Search

The Pioneers of Underwater Acoustics Medal is presented to an individual irrespective of nationality, age, or society affi liation, who has made an outstanding contribution to the science of underwater acoustics, as evidenced by publication of research in professional journals or by other accomplishments in the fi eld. The award was named in honor of fi ve pioneers in the fi eld: H. J. W. Fay, R. A. Fessenden. H. C. Hayes, G. W. Pierce, and P. Langevin.

A rupture induced underwater sound source (RIUSS) is being developed as an alternative to other impulsive sound sources commonly utilized in underwater acoustics experiments and surveys. The device is comprised of a graphite rupture disk mounted over an evacuated chamber. After the disk breaks, an inrush of water creates a high amplitude acoustic pulse. A field test was conducted to measure the acoustic output as a function of depth for a given source configuration, and high speed underwater video was simultaneously captured with an acoustic recording system to correlate the features of the acoustic output to the ensuing bubble activity.

Classical ocean acoustic experiments involve the use of synchronized arrays of sensors. However, the need to cover large areas and/or the use of small robotic platforms has evoked interest in single-hydrophone processing methods for localizing a source or characterizing the propagation environment. One such processing method is "warping," a non-linear, physics-based signal processing tool dedicated to decomposing multipath features of low-frequency transient signals (frequency f 500 Hz), after their propagation through shallow water (depth D 200 m) and their reception on a distant single hydrophone (range r 1 km). Since its introduction to the underwater acoustics community in 2010, warping has been adopted in the ocean acoustics literature, mostly as a pre-processing method for single receiver geoacoustic inversion. Warping also has potential applications in other specialties, including bioacoustics; however, the technique can be daunting to many potential users unfamiliar with its intricacies. Consequently, this tutorial article covers basic warping theory, presents simulation examples, and provides practical experimental strategies. Accompanying supplementary material provides matlab code and simulated and experimental datasets for easy implementation of warping on both impulsive and frequency-modulated signals from both biotic and man-made sources. This combined material should provide interested readers with user-friendly resources for implementing warping methods into their own research.

Coupled-mode methods have been used in underwater acoustics to compute three-dimensional sound propagation and scattering. Significant computational simplifications are possible for media with a lateral variation restricted to cylindrically symmetric anomalies, such as seamounts, and also for media which are invariant in one of the horizontal directions. Typically, the upper and lower depth boundaries of the medium have then been horizontal and flat. This paper generalizes the discrete coupled-mode method with a reflection- (or scattering-) matrix formulation to media with irregular and locally reacting boundaries. Horizontal and vertical segments thereby approximate a sloping boundary. Incorporation of the boundary conditions in a correct way necessitates modifications of the basic equation systems. Additional coupling matrices appear, involving integration of normal-mode products over the depth increments for adjacent regions of the medium. The paper includes three computational examples. One is from underwater acoustics with an island that rises above the sea level. The other two are from atmospheric acoustics with sound propagation over a locally reacting irregular ground surface. Using nonlinear optimization, it is possible to select a suitable artificial absorbing medium termination for a mode representation of the field.

Underwater acoustics is one of the primary areas of emphasis in the Ocean Engineering Department at the University of Rhode Island, the first Ocean Engineering program in the United States. The program offers Bachelors, Masters (thesis and non-thesis options) and Ph.D. degrees in Ocean Engineering. These programs are based at the Narragansett Bay campus, providing access to a living laboratory for student learning. Some key facilities of the program are an acoustics tank, a 100-foot-long wave tank, and currently the R/V Endeavor, a UNOLS oceanographic research vessel operated by the University of Rhode Island. A new Regional Class vessel is anticipated in 2021. At the graduate level, students are actively involved in research focused in areas such as acoustical oceanography, propagation modeling, acoustic positioning and navigation, geoacoustic inversion, marine mammal acoustics, ocean acoustic instrumentation, and transducers. An overview of classroom learning and ongoing research will be provided, along with information regarding the requirements of entry into the program.

