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An autonomous surface vehicle known as a wave glider, instrumented with a low-power towed hydrophone array and embedded digital signal processor, is demonstrated as a viable low-noise system for the passive acoustic monitoring of marine mammals. Other key design elements include high spatial resolution beamforming on a 32-channel towed hydrophone array, deep array deployment depth, vertical motion isolation, and bandwidth-efficient real-time acoustic data transmission. Using at-sea data collected during a simultaneous deployment of three wave glider-based acoustic detection systems near Stellwagen Bank National Marine Sanctuary in September 2019, the capability of a low-frequency towed hydrophone array to spatially reject noise and to resolve baleen whale vocalizations from anthropogenic acoustic clutter is demonstrated. In particular, mean measured array gain of 15.3 dB at the aperture design frequency results in a post-beamformer signal-to-noise ratio that significantly exceeds that of a single hydrophone. Further, it is shown that with overlapping detections on multiple collaborating systems, precise localization of vocalizing individuals is achievable at long ranges. Last, model predictions showing a 4× detection range, or 16× area coverage, advantage of a 32-channel towed array over a single hydrophone against the North Atlantic right whale upcall are presented for the continental shelf environment south of Martha's Vineyard. [ABSTRACT FROM AUTHOR]

A distributed fiber optic hydrophone (DFOH) is a new type of fiber optic hydrophone (FOH) with adjustable structure. The dependence of the directivity of a DFOH on array structure is theoretically and experimentally studied. The directivity function of a sensing channel and that of a DFOH are derived. Based on the directivity function, the simulations are performed. Finally, the theoretical analysis is demonstrated by the experiments performed on Qingyang lake, and the results reveal that the longer sensing channel length guarantees the lower first-order side lobe and the narrower main lobe. As the channel length increased from 1 to 3, the main lobe width and first-order side lobe height decreased by 4.9° and 6 dB, respectively. In addition, channel spacing is irrelevant to directivity as the spacing is shorter than the wavelength. As the channel spacing increased from 0 to 1, the variations of the main lobe width and first-order side lobe height are lower than 0.5° and 0.94 dB, respectively. This study would provide guidance for the structure design of a distributed fiber optic hydrophone in signal processing. [ABSTRACT FROM AUTHOR]

This paper focuses on the problem of transient signal detection using a single vector hydrophone in a horizontal isotropic acoustic field. The covariance matrix of the three sensor output quantities is analyzed in terms of its trace and determinant principal invariants, and the mathematical relationship between the signal-to-noise ratio and these invariants is established. Both the signal power and the ambient-noise power can be accurately estimated directly from the covariance matrix of the received data, and the signal-to-noise ratio can further be obtained. A signal power detector is proposed based on the theoretical analysis, and simulation results show that (i) it can detect signals with low signal-to-noise ratio and (ii) its performance is greatly improved compared with that of an energy detector under the condition of a nonstationary ambient-noise field. An experiment was performed to verify the theoretical analysis and the conclusions of the numerical simulation, and the data-processing results show that this method has good applicability in signal detection. [ABSTRACT FROM AUTHOR]

Advanced oxidation processes are promising techniques for water remediation and degradation of micropollutants in aqueous systems. Since single processes such as sonolysis and photocatalysis exhibit limitations, combined AOP systems can enhance degradation efficiency. The present work addresses the synergistic intensification potential of an ultrasound-assisted photocatalysis (sonophotocatalysis) for bisphenol A degradation with a low-frequency sonotrode (f = 20 kHz) in a batch-system. The effects of energy input and suspended photocatalyst dosage (TiO2-nanoparticle, m = 0–0.5 g/L) were investigated. To understand the synergistic effects, the sonication characteristics were investigated by bubble-field analysis, hydrophone measurements, and chemiluminescence of luminol to identify cavitation areas due to the generation of hydroxyl radicals. Comparing the sonophotocatalysis with sonolysis and photocatalysis (incl. mechanical stirring), synergies up to 295% and degradation rates of up to 1.35 min−1 were achieved. Besides the proof of synergistic intensification, the investigation of energy efficiency for a degradation degree of 80% shows that a process optimization can be realized. Thus, it could be demonstrated that there is an effective limit of energy input depending on the TiO2 dosage. [ABSTRACT FROM AUTHOR]

In this letter, a scandium-doped aluminum nitride (Scx ${\mathrm {Al}}_{1-x}\text{N}$ , x = 9.5%) piezoelectric high-frequency MEMS vector hydrophone with large operation bandwidth is presented, which measures acoustic pressure gradient underwater. The hydrophone sensor consists of four groups of sensing cell arrays. Each sensing cell is composed of a hexagonal sensing diaphragm made up of a piezoelectric stack of ScAlN piezoelectric thin-film sandwiched by Mo and highly doped single-crystal silicon, with a top and a bottom oxide film surrounding the piezoelectric stack. The hydrophone sensor can operate in not only omnidirectional receive mode, but also dipole receive mode, depending on the signal processing algorithm used for the four array groups. The vector hydrophone operates in the frequency range of 20 kHz to 160 kHz, which is specially designed to monitor the dolphin calls. The measured maximum receive sensitivity is -175 dB (re: 1 V/ $\mu $ Pa) in the dipole receive mode at 160 kHz and −169±2 dB (re: 1 V/ $\mu $ Pa) in the omnidirectional receive mode in the operation bandwidth, respectively. The reported vector hydrophone also presents a smooth directivity pattern at 100 kHz. The experiments show the reported MEMS vector hydrophone has good potential in conservation of dolphins and other underwater detection at high frequency between 20 kHz to 160 kHz. [ABSTRACT FROM AUTHOR]

