitted results between NLs and oceanic parameters were different for "before typhoon" and "after typhoon". The fitted slopes of linear regression showed a linear relationship with the logarithm of frequency. The average observed typhoon-generated NLs were 5 dB lower than the Wenz curve at the same wind force due to the insufficiently developed sea state or the delay between NLs and WS. The cross-correlation coefficient of FCM, which can be utilized in the identification of noise sources in different bands, exceeded 0.8 at frequencies higher than 250 Hz. Furthermore, standard deviation increased with frequency. The kurtosis was equal to 3 at >400 Hz approximately. The characteristics of NLs showed good agreement with the results of FCM. [ABSTRACT FROM AUTHOR]

Cartilaginous fishes (Chondrichthyes), including sharks, skates, rays, elephant fishes and chimaeras, have been in existence for over 400 million years and represent early stages of the evolution of extant jawed vertebrates. The sensory systems of cartilaginous fishes, including their hearing apparatus, have adapted to a diverse range of ecosystems, from the deep ocean to freshwater rivers, revealing high levels of morphological diversity. Since sound travels such long distances underwater, this environmental cue may represent an important signal in the behavior and survival of this group of fishes but there are still many gaps in our understanding of their hearing system. Based on current knowledge, cartilaginous fishes are most sensitive to low frequency sounds (< 1500 Hz) and there is abundant support that their inner ears use only particle displacement detection rather than the detection of sound pressure, but further studies are needed to corroborate the observations of long distance detection of sound sources that might be based on the detection of pressure oscillations. This review investigates the diversity and functional significance of the inner ear of chondrichthyans from a range of habitats and explores what is known about their hearing capabilities. How underwater sounds are processed by the central nervous system, the impacts of sound on acoustic ecology and behavior, and the potential effects of anthropogenic sound are also examined. Some suggestions for future work are presented to fill the large gaps in our knowledge of the hearing abilities of this important group of vertebrates. [ABSTRACT FROM AUTHOR]

Numerical simulation is applied to consider the propagation of a low-frequency sound pulse in a shallow-water waveguide, which consists of three fluid layers: a water layer (30 m thick, with a sound speed of 1500 m/s), a gas-saturated sediment layer (0.4 m thick, 300 m/s), and a lower half-space (1560 m/s). A similar situation takes place in Lake Kinneret. It is shown that when the source and receiver are located near the bottom at a distance of about 10 waveguide depths from each other, two types of pulses are well observed in the received signal, significantly separated by the arrival time. First, a group of pulses arrives, corresponding to the sound speed in the water layer, then a single pulse, the postcursor, arrives with a significant delay and corresponds to sound propagation in the low-velocity gas-saturated sediment layer. Mode analysis shows that the postcursor is a very unusual first mode (a so-called inleak normal mode) trapped in the gas-saturated layer like an oceanic underwater sound channel. The propagation velocity of the pulse of the first mode is close to the sound speed in the gas-saturated layer in a quite wide frequency range, which makes it possible to estimate this speed from the arrival time of the postcursor. [ABSTRACT FROM AUTHOR]

The paper discusses representation of the wave field on a vertical array in an underwater sound channel as the superposition of components stable with respect to large-scale perturbations of the sound velocity field. Each such component is formed by a narrow beam of rays incident on the array aperture. This representation makes it possible to modify traditional matched field processing by switching from comparison of the measured and calculated fields at the array to comparison of the stable components of these fields. It is shown that the modified approach in solving the problem of source localization makes it possible to weaken the requirements on the accuracy of the mathematical model of the medium. [ABSTRACT FROM AUTHOR]

In this paper, the process of vertical downward underwater supersonic gas jets and its acoustic characterization is investigated. This is because the acoustic properties of these jets are not well characterized to date; numerical simulations are conducted for three different jet conditions, for one case with an underexpanded jet and for two cases with overexpanded jets. For numerical validation, experimental overall sound pressure level measurements are available, giving a good agreement between simulations and experiments. The numerical data provide information about the formation of the jet pattern depending on the depth of penetration of the gas jet into the water. Adaptive filtering technique and time–frequency analyses are applied to evaluate the numerical acoustic data. Combined with the study of evolution of flow structures, the spectrum and spectrogram indicate that the formation of shock waves and its destruction by vibrations of the jet elevate pressure and sound pressure fluctuations in the flowfield. As a result of these investigations, it follows that computational fluid dynamics could be applied to well understand the physics behind underwater gas jet noise, and it is proposed to use computational fluid dynamics for further studies of gas jets blowing into water. [ABSTRACT FROM AUTHOR]

In this work, the acoustic absorption performance of polyurethane (PU) elastomer consisted of different molecular masses of polypropylene glycol (PPG), and a PU composite sample was made by adding multi-wall carbon nanotubes (MWCNT) was studied. Using the acoustic pulse tube device and spectrum of dynamic mechanical thermal analysis, modulus, glass transition temperature (Tg), damping, and acoustic absorption properties of PU with different components were analyzed. The sound absorption results in the frequency range of 2–18 kHz showed that the PU_2 (PPG 2000, TDI) had good acoustic absorption properties. The average acoustic absorption coefficient of PU_2 was 0.8, and the maximum acoustic absorption coefficient was 0.95. The PU_3 (PPG 1000, TDI + 0.1 mass% MWCNT) had lower acoustic absorption properties than PU_2, while the sample had a lower acoustic reflection coefficient compared to the samples PU_1 (PPG 1000, TDI) and PU_3. The damping properties showed that, with an increase in polyether molecular mass and also with the addition of MWCNT, the maximum value of tan(δ) was increased, while the glass transition temperature Tg decreased and increased compared to PU_1, respectively. The PU samples' hardness showed that the increase in molecular mass increases the soft segment and decreases the hardness, while the PU_3 hardness is superior to PU_2. [ABSTRACT FROM AUTHOR]

Longitudinal training dams (LTDs) in the river Waal are novel river training structures that protect the littoral zone from the adverse effects of navigation providing new habitats for riverine macroinvertebrates. In order to inform river management and to better understand their ecological value for native and alien mussel species, it is important to assess the habitat suitability of the protected LTD shore channels. We applied spatial hydroacoustics surveys consisting of side-scan sonar (SSS) and acoustic Doppler current profiler (ADCP) of the substrate type, water depth and flow velocity in three shore channels in combination with species sensitivity distributions (SSDs) to predict habitat suitability for native and alien mussel species. SSDs allowed for the prediction of habitat suitability as a potentially occurring fraction (POF) of a species pool. High substrate type, water depth, and near-bottom flow velocity POFs were found for ≥ 70%, 100%, and 4–51% of the total shore channel area, respectively, suggesting that shore channels provide suitable habitat for both native and alien mussel species. To enhance the shore channels as habitat for native mussel species, we recommend increasing shallow areas dominated by fine (silt/clay) and sand substrate types with low near-bottom flow velocities (near 0 m/s). In contrast, the total area of hard substrate (e.g., boulders) in the shore channels should be reduced as it strongly favored invasive alien mussel species in our study. Future research should include additional abiotic parameters to enhance the habitat suitability predictions and compare the results for different riverine habitats. [ABSTRACT FROM AUTHOR]