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• A theoretical method is developed to get the sound absorption performance of the FG coating excited by a phased array. • 2. FFT parameters in the angular spectrum method is discussed in detail. • 3. Parametric analyses give some new phenomena and meaningful conclusions. • 4. The angular spectrum decomposition of the incident sound field is utilized for mechanism analysis. Laying anechoic coatings on the surface of underwater vehicles is an effective technology to improve the acoustic stealth performance. Meanwhile, the detection waves may have complex directivity, and it is important to study their influence on the sound absorption of the coating. This article introduces a functionally graded coating under a phased arc array incidence, which simulates a novel anechoic coating on an underwater vehicle to absorb sound waves emitted by active sonars. The incident field of the phased array is constructed by spherical harmonic expansion and the superposition principle. The wave propagation of the functionally graded coating is analyzed through an equivalent laminated model and the reverberation matrix method. Utilizing the angular spectrum method, the reflected sound field and absorption coefficient of the coating under the phased arc array incidence are predicted and the theoretical method is formed. Then the selections of parameters in Fourier transforms of the angular spectrum method are discussed for achieving converged results. A finite element analysis is applied to verify the effectiveness and calculation speed of the proposed method through the establishment of detailed quarter or half model. The effect of phased arc array parameters and the coating on the absorption and reflection performances are further studied. The angular spectrum decomposition of the incident sound field explains the changing trends of the sound absorption curves. The present work provides an important way to deal with this complex underwater sound propagation problem. [ABSTRACT FROM AUTHOR]

In this research, the underwater sound absorption performance of porous media saturated with water is investigated through long straight tubes (LSTs) with circular cross-section and sintered fibrous metals (SFMs) containing complex pores, and effective underwater sound absorption capability is demonstrated theoretically, numerically and experimentally. Specifically, the theoretical model for LSTs, verified by direct numerical simulations (DNSs), reveals that much smaller pore diameter and much larger layer thickness of porous material is needed to gain high waterborne sound absorption coefficient than to obtain large airborne sound absorption coefficient. Besides, to examine the sound absorption capacity of realistic porous materials, SFMs backed with a finite cavity are experimentally measured under 7 different hydrostatic pressures in a water-filled tube, and numerically characterized via multi-physics couplings. Insensitivity of effective sound absorption capability to high pressure is established, which stems from the different underwater wave energy consumption mechanism (i.e. viscous-thermal effect) of porous materials from conventional rubber kind of underwater sound absorption materials. Physically, porous materials possess great potential in developing the next generation of high-performance underwater sound absorption materials. • Effective underwater sound absorption of water-saturated porous media is shown. • Viscous-thermal dissipation effect is insensitive to high hydrostatic pressure. • A porous material sample is tested in the water-filled tube for sound absorption. [ABSTRACT FROM AUTHOR]

This paper introduces a novel approach to expand the working frequency band of underwater acoustic transducers through multi-modal coupling. We designed a broadband multi-modal coupled piezoelectric capsule-shaped transducer structure by leveraging the advantages of conventional piezoelectric spherical-shell and piezoelectric cylindrical-tube transducers. First, the finite element method (FEM) was employed to scrutinize the influence of key structural parameters on the acoustic performance of the capsule-shaped transducer, thereby establishing the foundation for the final transducer design. Subsequently, the designed transducer was characterized through modal analysis and harmonic response analysis of electrical-mechanical-acoustical multiphysics field coupling. Finally, a prototype of the capsule-shaped transducer was fabricated and measured, with the results compared with those of a spherical-shell transducer. A comparative analysis of the measured outcomes revealed that the capsule-shaped transducer exhibits coupling among three modes within the frequency range of 10 kHz to 30 kHz. The transmitting voltage response (TVR) values exceeded 143.5 dB, reaching a maximum of 151.9 dB, and the working frequency bandwidth achieved 18 kHz. Compared with the spherical-shell transducer, the average TVR value demonstrates a 3.7 dB improvement, and the bandwidth exhibited an 8.4 % increase. According to the experimental findings, the proposed capsule-shaped piezoelectric transducer offers advantages such as high voltage response, a broad working frequency band, compact size, low weight, and easy installation. This technology can be further applied to small underwater platforms, enhancing their acoustic detection range and spatial coverage performance. [Display omitted] • A new multi-modal coupled transducer was proposed to expand the working frequency band. • The structure of the transducer was designed and optimized through finite element method. • The validity of the design was verified by fabricating and measuring the prototype transducer. • Compared with the spherical-shell transducer, the electroacoustic performance of the proposed capsule-shaped transducer is improved. [ABSTRACT FROM AUTHOR]

The sound speed profile (SSP) is an important factor affecting the acoustic propagation characteristics of the ocean, making the accurate acquisition of SSP a crucial step in the interdisciplinary research of oceanography and underwater acoustics. Limited by the cost of in-situ measurement and the performance of the instrument itself, direct measurement of SSP inevitably leads to insufficient depth or even missing information. In this paper, we propose using partial underwater prior information (UWPI) only including underwater sound speed to obtain preliminary reconstruction results of global SSP for the first time. The empirical orthogonal function (EOF) reconstruction algorithm is optimized by employing assimilated SSP as the background SSP to further reduce reconstruction errors. The maximum global average reconstruction error and root mean square error (RMSE) after optimization decrease by >51% and 71%, respectively, which indicates that the performance of the optimized algorithm combined with partial UWPI is further improved. Finally, the performance of the optimized algorithm is discussed from the perspective of acoustic propagation. This research provides a reliable technical approach for SSP reconstruction under incomplete depth conditions, which can be applied in underwater sound field prediction and acoustic detection in the future. [Display omitted] • Global scale SSP large-depth fast reconstruction is realized based on partial UWPI for the first time. • The algorithm is optimized and the maximum reconstruction error and RMSE decrease by >51% and 71%, respectively. • Providing a reliable technical approach for SSP reconstruction under incomplete depth conditions. [ABSTRACT FROM AUTHOR]

Ultrasonic wave propagation and scattering involving both solids and fluids underpins many key configurations in non-destructive testing and underwater acoustics. The resulting interactions are highly dependent on both material parameters and geometries and are difficult and expensive to investigate experimentally. Modelling capabilities are often used to overcome this, but these are also complex and computationally expensive due to the complexity of the fluid-solid interactions. We introduce a novel explicit time-domain finite element method for simulating ultrasonic waves interacting with fluid-solid interfaces. The method is displacement-based, and relies on classical hourglassing control, in addition to a modified time-stepping scheme to damping out shear motion in an inviscid fluid. One of the key benefits of the displacement-based approach is that nodes in the fluid have the same number of degrees of freedom as those in the solid. Therefore defining a fluid-solid model is as easy as defining an all-fluid or all-solid model, avoiding the need for any special treatments at the interfaces. It is thus compatible with typical elastodynamic finite element formulations and ready for implementation on a graphical processing unit. We verified the method across a range of problems involving millions of degrees of freedom in fields such as non-destructive testing and underwater acoustics. • A finite element formulation for solving elastic wave problems is presented. • The method is displacement based and explicit, allowing for ready implementation on GPUs. • Compatible with solid material finite element solvers due to continuity across the solid fluid interface. • It is suitable for simulations across a wide range of fields, with different scales. [ABSTRACT FROM AUTHOR]

Underwater sound absorption coatings capable of absorbing sound energy is essential to conceal underwater vehicles. While the accumulation of marine fouling on surfaces of ship hulls, platforms, and submarines can significantly alter their acoustic properties. Currently, the precise impact that different stages of marine fouling have on underwater sound absorption coatings remains unclear. In this study, a novel lab assessment method was developed to investigate the acoustic effect of marine fouling on the underwater sound absorption coatings. Four distinct stages of marine fouling were artificial fabricated and characterized, including conditioning film, microfouling, medium macrofouling, and heavy macrofouling. These effects are quantified through comprehensive tests conducted in a water-filled impedance tube. It is found that the first two stages of marine fouling have a negligible impact on the underwater sound absorption performance within the low frequency range of 500 Hz and 2000 Hz. While a limited impact is observed in the frequency range of 2000 Hz–5000 Hz, where the sound absorption coefficient remains approximately 0.6. Conversely, in the third and fourth stages of fouling, the average sound absorption coefficient drops from 0.8 to 0.2. Further, a new acoustic finite element method (FEM) model of hard fouling was developed to predict the acoustic effect, and the numerical results show the same trend with the experimental results. These findings emphasize the importance of considering antifouling strategies during the design of underwater sound absorption coatings, with a particular focus on targeting hard fouling organisms as the primary objective. • A laboratory-based artificial fouling fabrication method was developed. • The effect of different marine fouling on underwater sound absorption coatings were systematically investigated. • An acoustic finite element method (FEM)model of marine fouling was established. • The underlying mechanisms governing the impact of marine fouling on underwater sound absorption coatings were explained. [ABSTRACT FROM AUTHOR]