Numerous studies have evaluated the acoustic characteristics of soniferous snapping shrimp, but a few are based on long-term mooring measurements. In this study, underwater ambient noise signals were collected from a hydrophone moored 10 m from the sea bed in the coastal sea of western Jeju, South Korea, from mid-September 2019 for 90 days to analyze the variation in the sound of snapping shrimp. The kernel signal and a threshold value were utilized to identify the snapping shrimp, and the snap rate per minute was computed for quantitative analysis. The results show that the mean and standard deviation of the snap rate in the western sea of Jeju was 2132 ± 432 per minute during the whole measurement period. The surface water temperature and tidal level decreased by 7 °C from 25 °C and 50 cm from 190 cm, respectively, over 90 days. The snap rate decreased from September mainly due to the decrease in water temperature by 71 times per minute for every 1 °C decrease. It showed a circadian cycle, increasing by 17~24% at sunrise and sunset compared to the daytime minimum. The snap rate at night was the highest in late summer but the rate dropped like the one during the day in late fall. The snap rate at high tide was 13% higher on average than at low tide. The circadian and tidal changes of the snapping shrimp sound from long-term mooring measurements may be used as primary data for underwater ambient noise and the ecological behavior of snapping shrimp. [ABSTRACT FROM AUTHOR]

Direction of arrival (DOA) estimation based on Maximum Likelihood is a common method in array signal processing, with many practical applications, but the huge amount of calculation limits the practical application. To deal with such an Maximum Likelihood (ML) DOA estimation problem, firstly, the DOA estimation model with ML for acoustic vector sensor array is developed, where the optimization standard in various cases can be unified by converting the maximum of objective function to the minimum. Secondly, based on the Invasive Weed Optimization (IWO) method which is a novel biological evolutionary algorithm, a new Improved IWO (IIWO) algorithm for DOA estimation of the acoustic vector sensor array is proposed by using ML estimation. This algorithm simulates weed invasion process for DOA estimation by adjusting the non-linear harmonic exponent of IWO algorithm adaptively. The DOA estimation accuracy has been improved, and the computation of multidimensional nonlinear optimization for the ML method has been greatly reduced in the IIWO algorithm. Finally, compared with Genetic Algorithm (GA), Particle Swarm Optimization (PSO), Differential Evolution (DE) method and Tuna Swarm Optimization(TSO) algorithm, numerical simulations show that the proposed algorithm has faster convergence rate, improved accuracy in terms of Root Mean Square Error (RMSE), lower computational complexity and more robust estimation performance for ML DOA estimation. The experiment with tracking the orientation of the motorboat by Microelectronic mechanical systems (MEMS) vector hydrophone array shows the superior performance of proposed IIWO algorithm in engineering application. Therefore, the proposed ML-DOA estimation with IIWO algorithm can take into account both resolution and computation. which can meet the requirements of real-time calculation and estimation accuracy in the actual environment. [ABSTRACT FROM AUTHOR]

Algorithms working in mode space instead of directly matching the received complex sound pressure were developed to improve computational efficiency and robustness, but these algorithms may be inconvenient to apply in practice because manual operations are often inevitable when performing modal filtering. Based on a phase-matched filter, an imperfect matching scheme named the modal phase based matched impulse response (MP-MIR) is proposed to estimate the source range rapidly and conveniently with a single hydrophone. The field to be matched is still the received complex sound pressure. The replica field is a sum of several "phase" modes, which can be efficiently and conveniently synthesized merely with the horizontal wavenumbers of normal modes and the source–receiver range. The effectiveness of the proposed MP-MIR was demonstrated in localizing 84 emissions along a weakly range-dependent track at ranges of 2.54–20 km in the South China Sea. Although it was found, from cross-correlation coefficients, that the received signals showed strong variation even between adjacent emissions, MP-MIR outperformed the classical matched impulse response (MIR) with a lower standard deviation in most cases, demonstrating good robustness and potential for practical applications. [ABSTRACT FROM AUTHOR]

Sensing technologies using optical fibers have been studied and applied since the 1970s in oil and gas, industrial, medical, aerospace, and civil areas. Detecting ultrasound acoustic waves through fiber-optic hydrophone (FOH) sensors can be one solution for continuous measurement of volumes inside production tanks used by these industries. This work presents an FOH system composed of two optical fiber coils made with commercial single mode fiber (SMF) working in the sensor head of a Michelson's interferometer (MI) supported by an active stabilization mechanism that drives another optical coil wound around a piezoelectric actuator (PZT) in the reference arm to mitigate external mechanical and thermal noise from the environment. A 1000 mL glass graduated cylinder filled with water is used as a test tank, inside which the sensor head and an ultrasound source are placed. For detection, amplitudes and phases are measured, and machine learning algorithms predict their respective liquid volumes. The acoustic waves create patterns electronically detected with resolution of 1 mL and sensitivity of 340 mrad/mL and 70 mvolts/mL. The nonlinear behavior of both measurands requires classification, distance metrics, and regression algorithms to define an adequate model. The results show the system can determine liquid volumes with an accuracy of 99.4% using a k-nearest neighbors (k-NN) classification with one neighbor and Manhattan's distance. Moreover, Gaussian process regression using rational quadratic metrics presented a root mean squared error (RMSE) of 0.211 mL. [ABSTRACT FROM AUTHOR]