In recent years, methods have been developed to estimate a variety of environmental parameters based on measurements of the ocean ambient noise. For example, noise has been used to estimate water depth using the passive fathometer technique and bottom loss estimated and used to invert for seabed parameters. There is also information in the noise about the water column sound speed, volume attenuation, and the sea-state. The Fisher information can be used to quantify the basic information available in the noise measurements and its inverse, the Cramér-Rao lower bound (CRLB), provides the lower limit on the variance of an unbiased estimator of a particular parameter. The CRLB can be used to study the feasibility of various measurement configurations and parameter sensitivities. In this paper, the CRLB is developed for ocean ambient noise and the environmental information contained in the measurements is determined. The CRLBs provide an estimate of the underlying information in the data, however, it is independent of the estimation methodology. This is useful to determine if a given estimation method is reaching the lower bound. Results illustrating the bounds as well as sensitivities and performance of estimators are demonstrated using both simulations and data.

When a beam emitted from an active monostatic sensor system sweeps across a volume, the echoes from scatterers present will fluctuate from ping to ping due to various interference phenomena and statistical processes. Observations of these fluctuations can be used, in combination with models, to infer properties of the scatterers such as numerical density. Modeling the fluctuations can also help predict system performance and associated uncertainties in expected echoes. This tutorial focuses on "physics-based statistics," which is a predictive form of modeling the fluctuations. The modeling is based principally on the physics of the scattering by individual scatterers, addition of echoes from randomized multiple scatterers, system effects involving the beampattern and signal type, and signal theory including matched filter processing. Some consideration is also given to environment-specific effects such as the presence of boundaries and heterogeneities in the medium. Although the modeling was inspired by applications of sonar in the field of underwater acoustics, the material is presented in a general form, and involving only scalar fields. Therefore, it is broadly applicable to other areas such as medical ultrasound, non-destructive acoustic testing, in-air acoustics, as well as radar and lasers.

Regulations for underwater anthropogenic noise are typically formulated in terms of peak sound pressure, root-mean-square sound pressure, and (weighted or unweighted) sound exposure. Sound effect studies on humans and other terrestrial mammals suggest that in addition to these metrics, the impulsiveness of sound (often quantified by its kurtosis β) is also related to the risk of hearing impairment. Kurtosis is often used to distinguish between ambient noise and transients, such as echolocation clicks and dolphin whistles. A lack of standardization of the integration interval leads to ambiguous kurtosis values, especially for transient signals. In the current research, kurtosis is applied to transient signals typical for high-power underwater noise. For integration time (t 2 - t 1), the quantity (t 2 - t 1) / β is shown to be a robust measure of signal duration, closely related to the effective signal duration, τ eff for sounds from airguns, pile driving, and explosions. This research provides practical formulas for kurtosis of impulsive sounds and compares kurtosis between measurements of transient sounds from different sources. © 2020 Acoustical Society of America.

This paper considers the importance of particle motion to fishes and invertebrates and the steps that need to be taken to improve knowledge of its effects. It is aimed at scientists investigating the impacts of sounds on fishes and invertebrates but it is also relevant to regulators, those preparing environmental impact assessments, and to industries creating underwater sounds. The overall aim of this paper is to ensure that proper attention is paid to particle motion as a stimulus when evaluating the effects of sound upon aquatic life. Directions are suggested for future research and planning that, if implemented, will provide a better scientific basis for dealing with the impact of underwater sounds on marine ecosystems and for regulating those human activities that generate such sounds. The paper includes background material on underwater acoustics, focusing on particle motion; the importance of particle motion to fishes and invertebrates; and sound propagation through both water and the substrate. Consideration is then given to the data gaps that must be filled in order to better understand the interactions between particle motion and aquatic animals. Finally, suggestions are provided on how to increase the understanding of particle motion and its relevance to aquatic animals.