To enhance vibroacoustic radiation and propagation predictions, a novel integrated computation model (ICM) combining the finite element method (FEM), equivalent source method (ESM), and normal mode method (NMM) is proposed. This model overcomes the complex computation domain (CCD) issue inherent in current ICMs, improving accuracy and reducing model complexity. Initially, the FEM is used to compute the vibracoustic radiation from the volume source. Subsequently, the ESM is introduced to estimate Green's function within the near field. Additionally, the fast multipole method (FMM) is introduced to streamline multipoint interaction to that between two elements and enhance the computation speed of the ESM. Furthermore, the NMM is used to compute Green's function in the far field. In addition, the Hamiltonian analysis method (HAM) is employed to search for broadband wave numbers of the NMM and facilitate fast solutions for far field propagation. The simulation results indicate that the FEM-ESM-NMM model effectively addresses the CCD issue encountered with current ICMs. Moreover, computation, once accelerated, has been reduced by 40% in comparison to the ESM and NMM, and has seen an 80% reduction relative to the FEM. • Examines the issue of complex computation domain in integrated models from a distance-based perspective. • Presents an integrated model efficiently calculating underwater target sound fields at all distances. • Optimization algorithms were introduced to significantly improve the computational efficiency of the integrated calculation model. [ABSTRACT FROM AUTHOR]

The three-dimensional (3-D) sound speed structure of the ocean is a fundamental environmental element in studying underwater sound propagation modeling and forecasting. The accurate classification of the sound speed fields (SSFs) provides a comprehensive understanding and analysis of the sound propagation pattern. To take advantage of the 3-D structure of the ocean SSFs, this paper presents a quick method based on tensor decomposition for classifying the ocean 3-D SSFs. Utilizing the WOA18 dataset, High Order Iterative Orthogonal (HOOI) decomposition of the 3-D SSFs is executed so as to accurately extract the characteristic information of the SSFs. The Fuzzy C-Means clustering (FCM) method is applied to classify the feature tensors, partitioning of regional categories in different seasons and revealing the typical SSFs structures. By combining the BELLHOP model with analysis of the characteristics of the first convergence zone of each category, it is concluded that there are six categories of SSFs in the Western Pacific Ocean. The SSFs across all categories are primarily latitudinally distributed, featuring distinct sound channel axes and surface sound speed variations. • Effective extraction of tensor decomposition in sound speed fields. • Automatic tensor-based classification of sound speed fields. • Impact of typical sound speed vertical structure on sound transmission in the western Pacific Ocean. [ABSTRACT FROM AUTHOR]

The motion of charged seawater in a submarine wake, which is affected by the variance in the ion mass and the presence of a background geomagnetic field, results in a consequential wake magnetic field. The magnetic field is critical for the detection and monitoring of underwater targets. Despite its importance, the intricate relationship among hydrodynamics, ion transport, and electromagnetic fields, which collectively affect the generation of magnetic fields in submarine wakes, has yet to be sufficiently and systematically investigated. Hence, we formulate a theoretical model of the submarine wake magnetic field. This innovative interdisciplinary model seamlessly integrates the effects of ion inertia and ion induction in the marine environment. By performing multiphysics coupling simulations, we meticulously replicate the evolution of the wake magnetic field induced by the movement of a submarine propeller system. We comprehensively examine the velocity and magnetic-field components to elucidate the intricate mechanism of wake magnetic field generation. Furthermore, we comprehensively analyze the effects of ion inertia and ion induction on the wake magnetic field. Our findings highlight the predominant role of the ion-induction effect in determining the wake magnetic field when a submarine maintains a constant speed. Additionally, we investigate the effect of the submarine heading on the wake magnetic field and discuss the magnetic-field characteristics and attenuation rules at various axial and radial positions in the wake for different headings. Our scientific insights contribute considerably to the understanding of wake magnetic fields and offer valuable perspectives that can potentially advance future underwater target-detection methodologies based on wake magnetic fields. • The wake magnetic field is critical for the detection and monitoring of underwater targets. • Based on ion-force analysis, a complete theoretical model of the submarine wake magnetic field in an oceanic environment is established. • The effect of the mass difference of ions on the wake magnetic field is investigated. • The effect of the submarine heading on the wake magnetic field is investigated. [ABSTRACT FROM AUTHOR]

• The unique characteristics of acoustic metamaterials (AMMs), which are not found in natural materials, have been explored. • The fundamental structures, application areas, opportunities, and challenges of AMMs have been examined in detail. • The importance of simulation in designing AMMs has been emphasized, and 3D printing has been examined for their fabrication. • It has been suggested that a materials-centered approach holds the potential for revolutionary developments in this field. Acoustic metamaterials (AMMs) are a form of man-made material that can be specifically developed to have a sub-wavelength periodic structure with extraordinary characteristics not found in nature. Recently, the ability to manipulate sound waves and elastic waves, thanks to these properties, has attracted great interest from scientists and engineers and has been the subject of intense research. This review presents general conceptual descriptions of these materials. We have summarized current literature and we investigated basic structures such as Helmholtz resonators, membrane-type structures, locally resonant and space-coiled structures, and application fields of acoustic metamaterials such as sound imaging, sound absorption, acoustical cloaking, noise reduction, and underwater acoustics. As simulation methods are essential for design and performance evaluation of acoustic metamaterials, the studies carried out in the literature were described and 3D printing technologies for the fabrication of these designs were investigated. Finally, we discuss the opportunities and challenges of acoustic metamaterials. While acoustic metamaterials have enormous ability for the field of sound control, AMMs also present challenges such as frequency-dependent operation, optimization requirements, and material costs. This review provides a comprehensive resource to guide researchers and engineers in understanding the importance and future potential of acoustic metamaterials and identifying gaps in the literature. [ABSTRACT FROM AUTHOR]

Underwater sound classification has been an area of interest in the research community because of its applications in military, commercial, and environmental domains. Underwater sound classification is a challenging task because of the high background noise and complex sound propagation patterns in the sea environment. For underwater sound classification, deterministic as well as stochastic techniques are being used. However, in recent years, stochastic techniques which are learning-based are getting a lot of attention. There exist few survey studies with a limited scope that cover the limited number of studies. In this study, we present the most comprehensive review of research and the latest developments in the field of underwater sound classification by highlighting the contributions and challenges from over 250 recent research papers. We discuss machine learning as well as deep learning-based methods for marine vessel sound classification and fish sound classification. The study also includes details of sources of underwater sound, features, classifiers, datasets, related techniques, challenges, and future trends. We hope that the study will benefit the general reader as well as the research community to have a complete picture of the latest research in the field. • A comprehensive review of research and the latest developments in the field. • Highlight the contributions and challenges from over 250 recent research papers. • Discuss methods for vessel sound classification and fish sound classification. • Details of sources of sound, features, datasets, challenges, and future trends. [ABSTRACT FROM AUTHOR]

Underwater acoustics has assumed a pivotal role in the detection, navigation, communication, and target identification of ships and submarines. In recent years, advancements in noise reduction from marine vessels have underscored the significance of underwater acoustic vector sensors. This paper presents a novel approach to the design, fabrication, and characterization of a two-dimensional Micro-Electro Mechanical Systems (MEMS) vector acoustic sensor tailored for underwater environments. Traditional underwater acoustic vector sensors often employ doped polysilicon or silicon/germanium sensing films on rigid silicon substrates. The inflexible nature of silicon substrates necessitates labor-intensive and costly chemical-mechanical polishing processes to achieve submicron thickness, often requiring extended clean-room usage. In contrast, this work proposes a simplified, cost-effective, and scalable alternative. The sensor design utilizes reduced graphene oxide (rGO) as the sensing material and Kapton as the substrate, chosen for their unique properties. rGO exhibits high carrier mobility and low intrinsic noise, ensuring clear signals with minimal external interference, thereby improving measurement accuracy in challenging environments. Kapton provides excellent thermal stability, mechanical strength, and chemical resistance, along with superior insulating properties that reduce electrical noise, enhancing the signal-to-noise ratio. The designed vector sensor leverages the piezoresistive transduction principle, incorporating a thin piezoresistive film of rGO on a flexible Kapton substrate via a straightforward drop-casting method. Inspired by the sensory mechanisms of aquatic life, the two-dimensional vector sensor not only detects pressure associated with acoustic signals but also discerns the direction from which these low and ultralow frequency signals originate. The device's performance characteristics, sensitivity, and directivity are comprehensively evaluated through static, dynamic, and acoustic tests conducted in both air and water mediums. Static tests reveal the piezoresistive behavior of rGO under applied displacement, yielding displacement sensitivities of 0.0279 V/mm and 0.0276 V/mm for axial and radial displacements, respectively. Dynamic and acoustic tests demonstrate the sensor's capability to detect low-frequency signals spanning from 0.25 Hz to 200 Hz, exhibiting a smooth and favorable frequency response. In air, the sensor exhibits receiving sensitivities ranging from −15.18 dB to −19.75 dB (X-channel) and −13.67 dB to −16.67 dB (Y-channel). In the underwater environment, these values range from −134.40 dB to −138.93 dB (X-channel) and −131.83 dB to −136.93 dB (Y-channel). Notably, the variation in receiving sensitivity across channels in both air and water is limited to within 6 dB. Furthermore, the device showcases a symmetrical directivity pattern resembling the shape of an "8," with an approximate sensitivity of −136.66 dB, a significant improvement compared to the −165 dB sensitivity of conventional polysilicon underwater vector sensors. These experimental findings affirm the feasibility of the proposed underwater vector sensor, based on the piezoresistive effect and MEMS technology, highlighting its two-dimensional directivity capabilities. [Display omitted] • This study introduces a 2D MEMS vector acoustic sensor for underwater use, using rGO on Kapton for enhanced sensitivity. • rGO offers high carrier mobility and low noise; Kapton provides thermal stability, mechanical strength, and enhanced signal quality. • The vector sensor detects low frequency acoustic signals, with static, dynamic tests confirming piezoresistive behavior. • Acoustic tests show improved sensitivity and directivity in air and water, surpassing polysilicon sensors with a symmetrical "8" pattern. [ABSTRACT FROM AUTHOR]