The military implications of Chinese control of Taiwan are understudied. Chinese control of Taiwan would likely improve the military balance in China's favor because of reunification's positive impact on Chinese submarine warfare and ocean surveillance capabilities. Basing Chinese submarine warfare assets on Taiwan would increase the vulnerability of U.S. surface forces to attack during a crisis, reduce the attrition rate of Chinese submarines during a war, and likely increase the number of submarine attack opportunities against U.S. surface combatants. Furthermore, placing hydrophone arrays off Taiwan's coasts for ocean surveillance would forge a critical missing link in China's kill chain for long-range attacks. This outcome could push the United States toward anti-satellite warfare that it might otherwise avoid, or it could force the U.S. Navy into narrower parts of the Philippine Sea. Finally, over the long term, if China were to develop a large fleet of truly quiet nuclear attack submarines and ballistic missile submarines, basing them on Taiwan would provide it with additional advantages. Specifically, such basing would enable China to both threaten Northeast Asian sea lanes of communication and strengthen its sea-based nuclear deterrent in ways that it is otherwise unlikely to be able to do. These findings have important implications for U.S. operational planning, policy, and grand strategy. [ABSTRACT FROM AUTHOR]

Underwater direction-of-arrival (DOA) tracking using a hydrophone array is an important research subject in passive sonar signal processing. In this study, considering that an unknown underwater environment results in uncertain disturbances to the measurements, robust underwater DOA tracking with regard to uncertain environmental disturbances was studied. Because the uniform circular array (UCA) is free from the port and starboard ambiguity problem, a UCA was used to obtain the measurements for a long-time tracking scenario. First, a kinematic model of an underwater target and a measurement model based on the received signal of the UCA were established. Then, a DOA tracking algorithm was derived based on the extended Kalman filter (EKF), whose performance is significantly affected by the accuracy of the measurement noise covariance matrix (MNCM). Finally, considering that uncertain disturbances carry out unstable measurement noise, the modified Sage–Husa algorithm was used to obtain accurate MNCMs during the process of the derived EKF-based DOA tracking algorithm. Thus, a robust DOA tracking method with uncertain environmental disturbances using a UCA was proposed. The accuracy and reliability of the suggested method was verified via Monte Carlo simulations of a DOA tracking scenario and an experiment in the South China Sea in July 2021. [ABSTRACT FROM AUTHOR]

The flow noise for the acoustic vector hydrophone in the flank array is studied in this paper. The hydrophones are usually mounted above the baffle and are protected by a dome. This paper simplifies the flank array to be an infinite dome placed above an infinite baffle model, and the acoustic vector hydrophone is located in the fluid layer between the dome and the baffle. The spectral reflection coefficient of the multilayer baffle is obtained by the transfer matrix and matched boundary conditions. The cross-spectral density matrix is derived by the wavenumber–frequency spectrum analysis method. In addition, the spectral transfer functions are verified by the finite element method. Numerical results are presented to illustrate the influence of the free-stream velocity, the dome parameters, the location of the acoustic vector hydrophone and the baffle on the auto-power spectra of each hydrophone. Besides, the cross-power spectra of each hydrophone and the spatial correlation are discussed in this paper. The particle velocity channels are more sensitive than the pressure channel to the flow noise below 4000 Hz if the hydrophone is near the dome. The cross-power spectra between the pressure and particle velocity are lower than the particle velocity power spectra in the whole frequency band, and that are lower than the pressure power spectra in the higher frequency. The spatial correlation radius of the pressure and the particle velocity of all directions is small. [ABSTRACT FROM AUTHOR]

The article discusses the results of measuring underwater noise generated by a large-amplitude solitary internal wave based on observations made near the Mascarene Ridge in the Indian Ocean. The measurements were taken from a ship drifting over large depths, equipped with a vertical string of hydrophones and equipment for recording internal waves. The parameters of the recorded 50 m internal wave and characteristics of the emitted underwater noise are given. Powerful orbital currents accompanying the internal wave form rips on the ocean surface: areas of chaotic, collapsing waves that increase the air bubble concentration in the near-surface layer and emit underwater noise. Analysis of the data showed that the passage of a rip caused by an internal wave over a hydrophone leads to a significant (up to 18 dB) increase in the level of underwater noise in the 5–15 kHz frequency range. [ABSTRACT FROM AUTHOR]

Identifying individual animals is critical to describe demographic and behavioural patterns, and to investigate the ecological and evolutionary underpinnings of these patterns. The traditional non-invasive method of individual identification in mammals—comparison of photographed natural marks—has been improved by coupling other sampling methods, such as recording overhead video, audio and other multimedia data. However, aligning, linking and syncing these multimedia data streams are persistent challenges. Here, we provide computational tools to streamline the integration of multiple techniques to identify individual free-ranging mammals when tracking their behaviour in the wild. We developed an open-source R package for organizing multimedia data and for simplifying their processing a posteriori—"MAMMals: Managing Animal MultiMedia: Align, Link, Sync". The package contains functions to (i) align and link the individual data from photographs to videos, audio recordings and other text data sources (e.g. GPS locations) from which metadata can be accessed; and (ii) synchronize and extract the useful multimedia (e.g. videos with audios) containing photo-identified individuals. To illustrate how these tools can facilitate linking photo-identification and video behavioural sampling in situ, we simultaneously collected photos and videos of bottlenose dolphins using off-the-shelf cameras and drones, then merged these data to track the foraging behaviour of individuals and groups. We hope our simple tools encourage future work that extend and generalize the links between multiple sampling platforms of free-ranging mammals, thereby improving the raw material needed for generating new insights in mammalian population and behavioural ecology. [ABSTRACT FROM AUTHOR]