Research on supervised deep learning methods have shown potential for ocean acoustic applications. Supervised methods require labeled data samples for training. However, ocean acoustics data often do not contain labels. In experimental situations it is possible to estimate source range and speed labels as inferred by GPS data, but the environmental labels may not be easily defined. Applications of machine learning in ocean acoustics will be greatly enhanced by developing ways to utilize semi-supervised learning. In other applications, semi-supervised learning has been accomplished via contrastive learning or self-attention methods. Our work applies semi-supervised learning by developing a contrastive learning framework for acoustic data. First the model does self-training using an augmentation policy on unlabeled data to initialize representations of key features in the data. Then a small set of cleanly labelled samples are given to a model for supervised learning to teach the desired prediction task. Our work in contrastive learning will be applied to transiting surface ship spectrograms to demonstrate the effectiveness for predicting source labels and seabed type. Preliminary work will be presented to illustrate the advantages of semi-supervised learning. [Work supported by ONR contract N00014-19-C-20001.]

Passive acoustic monitoring is a useful technique for studying aquatic animals, but sustained observing systems require automated algorithms for detecting and classifying sounds of interest. In the last decade, deep neural networks have proven highly successful at solving a wide range of pattern recognition tasks, and recently, we have seen the first promising applications of deep neural networks to detection and classification tasks in marine bioacoustics. Deep neural networks exhibit a high degree of versatility and adaptability: the same network architecture can be trained to accomplish a multitude of tasks by feeding appropriate training data to the network without the need to modify the underlying algorithm. Thus, neural networks have the potential to transform our approach to developing acoustic detection and classification programs, enabling researchers in the field to develop or re-purpose their own programs. MERIDIAN is contributing towards this goal through the development of the open-source Python package Ketos, which provides a high-level programming interface for building training datasets and developing neural network based detectors and classifiers for analyzing underwater acoustics data. In this contribution, an overview of the software package will be given and its functionalities will be demonstrated through case studies.

To evaluate the performance of an equipment or object in underwater acoustics, the acoustic properties of acoustic materials should be accurately predicted and measured. The measurement of acoustic properties via panel testing in water tank require a narrow beam due to unwanted signals such as reflection from walls and diffraction signal. However, in the case of low frequency sound source, it generally has broad beam, and in order to have narrow beam, it is difficult to operation and physical limitation. These limitations and problems can be overcome by using a small size of parametric array capable of generating high-directional low-frequency signals. In order to use the parametric array as a sound source for an acoustic measurement system, it is necessary to understand the characteristics of the parametric array in near field. So this study was performed an experimental investigation on parametric arrays and study to construct an optimal system. Numerous factors such as the effect of pseudo sounds and the optimal distance between the source and receiver were considered to construct the system, and the results of experimental investigations, such as an efficient method to remove the effects of pseudo sounds and effective length, were confirmed.

The frequency bandwidth of the sound from a light helicopter, such as a Robinson R44, extends from about 13 Hz to 2.5 kHz. As such, the R44 has potential as a low-frequency sound source in underwater acoustics applications. To explore this idea, an experiment was conducted in shallow water off the coast of southern California in which a horizontal line of hydrophones detected the sound of an R44 hovering in an end-fire position relative to the array. Some of the helicopter sound interacted with seabed to excite the head wave in the water column. A theoretical analysis of the sound field in the water column generated by a stationary airborne source leads to an expression for the two-point horizontal coherence function of the head wave, which, apart from frequency, depends only on the sensor separation and the sediment sound speed. By matching the zero crossings of the measured and theoretical horizontal coherence functions, the sound speed in the sediment was recovered and found to take a value of 1682.42 ± 16.20 m/s. This is consistent with the sediment type at the experiment site, which is known from a previous survey to be a fine to very-fine sand.