Migratory fish populations have experienced great declines, and considerable effort have been put into reducing stressors, such as chemical pollution and physical barriers. However, the importance of natural sounds as an information source and potential problems caused by noise pollution remain largely unexplored. The spatial distribution of sound sources and variation in propagation characteristics could provide migratory fish with acoustic cues about habitat suitability, predator presence, food availability and conspecific presence. We here investigated the relationship between natural soundscapes and local river conditions and we explored the presence of human-related sounds in these natural soundscapes. We found that 1a) natural river sound profiles vary with river scale and cross-sectional position, and that 1b) depth, width, water velocity, and distance from shore were all significant factors in explaining local soundscape variation. We also found 2a) audible human activities in almost all our underwater recordings and urban and suburban river parts had elevated sound levels relative to rural river parts. Furthermore, 2b) daytime levels were louder than night time sound levels, and bridges and nearby road traffic were much more prominent with diurnal and weekly patterns of anthropogenic noise in the river systems. We believe our data show high potential for natural soundscapes of low-land river habitat to serve as important environmental cues to migratory fish. However, anthropogenic noise may be particularly problematic due to the omnipresence, and relatively loud levels relative to the modest dynamic range of the natural sound sources, in these slow-flowing freshwater systems. [Display omitted] • Underwater sounds carry information on river characteristics and soniferous animals. • Acoustic information may be used in behavioural decisions of fish, especially when migrating. • Traffic noise leads to diurnal cycles of increased underwater sound levels. • We raise serious concern about the potential impacts of traffic noise on aquatic life. [ABSTRACT FROM AUTHOR]

With the gradual opening of the Arctic channel, the underwater radiated noise induced by the ship-ice collision cannot be ignored. However, the underwater radiated noise induced by ship-ice collision is significantly different from that of ships navigating in open water. To explore the response rule of transient underwater radiation noise induced by ship-ice collision, this paper proposes a simplified numerical analysis method of ship-ice collision-induced transient underwater radiated noise in the full frequency band by combining the explicit dynamic method and acoustic analysis method. The ship-ice-water-air coupled collision analysis method based on the S-ALE algorithm is used to simulate and verify the ice load in the process of ship-ice continuous collision at different speeds. Then, the frequency-domain acoustic analysis method is used to predict the transient underwater radiated noise response in the full frequency band under icebreaking excitation and evaluate it by referring to the steady-state underwater radiated noise standard. The results show that the simplified analysis method can effectively predict the transient underwater radiation noise under icebreaking excitation. Under the icebreaking excitation, the radiation shape is irregularly scattered and the radiation direction is concentrated towards the bottom and bow of the ship with the frequency increase. The influence range of transient underwater radiated noise is mainly within 1000 Hz at different speeds, and the variation in the ship's speed affects the amplitude of the sound power level and the smoothness of the periodic response. The synthesized total sound power levels are 172.56 dB (V = 2.0kn), 174.15 dB (V = 2.5kn), 175.70 dB (V = 3.0kn), and 182.49 dB (V = 4.0kn), respectively. According to the comparison with the evaluation standard, it is found that the transient underwater radiation noise induced by ship icebreaking has a greater impact on marine organisms. It should give more detailed requirements within 10–1000 Hz when using relevant standards to evaluate underwater radiated noise induced by polar icebreaking. This paper can provide a reference for the research of underwater radiated noise induced by ship-ice collision and the formulation of relevant standards. • A process of the numerical calculation method for ship-ice collision-induced transient underwater radiated noise in the full frequency band is given based on IBEM and SEA methods. The process can be realized to forecast the transient underwater radiated noise of large-scale structures under icebreaking excitation. • To consider the influence of fluid and gravity on the ship-ice collision, the ship-ice-water-air four-phase coupling is realized using the S-ALE algorithm. Based on the numerical results, the model experiment, and the empirical formula (Lindqvist, Riska, Jeong), the longitudinal ice resistance values are compared and verified under different speeds. It is found that the numerical results are reliable as the sound source of ice-induced underwater radiation noise. • The transient underwater radiated noise of a polar transport vessel in the full frequency band was calculated for icebreaking excitation (force source). The frequency influence range of underwater radiated noise induced by icebreaking, the varying trends of radiated sound pressure, shape, and direction, and the influence of the ship's speed on radiated sound pressure, frequency influence range, and periodic response are analyzed. • Based on DNV and CCS standards, the ice-induced underwater radiated noise levels at different speeds are evaluated. It is concluded that the ship-ice collision will affect the survival of marine life to varying degrees. [ABSTRACT FROM AUTHOR]

• A photoacoustic communication system based on a pulsed laser and a highly sensitive broadband underwater hydro-acoustic sensor is developed for direct air-water communication (DAWC). • An enhanced single-path differential divide and differential self-multiplication (SDD-DSM) algorithm in conjunction with a phase-generate carrier (PGC) type demodulation technique is used to improve the sensitivity of hydro-acoustic sensor for the laser-generated sound (LGS) detection. • The feasibility of LGS-DAWC modulated by way of on-off keying (OOK) modulation and pulse position modulation (PPM) has been demonstrated. The selection of photoacoustic conversion mechanism, modulation method, and underwater sound detection method is critical in solving the problem of Air-Water Cross-Medium Communication (AWCMC) based on laser-generated sound (LGS) technology. The primary objective of this work is to provide a photoacoustic system used for AWCMC based on a pulsed laser and a fiber optic hydro-acoustic sensor. The pulsed laser is modulated by the way of on-off keying (OOK) modulation and pulse position modulation (PPM). Then, thermal expansion was suggested to generate the sound signal at the air-water interface. A novel hydro-acoustic sensor with high sensitivity was developed by utilizing the distributed feedback fiber laser (DFB-FL). The hydro-acoustic sensor uses an enhanced single-path differential divide and differential self-multiplication (SDD-DSM) algorithm in conjunction with a phase-generate carrier (PGC) type demodulation technique to detect LGS signals with great sensitivity. After filtering, envelope extraction, and digital shaping of LGS signals, the modulated information was successfully decoded, demonstrating the potential of OOK-LGS and PPM-LGS based on the given photoacoustic communication system applied to the AWCMC. [ABSTRACT FROM AUTHOR]

Ocean fronts, as typical marine phenomena, significantly impact the marine environment and the distribution of organisms. However, the research on their tilted structure is limited. This study considered the frontal slope as a key parameter for describing the tilting characteristics of ocean fronts. A novel method, based on high-resolution in situ observations and reanalysis data, was proposed to extract global frontal slopes using contour lines and surfaces. To validate this approach, a mathematical model was developed for the oceanic front slope and verified through in situ observations. The frontal slope equation produced reliable results, indicating that the front tilting primarily arises from shear flow and density differences. Furthermore, a modified oceanic front gradient parameter was proposed based on the frontal slope, and its significance in understanding the impacts of ocean fronts on underwater acoustic propagation was validated through numerical simulation and statistical analysis. The distribution characteristics of frontal slope for ocean fronts on a global scale were analyzed for the first time. Marginal ice zone fronts had the highest slope (>1.5 m/km), followed by tidal mixing fronts, western boundary current fronts, and water mass convergence fronts (1–1.5 m/km). In contrast, upwelling fronts and estuarine plume fronts generally had slopes below 1 m/km. This parameter provides a new perspective on ocean front systems and their influence on dynamic structures and ecological resource distributions. This study helps in a comprehensive understanding of the tilting characteristics of the ocean front and their implications. • A novel method extracted global ocean front slopes, revealing variations among different types of fronts. • The study validated a mathematical model for oceanic front slope through in situ observations. • The study introduced a modified ocean front gradient parameter, offering new light on underwater acoustics. [ABSTRACT FROM AUTHOR]