High intensity focused ultrasound has gained rapid clinical acceptance as a noninvasive treatment for solid tumors. As implied by the name, the intensity of sound at the focus is generally large. In a nonlinear ultrasound field, where the acoustic spectrum contains a considerable spread of harmonics, the pressure waveform is asymmetrically distorted, with a discrepancy between the peak compressional pressure and the peak rarefactional pressure, which are required in FDA and IEC regulations. Therefore, the amplitude and phase of the harmonics matter. In order to better understand nonlinear focused ultrasound, the amplitude and phase relation of the harmonics are investigated through both numerical simulations and measurements. The first three harmonics are extracted from the distorted wave by a zero-phase band-pass filter. It is demonstrated that, as the source pressure increases, the focusing gain for the fundamental component tends to decrease while the focusing gains for the second and third harmonics rise. The relative phases show very little change. There is a substantive agreement between the simulated and measured results for the focusing gain of the harmonics. The relative phase of the harmonics needs to be further verified after the calibration of the phase response of the hydrophone is well-developed. [ABSTRACT FROM AUTHOR]

Passive acoustic monitoring (PAM) involves recording the sounds of animals and environments for research and conservation. PAM is used in a range of contexts across terrestrial, marine and freshwater environments. However, financial constraints limit applications within aquatic environments; these costs include the high cost of submersible acoustic recorders. We quantify this financial constraint using a systematic literature review of all ecoacoustic studies published in 2020, demonstrating that commercially available autonomous underwater recording units are, on average, five times more expensive than their terrestrial equivalents. This pattern is more extreme at the low end of the price range; the cheapest available aquatic autonomous units are over 40 times more expensive than their terrestrial counterparts. Following this, we test a prototype low‐cost, low‐specification aquatic recorder called the 'HydroMoth': this device is a modified version of a widely used terrestrial recorder (AudioMoth), altered to include a waterproof case and customisable gain settings suitable for a range of aquatic applications. We test the performance of the HydroMoth in both aquaria and field conditions, recording artificial and natural sounds, and comparing outputs with identical recordings taken with commercially available hydrophones. Although the signal‐to‐noise ratio and the recording quality of HydroMoths are lower than commercially available hydrophones, the recordings with HydroMoths still allow for the identification of different fish and marine mammal species, as well as the calculation of ecoacoustic indices for ecosystem monitoring. Finally, we outline the potential applications of low‐cost, low‐specification underwater sound recorders for bioacoustic studies, discuss their likely limitations, and present important considerations of which users should be aware. Several performance limitations and a lack of professional technical support mean that low‐cost devices cannot meet the requirements of all PAM applications. Despite these limitations, however, HydroMoth facilitates underwater recording at a fraction of the price of existing hydrophones, creating exciting potential for diverse involvement in aquatic bioacoustics worldwide. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

We illustrate the implementation and results of a field experiment, consisting of recording continuous signal from a hydrophone 3 m deep in the Venetian lagoon. We simultaneously recorded audio signal through a microphone placed on a nearby pier. We investigate the potential of this simple instrumental setup to explore the small touristic boat traffic contribution to the underwater noise. The ultimate goal of our work is to contribute to quantifying underwater noise pollution due to motorboat passages and its impact on the ecosystem. Efforts such as ours should help to identify measures that could diminish noise pollution, focusing specifically on the aspects that are most disruptive to underwater life. After this preliminary test, more work can be planned, involving the deployment of a larger network of similar instruments around the lagoon. At this point, we can conclude that (i) our instruments are sensitive enough to detect motorboats and identify some of their characteristics; (ii) the area of interest is characterized by a large (approx. 20 dB) day/night difference in ambient noise; and (iii) the historic center of Venice and its immediate surroundings are particularly noisy, in comparison to other similarly studied locations. [ABSTRACT FROM AUTHOR]

In this article, we consider underwater passive localization for vertically moving targets in shallow water. A method that utilizes parameters derived from interference structures is introduced, which enables the implementation of both range and depth estimations for targets with a single hydrophone. Underwater moving targets radiate broadband continuous spectrum signals by forming multipath signals through interface reflections, resulting in stable interference structures in the time-frequency domain of the received signals. Ray theory is utilized to derive the trajectory equations of the interference fringes. Considering the periodic characteristics of interference structures, a specialized homomorphic filter is designed to effectively restore the peaks of the fringes to reduce localization errors. Furthermore, with a priori information on the target's initial depth, the Hough Transform is used to extract the target's location information within interference fringes formed by direct signal and surface-reflected signal. Addressing the issue of multi-peaks in parameter domain, parameter information across multiple fringes is fused. The effectiveness and robustness of the proposed method have been demonstrated through numerical simulations, lake trial, and error assessments, even when underwater environments information is not precisely known. • The trajectory equations of interference fringes in the time-frequency domain for vertically moving targets are derived. • A specialized homomorphic filter is designed to effectively reduce localization errors based on the characteristics of the interference period. • Demonstrates good robustness to high-speed targets and uncertain underwater environments. • Lake trial data support with practical significance. [ABSTRACT FROM AUTHOR]