USNA’s Underwater Acoustics and Sonar (SP411) course, taught in a studio classroom, can seat 36 students with 5 experimental/computer workstations on each side of the classroom. This non-laboratory course relies on hands-on demos and in-class mini-laboratory experiences bringing theoretical concepts and experimental aspects of sound propagation and detection theory to life. Experiments include: (a) sound speed versus temperature in water, (b) beam pattern functions versus angle for multi-element arrays, (c) Fourier analysis of periodic waveforms including rectangular pulses, and (d) square law detection of signals plus noise—generating receiving operating characteristic ROC curves. Computer visualizations using Mathematica included phasor addition of signals, N-element array superposition for directivity analysis and a user friendly ray tracing program implementing any sound speed versus depth profile. On 20 March, 2020, the USNA entered the pandemic era by requiring remote learning for all 4100 midshipman, commencing shortly after spring break. Remote learning in SP411 was asynchronous, devoid of lectures, including handouts and videos of wave effects and worked examples. Mathematica visualizations replaced experiments and extra instruction (in different time zones) was always available. Co-author LEB’s photoacoustic imaging experiments, done remotely, used homemade electronics (pulsed LED driver and low-noise amplifiers)—keeping future midshipmen research going.

This talk will provide an overview of the research carried out in ocean acoustics in India. The topics of special interest for international collaboration identified by some of the institutions involved in ocean acoustics research will be highlighted. The status of underwater acoustics education in India will be reviewed. Two institutions located in Chennai, India (National Institute of Ocean Technology (NIOT) and Indian Institute of Technology (IIT), Madras) are heavily involved in ocean acoustics research and their research activities will be discussed in detail. The ocean acoustics research at these two institutions has a strong field experimental component involving international collaboration. Past and ongoing research with international collaboration will be highlighted. Challenges faced by these and other institutions in fostering collaboration with international counterparts will be discussed. The scenario of ocean acoustics education and training in India will be surveyed and outlook for the future will be discussed.

Development of acoustics in research and field applications in Taiwan has been well established and aligned with developed countries world-wide. Taiwan has comprehensive acoustic regulations and standards for residential and hospitality building types and stringent environmental noise protection ordinances. It is the only country that has specific noise regulation for 20 Hz to 200 Hz. The Taiwan Acoustical Association (TAA) was established on 1987. Since then, thirty-two biannual meetings have been successfully held. Auditory and speech communication, building acoustics, mechanical acoustics, ultrasound, underwater acoustics, noise and vibration, as well as bio-acoustics and other related acoustic topics were covered in these meetings. According to the Taiwan Patent Search System (English version), about 187 acoustic related patents have been granted since 2004. In terms of research institutes, at least 14 top universities in Taiwan have specific major for acoustic studies. At National Taiwan University of Science and Technology, there is an architectural acoustics program under the Department of Architecture. Its team has been involved in the world-renowned performing arts center projects such as Taichung Metropolitan Opera House, Weiwuying National Kaohsiung Center for the Arts, Taipei Performing Arts Center and Taipei Pop Music Center. It can be concluded that research and field applications of acoustics in Taiwan has been valued by the people and will be continued and tuned to world-wide standards!

The propagation of sound in a density-stratified fluid is examined in an experiment with a tank of salty water whose density increases continuously from the fluid surface to the tank bottom. Measurements of the height dependence of the fluid density are used to calculate the height dependence of the fluid salinity and sound speed. The height-dependent sound speed is then used to calculate the refraction of sound rays. Sound propagation in the fluid is measured in three dimensions and compared with the ray analysis. This study provides a basis for laboratory modeling of underwater sound propagation in the fluctuating stratified oceans.

The cross-spectral density of ocean ambient noise is usually estimated from the product of the complex hydrophone signals, each of which already corresponds to the summed responses of sources from all angles. The true coherence is the integral over all angles of the angle-dependent product. The influence of this distinction on necessary time integration in geoacoustic inversion and passive fathometry is explored, and a meaningful separation of the cross-spectral density matrix into Toeplitz and Hankel parts is proposed. Various processing techniques are applied to synthesized data and some experimental vertical array data in a passive fathometry context. Passive fathometry is only sensitive to the Hankel part of the matrix.