Auclair et al. (2021) analyzed the propagation of acoustic-gravity waves (AGWaves) in the ocean and showed that AGWaves dispersion can be described based on the inner and boundary dispersion relations. A major limitation to their two-dispersion-relation model is the assumption of a rigid bottom boundary since acoustic waves can cross the ocean bottom and propagate in the sediment. An extension of their AGWaves-dispersion model is consequently proposed toward a more realistic two-layers fluid model. This improvement enables the evaluation of the perspectives opened by the new generation of compressible ocean models for ocean-acoustics applications. The acoustic regimes in this resulting model are shown to be in agreement with underwater acoustics literature. In addition, the free-surface boundary condition is in turn compared to the pressure release boundary condition to establish a bridge with classical acoustic dispersion models. • The sediment bottom largely influences the acoustic modes propagation in the ocean. • Free-surface and pressure-release are alike for acoustic waves propagation. • The free-surface sediment-bottom wave guide is adequate for acoustic gravity waves. [ABSTRACT FROM AUTHOR]

• Broadband waterborne absorptive metamaterial with hydrostatic pressure resistance is proposed. • Acousto-electric analogy is adopted to determine the equivalent parameters for transfer matrix method. • Oscillators of helical configurations demonstrate strong hydrostatic pressure resistance. • Experimental results confirm the superior performance of the configurations at 3 MPa. Underwater absorptive metamaterials are rapidly developed duo local resonance mechanisms, but they still suffer from the disadvantages of poor hydrostatic resistance. The poor acoustic absorption performance at high hydrostatic pressure mainly attributes to the decreased damping coefficient and increased modulus of substrate at high hydrostatic pressure. Acoustic metastructures with helical oscillators are proposed and optimized with simulating annealing algorithm, where the helical oscillators are beneficial for easing the stress in the polymer substrate and resulting in good acoustic absorption abilities. Four acoustic absorptive metastructures without any oscillators (pure polymer substrate), with cylindrical oscillators, with helical oscillators, with both cylindrical and helical oscillators are proposed and experimentally verified in this study. All three metastructures with oscillators demonstrate excellent acoustic absorption coefficient with the absence of the hydrostatic pressure, while the metastructures with helical oscillators exhibit better hydrostatic pressure resistance compared with the metastructures with cylindrical oscillators. Besides, the coupling of helical oscillators and helical oscillators enhanced the acoustic performance furtherly. It is revealed that the acoustic metamaterials with a combination of cylindrical oscillator and helical oscillator exhibit superior performance under hydrostatic pressure up to 3 MPa in the frequency range of 0.5kHz∼10 kHz compared with the others, demonstrating a new avenue for underwater acoustic metamaterials application. [Display omitted] [ABSTRACT FROM AUTHOR]

Due to the considerable noise and high annotation cost of passive sonar acquisition data challenges, underwater acoustic target recognition (UATR) tasks require novel solutions. In this paper, we propose a self-supervised dual-channel self-attention acoustic encoder (DSAE) for UATR tasks. First, to address the annotation challenge, we design a dual-channel self-attention acoustic encoder to unify features into self-supervised learning. Meanwhile, we propose a dynamic positive sample memory module (DMM) to enable the training samples comprehensive and balanced in self-supervised learning. Second, to address the noise challenge, we utilize a time–frequency mask to obtain space–time enhanced features. Based on our experimental results, DSAE improves the recognition accuracy and anti-noise robustness compared with other advanced acoustic learning methods. The results demonstrate that DSAE has the potential to offer significant value as a tool for UATR tasks. • Proposing a novel network for underwater acoustic target recognition. • Devising a dual-channel acoustic encoder to address annotation challenges. • Proposing a sample balancing strategy to optimize model training. • Proposing a feature enhancing strategy to address noise challenges. • Achieving promising recognition accuracy and anti-noise robustness. [ABSTRACT FROM AUTHOR]

• Liquid-solid synergistic (LSS) mechanism is proposed for sound absorption. • 3 mm coating efficiently absorbs sound from 1 to 1000 Hz. • Optimal channel widths are obtained corresponding to viscosities of viscous liquid. • Three conditions for implementing LSS mechanism are given. The liquid-solid synergistic (LSS) mechanism is put forward for sound absorption. An ultra-thin LSS anechoic coating model is established to achieve significant low-frequency and broadband sound absorption. Based on the LSS mechanism, the coating converts longitudinal acoustic waves into shear wave of viscous liquids and bending wave of viscoelastic solids. The wavelengths of shear wave in viscous liquids and bending wave in viscoelastic solids are both very short, but not equal to each other. Through liquid-solid synergistic coupling, low-frequency broadband water acoustic waves are attenuated and absorbed by the ultra-thin LSS coating. The LSS mechanism is analyzed in detail and further compared with that of Helmholtz resonator. Three conditions for implementing LSS mechanism are given. The effects of structural parameters, and material parameters of the solid and the liquid on the sound absorption capacity are investigated. The anechoic coating owns the sound absorption coefficient greater than 0.5 in the band of 1 to 1000 Hz, with a thickness of only 3 mm, which is far less than 1‰ of the incident wavelength at 100 Hz. The innovative mechanism provides a solution to the challenge of achieving thin thickness and low-frequency broad band sound absorption of underwater anechoic coatings. [Display omitted] [ABSTRACT FROM AUTHOR]

International sailing regattas are major sporting events often held within coastal marine environments which overlap with the habitats of marine species. Although races are confined to courses, the popularity of these events can attract large spectator flotillas, sometimes composed of hundreds of motorized vessels. Underwater noise from these flotillas can potentially alter soundscapes experienced by marine species. To understand how these flotillas may alter soundscapes, acoustic recordings were taken around racecourses during the 36th America's Cup in the Hauraki Gulf, New Zealand in 2021. Sustained increases in broadband underwater sound levels during the regatta (up to 17 dB re 1 μPa rms; 0.01–24 kHz) that extended beyond racecourse boundaries (>8.5 km) and racing hours were observed; very likely attributable to the increase in regatta-related vessel activity. Underwater noise pollution from spectator flotillas attending larger regattas should be considered during event planning stages, particularly when events occur in ecologically significance areas. • International regattas can attract a large number of spectators in motorized vessels. • Spectator vessels increase underwater noise well beyond course boundaries and racing hours. • Underwater noise from spectator flotillas should be considered during event planning. [ABSTRACT FROM AUTHOR]

The utilization of machine learning in analyzing ship radiated noise (SR-N) is undergoing rapid evolution. Because the omnipresent background noise strongly depends on the highly variable environment, the application of such techniques poses challenges. Furthermore, publicly available labeled datasets are scarce. Motivated by this, there has been a surge in the number of publications regarding the implementation of machine learning in the monitoring of SR-N within the past few years. This comprehensive survey delineates the state-of-the-art machine learning techniques applied to SR-N, with a specific focus on passive measurements. Recent developments are categorized into several sub-areas, namely; publicly available datasets, data augmentation, signal denoising, feature extraction, detection, localization, and recognition of SR-N. Additionally, future research directions are explored. • Machine learning (ML) in underwater sound analysis is rapidly evolving. • This survey overviews state-of-the-art ML techniques in Ship Radiated Noise. • Several preprocessing methods are discussed. • Automatic detection, localization, and recognition of ships are reviewed. • The survey ends with a forward-looking perspective for future research directions. [ABSTRACT FROM AUTHOR]

In this study, to address the problem of insufficient research on the transient noise of a power section for underwater vehicles, the vibration and acoustic radiation characteristics of a power cabin under transient impact loading are predicted based on the coupled time-domain finite element method and boundary element method (FEM/BEM). Corresponding experimental tests of vibration and acoustic radiation in air are carried out, and the trend of the numerical results based on the time-domain FEM/BEM better agrees with the test results, which verifies the correctness of the numerical model. Furthermore, the transient impact response has broadband and random characteristics, which significantly affect the acoustic stealth performance of underwater vehicles. Thus, star-shaped metamaterial components are designed for broadband transient vibration and noise control in this paper. The band gap properties of the star-shaped functional elements are calculated, and the vibration attenuation characteristics of the star-shaped metamaterial components are evaluated by the indices of the impact isolation coefficient and the vibration level difference (VLD). The vibration and noise reduction properties of the power cabin with star-shaped metamaterial components are analyzed in terms of the insertion loss and overall level. The results show that the vibration and acoustic radiation of a power cabin with star-shaped metamaterial components are significantly reduced at the band gap frequency, especially at 1500–4500 Hz, the maximum attenuation of sound pressure up to 75 dB. Compared with those in the power cabin with original components, the overall sound pressure levels in air and in water in the power cabin with star-shaped metamaterial components are 15.78 dB and 19.21 dB lower, respectively. Finally, the vibration and noise reduction characteristics of the power cabin with star-shaped metamaterial components are verified experimentally. In this paper, prediction, test and control steps for the vibration and noise of a power cabin for underwater vehicles under transient impacts are established, which provide support for the study of transient vibration and acoustic radiation in underwater vehicles. • Broadband transient vibro-acoustic characteristics are predicted based on the coupled time-domain FEM/BEM. • The star-shaped metamaterial components effectively improve the broadband vibration and noise performance. • The vibro-acoustic reduction performance of the star-shaped metamaterial components were experimentally verified. [ABSTRACT FROM AUTHOR]