This paper demonstrates the cost-effective development of a hydrophone that covers a spectrum range from 0.1 Hz to 100 kHz. The sensing element underwent characterization techniques, such as X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray (EDX) in order to examine the morphology, consistency, microstructure, and crystallography of traditional lead zirconate titanate (PZT) piezoelectric discs, which served as sensing components. Additionally, an electronic circuit with a microcontroller was designed for enhancing the hydrophone's performance and enabling further analysis on a personal computer. This circuit comprises stages for preamplification and filtering, Direct Current (DC) biasing, and Analog-to-Digital Conversion. The digitized signal is subsequently analyzed using specialized audio software. To calibrate the hydrophone, the substitution method was employed, comparing its acoustic response with that of a commercial hydrophone. [Display omitted] • Design and development of hydrophone device consisting of piezoelectric sensing element (PZT) and the appropriate electronics. • The hydrophone has high sensitivity at the range of 0.1 Hz to 100 kHz. • The sensor can be used for underwater monitoring, being low-cost and fully portable. • The acoustic signals can be sent and analyzed via a personal computer. [ABSTRACT FROM AUTHOR]

Array shape calibration is commonly categorized into active calibration and self-calibration, yet both methods have their drawbacks and are generally considered less optimal. This paper proposes a novel approach based on the coherence of noise radiated from non-cooperative ships. Firstly, the time-varying law of the frequency domain coherence of ocean ambient noise received by a titled hydrophone pair when a ship passes by is investigated, which reveals the relative location of the hydrophones and passing ship. The array shape can be calibrated by analyzing the ambient noise coherence and horizontal acoustic path difference between passing ships and different hydrophones. The calibration results are consistent which are obtained from the three sections of data collected near the shipping lanes in the Qingdao offshore. One of the calibrated result is employed to simulate the coherogram of ambient noise when the ship passes by. It is shown that the simulation is in good agreement with the measured results. The main error sources are also analyzed and simulated in this paper, and the results show that the method is basically feasible and reliable. • Noise coherence coefficient model in water near shipping lanes. • Array shape calibration based on coherence coefficients of ship-radiated noise. • Simulation of noise coherogram during ships passage. [ABSTRACT FROM AUTHOR]

Cetaceans are an important part of the ocean ecosystem and are widely distributed in seas across the world. Cetaceans are heavily reliant on acoustic signals for communication. Some Odontoceti can perceive their environments using their sonar system, including the detection, localization, discrimination, and recognition of objects. Acoustic signals are one of the most commonly used types of data for Cetacean research, and it is necessary to develop Cetacean acoustic signal detection methods. This study compared the performance of a manual method, short-time Fourier transform (STFT), and wavelet transform (WT) in Cetacean acoustic signal detection. The results showed that WT performs better in click detection. According to this research, we propose using STFT for whistle and burst-pulse marking and WT for click marking in dataset building. This research will be helpful in facilitating research on the habits and behaviors of groups and individuals, thus providing information to develop methods for protecting species and developing biological resources.

The study of sound in the natural environment provides interesting information for researchers and policy makers driving conservation policies in our society. The soundscape characterises the biophony, anthrophony and geophony of a particular area. The characterisation of these different sources can lead to changes in ecosystems and we need to identify these parameters in order to make the right decision in relation to the natural environment. These values could be extrapolated and potentially help different areas of ecoacoustic research. Technological advances have enabled the passive acoustic monitoring (PAM) of animal populations in their natural environment. Recordings can be made with little interference, avoiding anthropogenic effects, making it a very effective method for some species such as cetaceans and other marine species in addition to underwater noise studies. Passive acoustic monitoring can be used for population census, but also to understand the effect of human activities on animals. However, recording data over long periods of time requires large storage and processing capacity to handle all the acoustic events generated. In the case of marine environments, the installation of sensors and instruments can be costly in terms of money and maintenance effort. In addition, if they are placed offshore, a data communication problem arises with coverage and bandwidth. In this paper, we propose a low-cost instrument to monitor the soundscape of a marine area using ecoacoustic indices. The instrument is called MASE and provides three echo-acoustic indices at 10 min intervals that are available in real time, which drastically reduces the volume of data generated. It has been operating uninterruptedly for a year and a half since its deployment, except during maintenance periods. MASE has been able to operate uninterruptedly, and maintain an adequate temperature inside while preserving its structural integrity for long periods of time. This has allowed the monitoring and characterisation of the soundscape of the test area in Gando Bay, Gran Canaria Island (Spain) without the need for human intervention to access the data on the instrument itself. Thanks to its integration with an external server, this allows the long-term monitoring of the soundscape, and it is possible to observe changes in the soundscape. In addition, the instrument has made it possible to compare the period of acoustic inactivity during confinement and the return of anthropogenic acoustic activity at sea. [ABSTRACT FROM AUTHOR]