Deep ocean ambient sound levels have been the subject of recent studies, with particular interest in the identification of long term trends. This paper describes a statistical method for performing long term trend analysis and uncertainty evaluation of the estimated trends. Uncertainties are needed if the quality of the estimates are to be assessed and if the results from different studies or different methods are to be compared. The measured data span 14 years, from 2003 to 2017, and originate from the Southern Ocean close to Cape Leeuwin, Australia. The method uses a flexible discrete model incorporating terms that capture seasonal variations in the data and a moving-average statistical model to describe the serial correlation of residual deviations, with uncertainties validated using bootstrap resampling. The method is applied to time series representing monthly and daily aggregated statistical levels for five frequency bands to obtain estimates for the change in sound pressure level over the examined period with associated uncertainties. The results show a statistically significant reduction in sound pressure levels over the examined period at that location. Possible explanations for these changes are postulated, including the effects of shipping, wind speed, sea surface temperature, and changes in Antarctic ice volume.

Remote unknown sound-source localization is a challenging task with applications in a variety of fields, such as underwater acoustics, atmospheric acoustics, structural acoustics, bio-medical ultrasound, animal bio-acoustics, and seismology. In nearly all cases, the localization algorithm is only successful when there is good cross correlation between remotely measured and predicted acoustic fields, a possibility that exits at low frequencies and/or short ranges in imperfectly known environments. However, at sufficiently high frequency and/or long range, the requisite measured-to-predicted-field correlation might never be high enough for successful source localization. However, recent investigations of the frequency-difference autoproduct, a quadratic product of two complex field amplitudes having different frequencies, suggest that it may have the phase structure of an acoustic field at the difference frequency. Thus, using sufficiently low difference frequencies, unknown sources may be localized at ranges where conventional techniques are unsuccessful by correlating measured and predicted (ideal) autoproducts. The underlying formulation of frequency-differencing techniques is presented along with examples drawn from simulations, laboratory experiments, and ocean propagation measurements that involve frequencies from fractions of a Hertz to more than 100 kHz, and propagation distances from tens of centimeters to hundreds of kilometers. [Sponsored by ONR, NAVSEA, and NSF.]

For over 50 years Principles of Underwater Sound by Robert Urick has been a widely used book on underwater acoustics and sonar for practicing engineers, scientists, technicians, project managers, teachers and students and it is in the last stages of an extensive update to be released this spring. The book encapsulates the fundamentals and phenomena of underwater sound as applied to the Sonar Equation, the heart of prediction of sonar performance, and the book’s contents lie squarely in the middle between theory at one end and practical technology at the other. The editor and contributors of the update have attempted to imbue the new edition with the same “blue collar” spirit of Urick’s original work but with so much new technology to address since the present edition was published in 1983, it’s been difficult to keep the updated printed edition to a convenient size. Therefore, in addition to the traditional printed book edition, we are introducing a new e-book edition which will provide more detail than the printed edition. This presentation includes: (1) Descriptions of the two new formats (2) Examples of updated technology, and (3) New subjects: sonar transmissions’ effect on the health of marine mammals; sea floor mapping; locating downed airliners; and search plans.

Estimating source-receiver range and predicting seabed type are important problems in underwater acoustics. Deep learning solutions for simultaneous predictions have been previously shown some success for these problems due to their ability to learn patterns from large datasets. However, one important question to consider in deep learning is how predictions should be made: A network can be configured to classify or regress predictions. In this study, that question is explored by comparing predictions from networks trained on simulated SUS charge pressure time-series configured to either regress or classify range and seabed predictions from measured data taken with the IVAR system during the 2017 Seabed Characterization Experiment. To further this inquiry, networks configured to regress range and classify seabed type (via “multi-task” learning) are also explored. Separating the two predictions proves to be useful, as the networks using multi-task learning perform better at predicting range and seabed class simultaneously than those configured only for classification or regression. The results of this experiment illustrate the need to use the proper type of network outputs depending on the desired predictions. [Work supported by the Office of Naval Research.]