In the context of acoustics, underwater sound propagation is intricately influenced by conditions of the marine environment. These conditions dynamically alter the sound speed profile (SSP), introducing a high level of variability and uncertainty into SSP predictions. Such uncertainties are incorporated into the acoustical model, affecting the accuracy of forecasting sound propagation characteristics, including ray trajectory and propagation loss. This study introduces and applies a stochastic ray model designed to estimate the ray trajectory and propagation loss within the oceanic environment. We specifically examine the effects of SSP changes on sound propagation. Our approach integrates ray theory with an adaptive dynamic orthogonal differential equation to model stochastic evolution fields. Initially, this research utilizes SSPs gathered from the Philippine Sea and Kuroshio Extension region. We investigates the impact of SSP perturbation on the ray trajectory and propagation loss. Following this preliminary analysis, we present a stochastic ray model tailored for rapid estimation of sound field characteristics, premised on effects of the SSP disturbance. The proposed model achieves high accuracy while substantially diminishing the computational complexity compared with the B e l l h o p model. • The model realizes nonlinear mapping from SSP to sound field characteristic. • The computing cost is significantly reduced for sound field estimation. • The stochastic ray model realizes the rapid calculation of the propagation loss. [ABSTRACT FROM AUTHOR]

Underwater object detection is considered as one of the most challenging issues in computer vision. In this paper, a weighted multi-error information entropy based YOLO (You Only Look Once) network is proposed to address underwater illumination noise affecting the detection accuracy. First, underwater illumination is essentially structural and non-uniform, and it is modeled as an independent and piecewise identical distribution, which is a generic noise model to describe the complex underwater illuminating environment and accommodates the traditional Gaussian distribution as a special case. Second, assisted by the proposed illumination noise model, a minimum weighted error entropy criterion, which is an information-theoretic learning method, is introduced into the loss function of YOLO network, and then the network parameters are trained and optimized to improve the detection performance. Furthermore, a multi-error processing strategy is simultaneously used to handle vector errors during information back-propagation in order to accelerate convergence. Experiments on underwater object detection datasets including URPC2018, URPC2019 and Enhanced dataset, show the proposed weighted multi-error information entropy based YOLOv8 network gets mean average precision (MAP) of 88.7%, 91.8% and 96.7% respectively, and average frames per second (FPS) of 116.6. These two evaluation metrics are better than the baseline YOLOv8 and the existing advanced non-YOLO approaches by at least 5.2% and 5.3% respectively. The results verify the effectiveness and superiority of the proposed network for underwater object detection in complex underwater environment. [ABSTRACT FROM AUTHOR]

Under the influence of the geomagnetic environment, charged seawater motion in the submarine wake generates a wake electromagnetic field, which is of great significance for underwater target detection and tracking. The generation of the wake electromagnetic field involves the coupling of multiple physical fields, such as fluid mechanics, ion transport, and electromagnetic fields, which have not yet been systematically studied. In this context, a coupling model involving the flow and electromagnetic fields is constructed to analyze the evolution of the far-field wake magnetic field of a full-size submarine propeller system. The flow field, current density, and far-field distribution of the wake magnetic field of the submarine propeller system at typical and rotational speeds are obtained. The three-component and total distributions of the far-field wake magnetic field are analyzed, and the longitudinal variation is discussed. The mechanism of the wake magnetic field is revealed based on this. In addition, the attenuation rule of the far-field wake electromagnetic field is studied, and the frequency characteristics of the far-field wake electromagnetic field are analyzed by a power spectrum. The results show that the wake electromagnetic field generated by the submarine propeller system in the far-field region has a wide distribution range (0.1 nT), and the corresponding frequency characteristics can be detected by a precision magnetometer. This study provides useful insight for the further evaluation of the far-field law of wake electromagnetic fields, which is conducive for future underwater target detection based on wake electromagnetic fields. [ABSTRACT FROM AUTHOR]

Spiny lobsters are targeted in capture fisheries due the high market demand and this situation has led to overfishing activities. Therefore, a smart monitoring system is imperative to be developed in handling this issue in order to store the distribution of marine spiny lobsters' information such as habitat preferences, population density and biology aspects of the lobsters. This study focused on separating the mixed underwater acoustic sound. An output of the separation system was used for Passive Acoustic Monitoring (PAM) application. The objective of this study is to provide estimated source signals from a recorded mixed acoustic signal. The feasibility of extracting spiny lobster sound as the target sound was observed during the experiment. In this study, Blind Source Separation (BSS) approach was employed to estimate the target sound (i.e., spiny lobsters) from the mixed underwater acoustic signal consisting of the sound of spiny lobsters and several man-made and natural sounds. In this investigation, two different blind source separation methods namely Fast Fixed Point Independent Component Analysis (FastICA) and Non Negative Matrix Factorization (NMF) were implemented. The mixture of one target sound which was spiny lobster and four interferences signals were used as input in this experiment,. The separated source sound by using FastICA with Negentropy, FastICA with Kurtosis and NMF algorithms were compared and evaluated based on bss_eval_sources toolbox metrics. These metrics consisted of signal to distortion ratio (SDR), signal to interferences ratio (SIR) and signal to artifacts ratio (SAR). In conclusion, the FastICA with Negentropy technique provides the best performance in separating mixed signal based on SIR, SAR and SDR measurement results that showed the FastICA with Negentropy generated the highest average values compared to the FastICA with Kurtosis and NMF. [ABSTRACT FROM AUTHOR]

Fig. 12. The flowchart of the proposed method. [Display omitted] • A modified uniform phase empirical mode decomposition algorithm (MUPEMD) is proposed. • A method of calculating the critical entropy value by entropy cross curve is proposed. • A filtering method named GA-DZPIF is proposed. • An evaluation index named SSIM has been established. • A new denoising strategy based on MUPEMD, GA-DZPIF, and critical entropy value is proposed. Noise reduction of underwater acoustic signal is of great significance in improving signal recognition capabilities of sonar system. In this paper, as a new decomposition algorithm, modified uniform phase empirical mode decomposition (MUPEMD) is proposed to improve the calculation efficiency and alleviate mode aliasing problem. After being decomposed by MUPEMD, mode components can be classified adaptively through critical entropy value. Dynamic zero point interval filtering method based on genetic algorithm (GA-DZPIF) is proposed to filter single mode component. Structure similarity of phase diagram is established as a new evaluation index to avoid the subjective difference which can enhance the persuasiveness of experimental result. Finally, a new noise reduction method for ship radiated noise signal based on MUPEMD, GA-DZPIF and entropy cross curve is proposed. Good denoising performance has been achieved for measured ship radiated noise signal, and it has a wide range of practical application. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel force-chain metastructure is proposed to resist hydrostatic pressure. • The effects of particle size and filling rate on the performance of metastructure are described in detail to guide the design of the metastructure. • It is verified that the metastructure has a wide range of engineering applications by 3D model. With the increase of water depth, the degradation of the performance of acoustic coating in underwater equipment has long plagued researchers. In this paper, a viscoelastic underwater sound-absorbing metastructure that can withstand 3 MPa underwater pressure is formed by embedding particulate matter in viscoelastic materials. Taking the two-dimensional structure as the research object, the factors affecting the force-chain properties are analyzed using simulations. It is important that the effectiveness of the filling of particulate matter in the 3D model is verified, which provides a possibility for the design and manufacture of water pressure insensitive underwater sound-absorbing materials. [ABSTRACT FROM AUTHOR]

• Aiming at the similarity law of underwater explosion model test, the deficiency of traditional impact factor and the similarity condition of single value of model test are analyzed by dimensional analysis method. • In view of the shortcomings of the traditional impact factor, this paper deduces a new impact factor from the perspective of single value similarity. • Based on the traditional and new impact factors, a scaled model test is designed to evaluate the effectiveness of the impact factors from multiple aspects by combining numerical and experimental methods. The dimensionless quantity is the invariant under the unit conversion, and the only choice to characterize the laws of physics. Aiming at the similarity law of underwater explosion model test, the deficiency of traditional impact factor and the similarity condition of single value of model test are analyzed by using dimensional analysis method and combining with domestic and foreign research. In view of the shortcomings of traditional impact factors, a new impact factor is derived from the perspective of single-value similarity based on the impact dynamic response characteristics of underwater explosion structures. Taking the midship cabin structure of a certain frigate as the prototype, the scale model tests are designed based on the traditional and new impact factors respectively. The effectiveness of the impact factors is evaluated from the aspects of structural damage characteristics, dimensionless deformation deflection, dimensionless velocity and dimensionless acceleration at the center of the structure. To account for the impact of structural characteristics, scale ratio, and proportional distance on the impact factor's effectiveness, various grillage, cabin, and hull girder model tests are conducted using ships of different tonnages as prototypes. The results of the model test, guided by the new impact factor, align well with the prototype test data. In contrast to the traditional impact factor, the model test exhibits greater precision for predicting structural responses, with a broad spectrum of potential applications. [ABSTRACT FROM AUTHOR]