Deep learning (DL)-based modulation recognition methods of underwater acoustic communication signals are mostly applied to a single hydrophone reception scenario. In this paper, we propose a novel end-to-end multihydrophone fusion network (MHFNet) for multisensory reception scenarios. MHFNet consists of a feature extraction module and a fusion module. The feature extraction module extracts the features of the signals received by the multiple hydrophones. Then, through the neural network, the fusion module fuses and classifies the features of the multiple signals. MHFNet takes full advantage of neural networks and multihydrophone reception to effectively fuse signal features for realizing improved modulation recognition performance. Experimental results on simulation and practical data show that MHFNet is superior to other fusion methods. The classification accuracy is improved by about 16%. [ABSTRACT FROM AUTHOR]

Four species of grouper (family Epinephlidae), Red Hind (Epinephelus guttatus), Nassau (Epinephelus striatus), Black (Mycteroperca bonaci), and Yellowfin Grouper (Mycteroperca venenosa) share an aggregation site in Little Cayman, Cayman Islands and produce sounds while aggregating. Continuous observation of these aggregations is challenging because traditional diver or ship-based methods are limited in time and space. Passive acoustic localization can overcome this challenge for sound-producing species, allowing observations over long durations and at fine spatial scales. A hydrophone array was deployed in February 2017 over a 9-day period that included Nassau Grouper spawning. Passive acoustic localization was used to find positions of the grouper-produced calls recorded during this time, which enabled the measurement of call source levels and evaluation of spatiotemporal aspects of calling. Yellowfin Grouper had the lowest mean peak-to-peak (PP) call source level, and Nassau Grouper had the highest mean PP call source level (143.7 and 155.2 dB re: 1 μPa at 1 m for 70–170 Hz, respectively). During the days that Nassau Grouper spawned, calling peaked after sunset. Similarly, when Red Hind calls were abundant, calls were highest in the afternoon and evening. The measured source levels can be used to estimate communication and detection ranges and implement passive acoustic density estimation for these fishes. [ABSTRACT FROM AUTHOR]

For the axial stretching fiber laser hydrophone, a fiber laser is often exposed to flexural interference. A theoretical model for the flexural vibration of a fiber laser was therefore constructed in this paper to explore the low frequency anti-interference mechanism for the frequency response of an axially tensioned fiber laser hydrophone (FLH). A specific packaging structure was used for finite element comparison and simulation. Packaged FLHs were tested for frequency response. The simulation and test results reveal that the flexible attachment at both ends of a fiber laser leads to lower amplitude of flexural vibration compared with rigid attachment, which therefore promotes a flat acoustic response curve of an FLH. The analysis given in this paper can be taken as a basis for improving the packaging technique. [ABSTRACT FROM AUTHOR]

We report on the extraction of deep ocean travel time variations from time‐lapse cross‐correlations between a hydrophone station and a three‐component broadband seismometer. The signals we cross‐correlate in this study result from repeated activity by the Monowai seamount, one of the most active submarine volcanoes of the Tonga‐Kermadec ridge. In particular, we introduce a specific workflow to exploit repetitive hydroacoustic underwater source activity, which we detail to such an extent that it serves as an example (or "cookbook"). For this reason, we have made the source code publicly available. The workflow proposed in this study (a) overcomes differences in instrument sensitivity and sample rates, (b) involves the selection of eligible cross‐correlations based on a source activity criterium as well as slowness analysis, and (c) extracts the travel time variations in distinct frequency bands. In our case, the two frequency bands are 3–6 and 6–12 Hz. We find that the estimated travel time variations in both frequency bands consist of a complex periodic pattern superimposed on a robust linear trend. This linear trend is decreasing, which we attribute to increasing water temperatures along the propagation path of the hydroacoustic signals. Plain Language Summary: We extract variations in the ocean travel time between submerged microphones and a seismometer on an island. The signals in this study result from repeated activity by the Monowai seamount, one of the most active submarine volcanoes of the Tonga‐Kermadec ridge. The travel time variations result from a time‐lapse similarity analysis. In particular, we introduce a specific workflow to exploit repetitive underwater source activity. We detail it to such an extent that it serves as an example (or "cookbook"). For this reason, we have made the source code publicly available. The workflow proposed in this study (a) overcomes discrepancies in instrument sensitivity and sampling rates, (b) involves the selection of qualified signals based on a source activity criterium as well as directional analysis, and (c) extracts the travel time variations in distinct frequency bands. The estimated travel time variations consist of a complex periodic pattern superimposed on a robust linear trend. Moreover, this linear trend is decreasing, which we attribute to increasing water temperatures along the propagation path of the hydroacoustic signals. Key Points: Travel time variations extracted from time‐lapse cross‐correlations between a hydrophone station and a three‐component broadband seismometerFrequency‐dependent acoustic travel time serves as path‐averaged temperature changes in the Sound Fixing and Ranging channelSelecting eligible cross‐correlations by source activity and slowness estimation is crucial for extracting reliable oceanic travel times [ABSTRACT FROM AUTHOR]

This article emphasis the importance of constructing an underwater vehicle monitoring system to solve various issues that are related to deep sea explorations. For solving the issues, conventional methods are not implemented, whereas a new underwater vehicle is introduced which acts as a sensing device and monitors the ambient noise in the system. However, the fundamentals of creating underwater vehicles have been considered from conventional systems and the new formulations are generated. This innovative sensing device will function based on the energy produced by the solar cells which will operate for a short period of time under the water where low parametric units are installed. In addition, the energy consumed for operating a particular unit is much lesser and this results in achieving high reliability using a probabilistic path finding algorithm. Further, two different application segments have been solved using the proposed formulations including the depth of monitoring the ocean. To validate the efficiency of the proposed method, comparisons have been made with existing methods in terms of navigation output units, rate of decomposition for solar cells, reliability rate, and directivity where the proposed method proves to be more efficient for an average percentile of 64%. [ABSTRACT FROM AUTHOR]