Accurate modeling of acoustic propagation in the ocean waveguide is important to SONAR-performance prediction. Particularly in shallow waters, a crucial contribution to the total transmission loss is the bottom refection loss, which can be estimated passively by beamforming the natural surface-noise acoustic field recorded by a vertical line array of hydrophones. However, the performance in this task of arrays below 2 m of length is problematic for frequencies below 10 kHz It is shown in this paper that, when the data are free of interference from sources other than wind and wave surface noise, data from a shorter array can be used to approximate the coherence function of a longer array. This improves the angular resolution of the estimated bottom loss, often making use of data at frequencies above the array design frequency. Application to simulated and experimental data shows that the technique, rigorously justified for a halfspace bottom, is effective also on more complex bottom types. Dispensing with active sources, small autonomous underwater vehicles equipped with short arrays can be envisioned as compact, efficient seabed-characterization systems. The proposed technique is shown to improve significantly the reflection-loss estimate of an array that would be a candidate for such application.

The application of ocean acoustic tomography in Fram Strait requires a careful assessment of the accuracy to which estimates of sound speed from tomography can be converted to estimates of temperature. The Fram Strait environment is turbulent, with warm, salty, northward-flowing North Atlantic water interacting with cold, fresh, southward-flowing Arctic water. The nature of this environment suggests that salinity could play an important role with respect to sound speed. The properties of sound speed with respect to temperature and salinity in this environment were examined using climatological and in situ glider data. In cold water, a factor of about 4.5 m s(-1) °C(-1) can be used to scale between sound speed and temperature. In situ data obtained by gliders were used to determine the ambiguities between temperature, salinity, and sound speed. Tomography provides a depth-averaging measurement. While errors in the sound speed-temperature conversion at particular depths may be 0.2 °C or larger, particularly within 50 m of the surface, such errors are suppressed when the depth is averaged. Using a simple scale factor to compute temperature from sound speed introduced an error of about 20 m °C for depth-averaged temperature, a value less than formal uncertainties estimated from acoustic tomography.

Underwater sound of rigid-hulled inflatable boats was recorded 142 times in total, over 3 sites: 2 in southern British Columbia, Canada, and 1 off Western Australia. Underwater sound peaked between 70 and 400 Hz, exhibiting strong tones in this frequency range related to engine and propeller rotation. Sound propagation models were applied to compute monopole source levels, with the source assumed 1 m below the sea surface. Broadband source levels (10-48 000 Hz) increased from 134 to 171 dB re 1 μPa @ 1 m with speed from 3 to 16 m/s (10-56 km/h). Source power spectral density percentile levels and 1/3 octave band levels are given for use in predictive modeling of underwater sound of these boats as part of environmental impact assessments.

Underwater noise associated with the installation and operation of hydrokinetic turbines in rivers and tidal zones presents a potential environmental concern for fish and marine mammals. Comparing the spectral quality of sounds emitted by hydrokinetic turbines to natural and other anthropogenic sound sources is an initial step at understanding potential environmental impacts. Underwater recordings were obtained from passing vessels and natural underwater sound sources in static and flowing waters. Static water measurements were taken in a lake with minimal background noise. Flowing water measurements were taken at a previously proposed deployment site for hydrokinetic turbines on the Mississippi River, where sounds created by flowing water are part of all measurements, both natural ambient and anthropogenic sources. Vessel sizes ranged from a small fishing boat with 60 hp outboard motor to an 18-unit barge train being pushed upstream by tugboat. As expected, large vessels with large engines created the highest sound levels, which were, on average, 40 dB greater than the sound created by an operating hydrokinetic turbine. A comparison of sound levels from the same sources at different distances using both spherical and cylindrical sound attenuation functions suggests that spherical model results more closely approximate observed sound attenuation.

In many underwater acoustics problems such as search and rescue, it is desirable to establish the source of the sonar pings. In this paper a methodology is proposed to watermark sonar pulses prior to transmission by embedding them with a chain of low power chirplets represented by a graph. Watermark detection is formulated using stretch processing where chirplet energies are focused on a single beat frequency only if the graph is traversed along the same path used at the source. The algorithm is tested by simulation in shallow water at varying ranges.