We here present a 3D mapping of the dissolved CO 2 at Lagoa das Furnas (São Miguel Island, Azores) obtained with an infrared-based probe that measures the partial pressure of CO₂ gas dissolved in liquids, and a multi-parametric probe to measure pH, ORP, conductivity, dissolved O 2 , and temperature. We interpolated the dissolved CO 2 at different depths and around the lake to calculate the total mass of CO 2 dissolved as CO 2 (aq) (109–176 tons). We identified different plumes of dissolved CO 2 that spatially correspond to previously detected shallow anomalies of the diffusive CO 2 degassing at the water-air (interface) and hydroacoustic signatures of the bubbling activity. Our result represents the first direct quantification of the total dissolved CO 2 in a volcanic lake, opening new perspectives for volcanic lake monitoring and related hazard assessment. • Heterogeneous distribution of dissolved CO 2 in volcanic lakes. • Measuring multiple vertical profiles of dissolved CO 2 and water physicochemical parameters. • Interpolating dissolved CO 2 measurements to obtain a 3d reconstruction of the CO 2 content. • Identifying areas with higher CO 2 emissions from the lake bottom and gas plumes. [ABSTRACT FROM AUTHOR]

Hydroacoustic analysis of three-dimensional, turbulent flow over superhydrophobic and oscillating cylinders has been performed at Re = 3900. A third-type Robin boundary condition has been utilized to apply superhydrophobicity over the surfaces including two different slip coefficients of 0.425 and 0.1. Regarding the acoustical analysis, the Ffowcs Williams–Hawkings acoustic analogy has been used to investigate the generation and propagation of sound. For the case of a stationary cylinder, superhydrophobicity leads to an 8 percent increase in the Strouhal number. Furthermore, the overall sound pressure level is reduced by an amount of around 6 decibels. It is indicated that at a constant amplitude, the mean drag coefficient and the root-mean-square lift coefficient experience a 40% and 750% increase, respectively. Also, the maximum and the overall sound pressure levels escalate as the frequency ratio increases. The maximum amount of elevation in terms of the generated sound is 25 decibels for the synchronized case. Further analysis of the sound directivity proves that both the stationary cylinder and the lock-in cases of the transversely oscillating cylinder demonstrate symmetrical patterns in the dipole form. However, the non-synchronized states of the oscillation exhibit asymmetrical trends and deviate from the dipole pattern. • The aim is to predict the effect of superhydrophobicity and boundary movement on SPL. • The force coefficients, the shedding frequency, the peak/overall SPLs are analyzed. • The shedding modes, behavior of the sound spectra in oscillating conditions are presented. • The maximum amount of elevation in terms of the generated sound is 25 dB. • The directivity of sound will be studied using polar diagrams. [ABSTRACT FROM AUTHOR]

Understanding the temperature-dependent sound scattering and acoustic radiation force of a spherical shell is crucial for a number of applications, including marine geophysics, underwater sonar systems, and underwater imaging. In the present study, a multiphysics vibroacoustic analytical framework is developed to investigate axially symmetric underwater temperature-dependent sound scattering and acoustic radiation force phenomena. The analysis focuses on a submerged structure consisting of a functionally graded (FG) sandwich spherical shell with piezoelectric layers subjected to a nonlinear thermal environment. The power-law distribution is assumed to describe how the FG core material's properties change as a function of thickness and temperature. The coupled vibroacoustic equations are obtained through the implementation of Hamilton's method, utilizing the principles of electromechanical coupling and the Kirchhoff-Love thin shell concept. After that, Fourier series expansions are used to solve the governing partial differential equations. The accuracy and adaptability of the method proposed in this work are shown by comparing the current results to those found in the literature and in COMSOL Multiphysics® finite element (FE) simulations. The far-field scattering form function and radiation force function versus frequency range are studied with a focus on the effects of electric voltage, temperature variations, and gradient index. • Increasing power-law index leads to lower backscattering and radiation force. • Decreasing shell temperature causes overall reduction in backscattering and radiation force. • Increasing the electrical voltage decreases the backscattering and radiation force. [ABSTRACT FROM AUTHOR]

We present a thorough procedure for measuring the rheological properties of soft, highly attenuating, visco-elastic materials at ultrasonic frequencies. The material chosen for this illustration is a crosslinked Polyurethane (PU) elastomer (Sika UR3440 type), which is widely used in the field of underwater acoustics. We determine its complex longitudinal modulus M and shear modulus G as function of frequency in the range 1–5 MHz and of temperature in the range 5–40 °C. M is determined from the measurement of the transmission of longitudinal, plane waves by a slab of PU immersed in water. G is determined by contact measurements from the transmission and reflection of transverse, plane waves by a slab of PU. This determination of G for such a soft and viscous material as PU is made possible by the use of thin slabs and the implementation of an original signal analysis. M , G and bulk modulus K are found to obey the time–temperature superposition principle and to be accurately described by a fractional derivative rheological model. This allows us to propose analytic formulas for the frequency and temperature dependence of M , G and K valid above PU glass transition temperature. • We present a technique giving access to the shear modulus of soft solids at ultrasonic frequencies. • Its combination with an immersion technique gives their longitudinal, shear and bulk moduli. • A crosslinked Polyurethane elastomer widely used in the field of underwater acoustics is studied. • All its moduli satisfy the time–temperature superposition principle. • A fractional derivative Kelvin–Voigt model precisely describes their frequency dependence. [ABSTRACT FROM AUTHOR]

The Canadian Department of National Defence (DND) has approved the resumption of military activities within 'Whiskey Hotel', a 330 km2 area in the Strait of Juan de Fuca that overlaps the critical habitat of endangered Southern Resident Killer Whales in the northeast Pacific Ocean. Multiple Canadian and United States defense organizations use the area for training. The renewal of small-arms training follows a three-year cessation self-imposed by DND to allow a third-party consultant to investigate the risk in-air and underwater training noises pose to marine mammals. Here, we summarize the findings and limitations of the commissioned report and discuss the broader implications for mitigating aquatic noise pollution. The report evaluated the potential for marine mammals to experience injury, behavioral disturbances, and acoustic masking, and proposed mitigation measures (e.g., avoidance zones) to limit impacts. However, the narrow scope of the commissioned report prevented the investigation of the potential effects on non-mammalian taxa and the inclusion of vessel noise generated during training. Resuming military activities within the habitats of imperiled species is contentious among stakeholders. Nevertheless, the DND's approach to examining noise pollution employs at least two of the precautionary principle's four components, and their decision to commission a third-party investigation and implement mitigation strategies represents a framework rarely applied when evaluating the impacts of pollutants. The importance of this approach is magnified given the absence of federal underwater noise regulations, the pending development of Canada's Ocean Noise Strategy, and multiple nations seeking to balance industrial and military activities with environmental stewardship. [Display omitted] [ABSTRACT FROM AUTHOR]

As awareness on the impact of anthropogenic underwater noise on marine life grows, underwater noise measurement programs are needed to determine the current status of marine areas and monitor long-term trends. The Joint Monitoring Programme for Ambient Noise in the North Sea (JOMOPANS) collaborative project was funded by the EU Interreg to collect a unique dataset of underwater noise levels at 19 sites across the North Sea, spanning many different countries and covering the period from 2019 to 2020. The ambient noise from this dataset has been characterised and compared - setting a benchmark for future measurements in the North Sea area. By identifying clusters with similar sound characteristics in three broadband frequency bands (25–160 Hz, 0.2–1.6 kHz, and 2–10 kHz), geographical areas that are similarly affected by sound have been identified. The measured underwater sound levels show a persistent and spatially uniform correlation with wind speed at high frequencies (above 1 kHz) and a correlation with the distance from ships at mid and high frequencies (between 40 Hz and 4 kHz). Correlation with ocean current velocity at low frequencies (up to 200 Hz), which are susceptible to nonacoustic contamination by flow noise, was also evaluated. These correlations were evaluated and simplified linear scaling laws for wind and current speeds were derived. The presented dataset provides a baseline for underwater noise measurements in the North Sea and shows that spatial variability of the dominant sound sources must be considered to predict the impact of noise reduction measures. • Measurement results from Joint Monitoring Programme for Ambient Noise in the North Sea (JOMOPANS) • Benchmark of underwater noise measurements at 19 locations • Characterization and comparison of ambient noise • Correlation of sound levels with shipping and wind noise • Quantification of flow noise in field measurements of underwater noise [ABSTRACT FROM AUTHOR]