Background: While microbubble contrast agents (MCAs) are commonly used in ultrasound (US), they are inherently limited to vascular targets due to their size. Alternatively, phase‐changing nanodroplet contrast agents (PNCAs) can be delivered as nanoscale agents (i.e., small enough to extravasate), but when exposed to a US field of sufficient mechanical index (MI), they convert to MCAs, which can be visualized with high contrast using nonlinear US. Purpose: To investigate the effect of perfluorocarbon (PFC) core composition and presence of cholesterol in particle coatings on stability and image contrast generated from acoustic activation of PNCAs using high‐frequency US suitable for clinical imaging. Methods: PNCAs with varied core compositions (i.e., mixtures of perfluoropentane [C5] and/or perfluorohexane [C6]) and two coating formulations (i.e., with and without cholesterol) were characterized and investigated for thermal/temporal stability and postactivation, nonlinear US contrast in phantom and in vivo environments. Through hydrophone measurements and nonlinear numerical modeling, MI was estimated for pulse sequences used for PNCA activation. Results: All PNCA compositions were characterized to have similar diameters (249–267 nm) and polydispersity (0.151–0.185) following fabrication. While PNCAs with majority C5 core composition showed higher levels of spontaneous signal (i.e., not due to US activation) in phantoms than C6‐majority PNCAs, all compositions were stable during imaging experiments. When activating PNCAs with a 12.3‐MHz US pulse (MI = 1.1), C6‐core particles with cholesterol‐free coatings (i.e., CF‐C6‐100 particles) generated a median contrast of 3.1, which was significantly higher (p < 0.001) than other formulations. Further, CF‐C6‐100 particles were activated in a murine model, generating US contrast ≥$ \ge $3.4. Conclusion: C6‐core PNCAs can provide high‐contrast US imaging with minimal nonspecific activation in phantom and in vivo environments. [ABSTRACT FROM AUTHOR]

The goal of this study was to characterize the detection range of a near real-time baleen whale detection system, the digital acoustic monitoring instrument/low-frequency detection and classification system (DMON/LFDCS), equipped on a Slocum glider and a moored buoy. As a reference, a hydrophone array was deployed alongside the glider and buoy at a shallow-water site southwest of Martha's Vineyard (Massachusetts, USA) over a four-week period in spring 2017. A call-by-call comparison between North Atlantic right whale upcalls localized with the array (n = 541) and those detected by the glider or buoy was used to estimate the detection function for each DMON/LFDCS platform. The probability of detection was influenced by range, ambient noise level, platform depth, detection process, review protocol, and calling rate. The conservative analysis of near real-time pitch tracks suggested that, under typical conditions, a 0.33 probability of detection of a single call occurred at 6.2 km for the buoy and 8.6–13.4 km for the glider (depending on glider depth), while a 0.10 probability of detection of a single call occurred at 14.4 m for the buoy and 22.6–27.5 km for the glider. Probability of detection is predicted to increase substantially at all ranges if more than one call is available for detection. [ABSTRACT FROM AUTHOR]

A single-sensor differential pressure vector hydrophone based on a radially polarized piezoelectric circular tube structure is proposed, which can realize the azimuth angle of the incident acoustic wave by using the scalar information of the underwater sound field through a single sensor without any priori conditions. It has the characteristics of simple structure and low cost. The proposed hydrophone is improved based on the piezoelectric circular tube structure. And its piezoelectric ceramics are divided into eight parts. Through the different response voltages of each part, the underwater sound field scalar information received by a single sensor is used, combined with signal processing algorithms, the azimuth angle of the incident sound wave is estimated. The finite element method is used to complete the simulation of the hydrophone, and the simulation results are analyzed and fitted. Through the mockup test, comparing the results, and combining with the fitting formula, it is verified that the hydrophone can estimate the azimuth angle of the incident sound wave without any priori conditions. [ABSTRACT FROM AUTHOR]

This article reports experimental validation for spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. The method was validated using linear and nonlinear beams transmitted by seven single-element spherically focusing transducers (2–10 MHz; ${F}$ /#: 1–3) and measured with five hydrophones (sensitive element diameters ${d}_{g}$ : 85–1000 $\mu \text{m}$), resulting in 35 transducer/hydrophone combinations. Exponential functions, exp($-\alpha {x}$), where ${x} = {d}_{g}$ /($\lambda _{{1}}{F}$ /#) and $\lambda _{{1}}$ is the fundamental wavelength, were used to model focal pressure ratios ${p}'/{p}$ (where ${p}'$ is the measured value subjected to spatial averaging and ${p}$ is the true axial value that would be obtained with a hypothetical point hydrophone). Spatiotemporal deconvolution reduced $\alpha $ (followed by root mean squared difference between data and fit) from 0.29–0.30 (7%) to 0.01 (8%) (linear signals) and from 0.29–0.40 (8%) to 0.04 (14%) (nonlinear signals), indicating successful spatial averaging correction. Linear functions, Cx + 1, were used to model FWHM’/FWHM, where FWHM is full-width half-maximum. Spatiotemporal deconvolution reduced ${C}$ from 9% (4%) to −0.6% (1%) (linear signals) and from 30% (10%) to 6% (5%) (nonlinear signals), indicating successful spatial averaging correction. Spatiotemporal deconvolution resulted in significant improvement in accuracy even when the hydrophone geometrical sensitive element diameter exceeded the beam FWHM. Responsible reporting of hydrophone-based pressure measurements should always acknowledge spatial averaging considerations. [ABSTRACT FROM AUTHOR]