This paper shows that energy conservation and the derivation of the two-way coupled mode range equations can be extended in three dimensions to complex mode functions and eigenvalues. Furthermore, the energy in the coupled mode formulation is conserved for finite thickness fluid ocean waveguides with a penetrable bottom boundary beneath any range dependence. The derivations rely on completeness and a modified orthonormality statement. The mode coupling coefficients are specified solely and explicitly by the waveguide range dependence. The statement of energy conservation is applied to a numerical coupled mode calculation.

Predictions of log-amplitude variance are compared against sample log-amplitude variances reported by White, Andrew, Mercer, Worcester, Dzieciuch, and Colosi [J. Acoust. Soc. Am. 134, 3347–3358 (2013)] for measurements acquired during the 2009 Philippine Sea experiment and associated Monte Carlo computations. The predictions here utilize the theory of Munk and Zachariasen [J. Acoust. Soc. Am. 59, 818–838 (1976)]. The scattering mechanism is the Garrett–Munk internal wave spectrum scaled by metrics based on measured environmental profiles. The transmitter was at 1000 m depth and the receivers at nominal range 107 km and depths 600–1600 m. The signal was a broadband m-sequence centered at 284 Hz. Four classes of propagation paths are examined: the first class has a single upper turning point at about 60 m depth; the second and third classes each have two upper turning points at roughly 250 m; the fourth class has three upper turning points at about 450 m. Log-amplitude variance for all paths is predicted to be 0.04–0.09, well within the regime of validity of either Born or Rytov scattering. The predictions are roughly consistent with the measured and Monte Carlo log-amplitude variances, although biased slightly low. Paths turning in the extreme upper ocean (near the mixed layer) seem to incorporate additional scattering mechanisms not included in the original theory.

Mike Buckingham’s contributions to underwater acoustics and acoustical oceanography span a broad range of topics, from field expeditions to measure Arctic ambient noise in the marginal ice zone to a complete model for propagation and attenuation in marine sediments. The hallmark of Mike’s work is the formulation of elegant analytical solutions to canonical problems, providing physical insight. I will attempt to illustrate Mike’s approach with examples from his work and illustrations of how it has motivated and influenced future approaches.

For 38 years, MSK has developed mentoring undergraduate students in the Physics Department. Mentorship is a unique dance, a growing tree—with kind branches embracing any challenge—a give and take enjoyed by both the mentor and mentee. It should be fun—filled with goals to achieve (often wrapped in a research challenge). “Acoustics” and “Underwater Acoustics and Sonar,” courses—inspire projects. Students have presented research at ASA meetings. Some become co-authors on published papers. For 35 years, MSK developed a high school mentorship program at USNA. Rising juniors and seniors may apply. HS mentees start their research on a project that evolves over one or two months working twice a week for 2.5–3 h sessions. Students often continue over the summer. Recently, MSK took HS mentorship students to the ASA Boston Meeting, the 17th ISNA Meeting (Santa Fe 2018), Louisville and Internoise 2019 in Madrid. Three HS seniors at the ISNA conference presented research (published in POMA) on (1) nonlinear acoustic landmine detection with wetted and dry beads [EVS], (2) acoustic levitation and nonlinear sound scattering by a bubble [KAH], and (3) the interaction of two crossed ultrasonic streaming jets [JMC]. Their dedication and insights are highly valued.

Belugas are widely known for their diverse and frequent calls and have been given the nickname, “sea canaries.” As the number of belugas has been living in aquariums and ocean parks, it is very important for us to provide a comfortable environment for these creatures. That means we should understand how sounds made by belugas vocalization and environmental noise related to belugas habitats, therefore designing the most suitable artificial pool for these creatures. This study is on sound acquisition and acoustic estimation near the living pool of captive belugas whales in a new Ocean Park. Besides different hydrophones using for passive acoustic recording, accelerometer sensors and sound meters are deployed to obtain structural vibrations and airborne noises, respectively. Acoustic signals in different sites and pools were recorded, and their characteristics are analyzed and compared to mainly understand the relationship between underwater acoustics and structural vibrations. In the future, more detail tests will design to find out what is the main noise source which may be influencing belugas’ living. This study is also helpful to apply appropriate and reasonable methods for noise reduction. [Work supported by National Natural Science Foundation of China (Grant No. 41374147).]