Anthropogenic (man-made) sound has the potential to harm marine biota. Increasing concerns about these effects have led to regulation and mitigation, despite there being few data on which to base environmental management, especially for fishes and invertebrates. We argue that regulation and mitigation should always be developed by looking at potential effects from the perspectives of the animals and ecosystems exposed to the sounds. We contend that there is currently a need for far more data on which to base regulation and mitigation, as well as for deciding on future research priorities. This will require a process whereby regulators and researchers come together to identify and implement a strategy that links key scientific and regulatory questions. Much of the current regulation and mitigation of the effects of anthropogenic sound is based on a very limited dataset. Future regulation and mitigation must be approached from the perspective of the animals. Which sounds affect the animals adversely, and what is the nature of these effects? Considerable data (and funding to get that data) is needed, particularly for fishes and invertebrates, before regulation and mitigation can be properly applied. Regulators and investigators should work together to develop a plan of action that focuses on the most important questions, to inform ways to protect animals and develop appropriate mitigation and regulation, and to inform the most effective way(s) to answer those questions. Future work should focus on a limited number of species, key research questions, and experimental approaches that allow easy comparison of data across studies, species, and sound sources. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate the performance of 16-ary orbital angular momentum shift keying (OAM-SK) decoders based on convolutional neural networks (CNNs) in an underwater optical wireless communication (UOWC) system. We verify the decoding accuracy of the OAM-SK system in a 1-m tank filled with flowing salty water. The oceanic turbulence is emulated by the random phase screen method by a spatial light modulator (SLM). The results show that the decoders are robust to short-distance water fluctuations induced by the water flow, with an accuracy of more than 99% in clean water. In turbid water, the decoders require input images with a greater number of pixels and can reach an accuracy of more than 99% with a pixel number of 64 × 64. The decoders are tolerant to weak turbulence, with accuracies greater than 96.78% and 99.75% for input pixel numbers of 32 × 32 and 64 × 64 in 20-m temperature-induced channels with an equivalent temperature structure parameter of C n 2 = 1 0 − 15 K 2 ⋅ m − 2 ∕ 3. As the strength of the oceanic turbulence increases, the accuracy decreases to lower than 85.5%. In both turbid channels and turbulent channels, large OAM modes are more likely to be incorrectly recognized. Increasing the pixel number of the input images or decreasing the order of the coding format can improve the decoding accuracy under moderate-to-strong turbulence conditions. This work will be beneficial for the design of UOWC systems. [ABSTRACT FROM AUTHOR]

In this work, a hydrophone has been integrated in a moving autonomous underwater vehicle (AUV) in order to have an autonomous controlled system capable of recording underwater audio. The performance of the system, in terms of best recording of the audio, has been analyzed by placing the hydrophone in two different positions: the front of the AUV and its lower part. From this analysis, it has been determined that the best position for the hydrophone (i.e., the one that receives the lowest engine noise) is the former. In addition, a signal processing based on an order analysis has been performed for the purpose of filtering the noise introduced by the engine of the AUV. This technique achieves the filtering of most of this noise without removing contributions from other sources of interest whose frequency spectrum overlaps with the noise of the engine. This is a fundamental characteristic of the new system because it allows for cleaner underwater recordings. The aim is to increase the capabilities for detection of sources emitting in the range of the AUV acoustic contribution by reducing the same and not so much by completely removing it. [ABSTRACT FROM AUTHOR]

In this paper, we present a deep learning based underwater acoustic (UWA) orthogonal frequency-division multiplexing (OFDM) communication system. Unlike the traditional receiver for UWA OFDM communication system that performs explicitly channel estimation and equalization for the detection of transmitted symbols, the deep learning based UWA OFDM communication receiver interpreted as a deep neural network (DNN) can recover the transmitted symbols directly after sufficient training. The estimation of transmitted symbols in the DNN based receiver is achieved in two stages: (1) training stage, when labeled data such as known transmitted data and signal received in the unknown channel are used to train the DNN, and (2) test stage, where the DNN receiver recovers transmitted symbols given the received signal. To demonstrate the performance of the deep learning based UWA OFDM communications, we generate a large number of labeled and unlabeled data by using an acoustic propagation model with a measured sound speed profile to train and test the DNN receiver. The performance of the deep learning based UWA OFDM communications is evaluated under various system parameters, such as the cyclic prefix length, number of pilot symbols, and others. Simulation results demonstrate that the deep leaning based receiver offers consistent improvement in performance compared to the traditional UWA OFDM receiver. [ABSTRACT FROM AUTHOR]

Wireless communication in an aquatic environment constitutes an active field of research and opens up new prospects for the future. Underwater wireless communication is practically unexplored field compared with terrestrial wireless communication. The growing interest in underwater acoustic communications is due to the expansion of applications, which were, at the beginning exclusively military and nowadays commercial ones. In this study, the fundamental of underwater acoustics (sound velocity, absorption, attenuation, SNR...) governing the wireless communication systems in seawater are presented, then some optimal parameters (optimal frequency, bandwidth, attenuation...) for a communication characterized by a weak loss of transmission are deduced. This investigation is based not only on Mackenzie's model, which provides a good representation of the sound velocity profile in seawater, but also on François and Garrison model taking into account all absorption phenomena. The simulation of Mackenzie's model by analyzing the effects of some parameters on sound velocity is carried out and due to the accuracy of the model, Mackenzie's equation can be selected as the basis of all propagation delay modeling. As for François and Garrison model based upon a large number of previous experimental results and theoretical studies, it is adopted in this study to analyze with an accurate assessment the frequency dependent component of SNR. [ABSTRACT FROM AUTHOR]

In this paper, we propose a computationally efficient multipath matching pursuit (MMP) channel estimation algorithm for underwater acoustic (UWA) transform domain communication systems (TDCSs). The algorithm, referred to as the MMP-DCD-CV algorithm, is based on the dichotomous coordinate descent (DCD) iterations and cross validation (CV). The MMP-DCD-CV sparse channel estimator in each iteration searches for multiple promising path candidates most relevant to a residual vector and chooses the best candidate. The DCD iterations are used to solve the corresponding least squares problem with low complexity and numerical stability. The CV provides a stopping criterion of the algorithm without a priori information on the channel sparsity and noise level and examines whether the algorithm overfits its data, thus improving the estimation accuracy. The performance of the proposed algorithm is evaluated under simulated sparse UWA channels. The numerical results show that the algorithm achieves better performance than the original MMP algorithm, has lower complexity, and does not require prior knowledge on the channel sparsity and noise level. We also propose an UWA TDCS with sparse channel estimation based on the proposed MMP-DCD-CV algorithm. The proposed UWA communication system is tested by the Waymark simulator, providing the virtual signal transmission in the UWA channel, with a measured Sound Speed Profile and bathymetry. Numerical results demonstrate that the UWA TDCS with the proposed sparse channel estimator offers considerable improvement in system performance compared to other TDCS schemes. [ABSTRACT FROM AUTHOR]

• A novel type of acoustic metamaterials with multi-resonators is proposed. • Coupled resonance is applied to improve acoustic absorption performance of LRSMs. • The effect of steel plate on sound absorption performance of LRSMs is studied. Generally, the better sound absorption characteristics can be achieved in the low frequency band by embedding resonance units into the conventional anechoic coating. However, in this way, the frequency range of effective sound absorption is relatively narrow, which limits the application of anechoic coating in engineering. In this paper, in order to optimize this sound absorption characteristics, a new type of underwater sound absorption metamaterials with multi-resonator are proposed based on the coupled resonance mechanism. The underwater sound absorption characteristics of the proposed metamaterials with steel plate backing are investigated by finite element method (FEM). The effect of coupled resonance between the resonators on the sound absorption characteristics of metamaterials is investigated. In addition, the effect of the geometric structure of steel plate backing on the sound absorption characteristics is studied as well. Numerical results show that the proposed metamaterials have many advantages over the conventional anechoic coating in underwater sound absorption due to the coupled resonance effect. Moreover, the steel plate backing has a significant effect on the sound absorption characteristics of the proposed metamaterials, especially, in the low frequency range. [ABSTRACT FROM AUTHOR]