This article reports spatiotemporal deconvolution methods and simple empirical formulas to correct pressure and beamwidth measurements for spatial averaging across a hydrophone sensitive element. Readers who are uninterested in hydrophone theory may proceed directly to for an easy method to estimate spatial-averaging correction factors. Hydrophones were modeled as angular spectrum filters. Simulations modeled nine circular transducers (1–10 MHz; F/1.4–F/3.2) driven at six power levels and measured with eight hydrophones (432 beam/hydrophone combinations). For example, the model predicts that if a 200- $\mu \text{m}$ membrane hydrophone measures a moderately nonlinear 5-MHz beam from an F/1 transducer, spatial-averaging correction factors are 33% (peak compressional pressure or ${p}_{c}$), 18% (peak rarefactional pressure or ${p}_{r}$), and 18% (full width half maximum or FWHM). Theoretical and experimental estimates of spatial-averaging correction factors to were in good agreement (within 5%) for linear and moderately nonlinear signals. Criteria for maximum appropriate hydrophone sensitive element size as functions of experimental parameters were derived. Unlike the oft-cited International Electrotechnical Commission (IEC) criterion, the new criteria were derived for focusing rather than planar transducers and can accommodate nonlinear signals in addition to linear signals. Responsible reporting of hydrophone-based pressure and beamwidth measurements should always acknowledge spatial-averaging considerations. [ABSTRACT FROM AUTHOR]

An acoustic survey of Cuvier's beaked whales (Ziphius cavirostris) was conducted off the U.S. West Coast in August and September 2016 using drifting recorder systems with a vertical array of two hydrophones at a depth of ~110 m. Recorders were deployed 22 times to representatively cover a 1,058,000 km2 study area from the shelf break to ~556 km offshore. Vertical angles to echolocation pulses were measured using the signal time‐difference‐of‐arrival on the two hydrophones. Echolocation pulses of Cuvier's beaked whales were identified from their arrival angles (always from below the array) and unique acoustic characteristics. The density and abundance of Cuvier's beaked whales were estimated using a group‐based point‐transect analysis with 2 min time snapshots. The area effectively surveyed was estimated using a maximum simulated likelihood approach to fit the observed distribution of signal arrival angles. The acoustic availability of whales during their dive cycle was estimated from the duration of acoustic encounters using a mark‐recapture approach. Overall, Cuvier's beaked whales were present during 0.60% of snapshots, and their estimated average density is 5.12 animals per 1,000 km2 (CV = 0.27). Their estimated abundance in the study area is 5,454 individuals (95% credibility intervals: 3,151 to 8,907). [ABSTRACT FROM AUTHOR]

We propose a new type of fiber hydrophone composed of an etched fiber Bragg grating and a special packaging structure for detecting acoustic waves in the low-frequency band under water. The operating mechanism is based on the mechanical vibration of the fiber Bragg grating from the induced vibrating stress of acoustic pressure. The induced pressure of acoustic waves pushes the silicone rubber thin film, causing its vibration and then stretching the fiber Bragg grating, thus resulting in the grating wavelength shift which is overlapped with a tunable laser. The variation in the overlapped light intensity is transferred to an electrical signal by using a photodetector. From the experimental results, we can determine that the smaller the fiber diameter, the higher the sensitivity and frequency response. In order to confirm that this FBG hydrophone has the ability to work in high-frequency acoustic waves, this fiber grating hydrophone and a standard piezoelectric hydrophone are experimentally compared to in the same test conditions in the frequency range from 4 to 10 kHz. According to the experimental results, the fiber grating hydrophone has better responsivity than that of the conventional hydrophone. Due to the unique sensing structure design, this wide-band fiber hydrophone can be useful in long-term continuous monitoring of acoustic waves. [ABSTRACT FROM AUTHOR]

High-resolution images are required for the exploration of mineral resources on and beneath the deep seafloor. Under the Strategic Innovation Promotion Program launched by the Council for Science, Technology, and Innovation in Japan, we developed a survey system using a deep-towed acoustic source and receivers that produces very high-resolution subseafloor images. Our system uses a subbottom profiler as the acoustic source and a hydrophone cable as receivers. These instruments are towed near the seafloor by a remotely operated vehicle (ROV). The subbottom profiler generates chirp signals in the frequency band of 700–2250 Hz, and 32 hydrophones with a receiver interval of 1 m record the reflected waves. The system also includes two recorder units that record the acoustic wavefield with a sampling interval of 0.1 ms. We carried out field surveys at two locations in Japan: one site is in the Sea of Japan where gas hydrates were discovered and the other site is near Okinawa Island where a plume from a hydrothermal smoker was recorded by an ROV camera. Pre-stack depth migration was applied to the processed data using velocity models with a constant velocity gradient below the seafloor. The vertical resolution was 0.5–1 m depending on the depth, and the horizontal resolution was about ten times larger than the vertical resolution. The resulting images show that detailed subseafloor structures can be clearly imaged by the deep-towed system, even at sea depths greater than 1000 m. [ABSTRACT FROM AUTHOR]