• A developed DFB-FL sensor was used to detect the underwater laser induced acoustic (LIA) wave. • The communication between the airborne platform and underwater targets (CAPUT) is completed by a photoacoustic communication system based on the pulsed laser and DFB-FL sensor. The Air-Water Cross-Medium Communication (AWCMC) is significant for ocean exploration. This work presents an approach to direct AWCMC based on the optoacoustic conversion technique, which utilizes a pulsed laser and a distributed feedback fiber laser (DFB-FL) hydrophone. On-off keying (OOK) is used to modulate the pulsed laser, and ASCII is selected as the signal coding format. The light signal after modulation is transmitted to the air–water interface, where it is converted into underwater sound signals by the optoacoustic effect. Next, the underwater acoustic signal containing the encoded information is detected and demodulated by the DFB-FL hydrophone, which proves the feasibility of the proposed method used for the AWCMC. The experimental results indicate that the ASCII-encoded OOK signals with varying repetition frequencies can be generated by modulating the laser. Then, the performance of laser-generated sound (LGS) communication is evaluated by measuring the data rate and bit error rate (BER) with the repetition frequency in the range of 1–500 Hz. It is found that both the data rate and BER exhibit a positive correlation with the repetition frequency of the pulsed laser. Therefore, when optoacoustic technology is applied to the AWCMC, optimization design is required for the indicators of data rate and BER. [ABSTRACT FROM AUTHOR]

Characteristics of laser-induced plasma depends strongly on its ambient pressure, and a deeper understanding of the high pressure impact on the underwater plasma emission is essential for the application of LIBS in the deep-sea. In this work, we investigated the temporal evolution of underwater plasma and cavitation bubble at the high pressures up to 50 MPa, by using fast imaging and shadowgraph techniques. It showed that as the ambient pressure increases, the plasma expansion is much suppressed with a stronger emission intensity and a slower emission decay, but it quenches much earlier and ends with a sharp drop at the higher pressures. From the shadowgraph imaging results, it showed that the increase in pressure has a great impact on the bubble evolution. Both the bubble maximum radius and the bubble lifetime decrease dramatically with the pressure as a consequence of faster dynamics. By comparing between the plasma and bubble results, we demonstrated the critical role of bubble dynamics on the characterization of plasma emission at high pressures. At relatively low pressures, the bubble expansion time is long enough to finish the plasma radiation. While at high pressures, strong bubble-plasma interaction occurs that will guide the plasma evolution behavior at the late stage. The plasma can gain energy from the confinement effect caused by the bubble shrinking process which leads to an increase in the emission intensity, but it will quench immediately after the bubble collapse. Such effects caused by bubble dynamics at high pressures explain well the evolution behaviors of underwater LIBS signals of OH, Ca II, Ca I, and CaOH, considering the different lifetimes of these emitting species in the plasma. This work reveals the complex impacts of high pressure on underwater plasma emissions and underwater LIBS signals. [Display omitted] • Evolution of underwater plasma and bubble was studied at high pressures up to 50 MPa. • Bubble dynamics play a key role on the underwater plasma emission at high pressures. • Bubble shrinking process at high pressures leads to an increase in plasma emission. • Plasma quenches earlier at higher pressure that immediately after the bubble collapse. • Impacts of high pressure on underwater LIBS signals rely on the analyzed lines. [ABSTRACT FROM AUTHOR]

The underwater sound absorption material for submarines should fulfill the demand for remarkable performance in medium-low frequency sound absorption at low thicknesses. Hence, in this paper, the innovative "filled-microperforated plate-like" (F-MPPL) structure was introduced to the hollow microballoons filled underwater sound absorption composite (HBF composite) using the tailor-made 3D spacer fabric. The test results show remarkable mechanical properties with a compression strength and compression modulus of 2256 KPa and 14.6 MPa for the new type of F-MPPL composite, respectively. In addition, the peak sound absorption coefficient of the F-MPPL composite reaches 0.77 at just 7.5 mm thickness based on the resonant sound absorption characteristic introduced by the F-MPPL structure. Moreover, the study investigated the effects of seawater aging on the F-MPPL composite and found that while there was a decrease in compression strength and average sound absorption coefficient, the degradation of the F-MPPL composite was significantly smaller than the HBF composite due to the inhibitory effect of spacer yarns on pore expansion. Finally, this study proposed the method of using compression strength to predict the long-term life of the F-MPPL composite under seawater aging, and a 60% retention rate after 20 months was displayed. [Display omitted] • The filled-microperforated plate-like (F-MPPL) structure endows material with resonant sound absorption characteristics. • The spacer yarns limits PU foaming and prevents pores expanding during seawater aging. • The F-MPPL composite exhibits lower porosity and water absorption, resulting in a reduced corrosion by seawater. • The compression strength retention rate of the F-MPPL composite remains 60% after 20 months of aging. [ABSTRACT FROM AUTHOR]

In research into underwater acoustics, the ability to differentiate submerged sources from surface sources could meet multiple applications. By using a bottom-mounted Horizontal Line Array (HLA), a solution to the problem of acoustic source depth discrimination in a downward-refracting underwater environment is presented. The testing problem is considered by beamforming in the modal domain. According to wavenumber offsets in the wavenumber spectra, a source depth feature is detected. The phenomenon is explained in terms of mode theory, using the concept of mode trapping. The decision metric is built based on the Accumulated Relative Wavenumber Offset (ARWO) derived by subarrays and broadband processing, then the simulation result reveals that the source can be correctly discriminated. Without mode matching or mode filtering, the method has high computational efficiency. In addition, the applicability for bottom-mounted HLA and the sediment independence of this method is proved. Finally, a moving surface source and a dipping submerged source during an experiment conducted in the South China Sea are taken into consideration. The source-depth characteristic is magnified by the ARWO. Over test trials, the classifier provides a satisfactory effect, with which the depth discrimination method is well explained. [Display omitted] • HLA samples the normal modes more adequately at the sea bottom. • Surface and submerged sources show different characteristics in the modal domain. • The classifier based on wavenumber spectra is computational efficient. • Performance of this method is robust against sediment and tolerant of environmental parameters. [ABSTRACT FROM AUTHOR]

There is a growing interest in the study of mesopelagic and bathypelagic species due to their implication in the active carbon flux and climate change. Underwater acoustics is one of the most efficient methods to study such widely distributed communities. However, there are several challenges among which the reduced spatial resolution with depth for vessel-mounted echosounders stands out. The increase in sampling volume with range precludes observation of fine structures within the scattering layers or the study of less-aggregated organisms far from the transducer face. Echosounders are thus increasingly being deployed in alternative platforms that get the transducer close to the layers of interest. We present here the comparison between two active acoustic platforms used to analyse mesopelagic and bathypelagic species: a vessel-mounted EK60 scientific echosounder and a 6000 m depth rated Acoustic Zooplankton Fish Profiler (AZFP) attached to a CTD rosette. Both systems depicted similar vertical distributions and scattering at 38 kHz for the main scattering layers recorded around the Canary Islands but the finer vertical resolution of the AZFP discerned a series of sub-layers within them that differed in scattering, avoidance and sparsity. The availability of high frequencies at depth also provided further insights on the frequency response of those layers. However, the horizontal orientation of the AZFP created sidelobe artefacts close to the surface and bottom that limit its use in those areas. Signal-to-noise ratio of the AZFP system in the bathypelagic zone also precludes applying echointegration analysis at those depths or outside the denser scattering layers. Finally, the visualization of horizontal or temporal patterns such as migration or mesoscale features is limited to a coarse resolution using data from closely spaced successive stations. New challenges encountered with vertically-deployed echosounders and recommendations for their use are pointed out. Although no scattering layers were detected in the bathypelagic zone, this study unveils an area never before recorded acoustically. • We present an acoustic device that can reach 6000 m of depth. • The horizontal beam orientation ensures a constant ensonified sampling volume but adds some artefacts. • Similar scattering layers are detected for coincident frequencies with respect to the vessel echosounder but distinct sublayers are revealed. • Layers' sparsity and avoidance could be potential discriminating properties. • Echointegration can only be applied within dense layers at close range. [ABSTRACT FROM AUTHOR]

Snake-like robots have attracted attention as robots that can travel over rough terrain where wheeled mobility mechanisms cannot. Previous research on snake-like robots has mainly focused on the biological movement of snakes; therefore, the issue of power consumption caused by driving a large number of actuators still remains unaddressed. In this study, we propose an efficient movement method using the movement of the center of gravity (COG) as a solution to the above-mentioned problem. In the proposed method, the snake-like robot is first transformed into a wheel shape, and then, some motors of the joint are moved to shift the COG and rotate the robot. Therefore, good movement efficiency can be achieved on leveled terrain by the rolling movement, and a snake-like undulating drive with high running performance can be selected on underwater and rough terrain. To realize the proposed movement method, we propose a method for switching between the rotational movement and undulation drive modes. However, the rolling motion with a COG shift needs a design of an appropriate orbit of the eccentric COG and timing of the motion stage transition. In this study, we demonstrate the rolling motion design with a simplified equation of motion. Next, experiments are conducted to verify the traveling efficiency in the proposed rolling mode, and it is proven that the method can achieve twice the traveling efficiency of the undulating motion. • We propose new rolling motion for a snake robot to improve traveling efficiency. • Simplified mathematical model is adopted to design COG orbit and kick-motion. • Rolling motion achieved twice the traveling efficiency of the undulating motion. [ABSTRACT FROM AUTHOR]