While machine learning has become increasingly popular as a means to learn information from large datasets, the question remains how different machine learning models can best be used to improve source ranging and environmental classification. In the current research, machine learning is used to predict the distance and depth of an impulsive source and the seabed type given a set of acoustic signals in a shallow-water ocean environment. Multiple machine learning models have been developed for this problem, including a support vector machine classifier. This support vector machine was trained on synthetic datasets of varying sizes and characteristics to predict a class corresponding to source-receiver range and seabed type. The trained model was able to classify synthetic acoustic signals with eighty-five to ninety percent average accuracy. The trained model is also applied to pressure time series signals recorded on the APL-UW Intensity Vector Autonomous Receiver system during SBCEX. Future work will continue to compare the efficiency and accuracy of support vector machines against other types of machine learning in underwater acoustics. [Work supported by the Office of Naval Research.]While machine learning has become increasingly popular as a means to learn information from large datasets, the question remains how different machine learning models can best be used to improve source ranging and environmental classification. In the current research, machine learning is used to predict the distance and depth of an impulsive source and the seabed type given a set of acoustic signals in a shallow-water ocean environment. Multiple machine learning models have been developed for this problem, including a support vector machine classifier. This support vector machine was trained on synthetic datasets of varying sizes and characteristics to predict a class corresponding to source-receiver range and seabed type. The trained model was able to classify synthetic acoustic signals with eighty-five to ninety percent average accuracy. The trained model is also applied to pressure time series signals recorded on the APL-UW Intensity Vector Autonomous Receiver system during SBCEX. Future work will cont...

In this invited talk, we report the experimental investigation of hybrid meta-gel, a class of designed hydrogel composites with tunable acoustic properties over broadband frequencies. The meta-gel consists of patterned channels in a tough hydrogel matrix, where air, water, or liquid metal can be purged through the channels to tune the meta-gel’s acoustic transmission over broadband frequencies on demand. We show that the acoustic properties of the meta-gel can be tuned by varying the volume ratio of the channels, properties and ratio of the different filler materials with combined experiments, theory, and simulations. We validated that the hybrid metamaterial significantly increased directivity and main lobe energy over a broad bandwidth both numerically and experimentally. The meta-gel enables novel functions such as adjustable imaging regions of ultrasound, demonstrating tangible applications in underwater acoustics and medical imaging.

Dr. Edmund J. Sullivan developed synthetic aperture towed-array processing. Confronted by many nay-sayers Dr. Sullivan persisted with his work and developed the first approach as the signal processing group leader (SPGL) at the SACLANT ASW Center (now CMRE) in La Spezia, Italy. The first notional passive synthetic aperture processor using an overlapped correlation method was jointly developed [“Extended Towed Array Processing by Overlapped Correlator,” (JASA, 1989)]. This idea was to blossom even further in his collaborative works (Stergiopolous et al.) extending these ideas to a fully, recursive (in-time) model-based passive-synthetic aperture processor [“Space-time array processing: A model-based approach” (JASA, 1997)]. He also began collaborations with the Swedish Navy where he performed joint experiments in model-based passive synthetic aperture evaluating its performance in the ocean and demonstrating its effectiveness. Dr. Sullivan began mentoring more researchers and teaching at the University of Rhode Island where he advised students in underwater acoustics and processing leading researchers to the model-based approach (Cousins), synthetic aperture processing (Edelson) [“On the performance of the overlap-correlator synthetic aperture technique” (JASA, 1991)] and broadband processing (Holmes) [“Broadband passive synthetic aperture” (JASA, 2006) ]. His ideas were summarized in his recent text, [Model-Based Processing for Underwater Acoustic Arrays (Springer, 2015)].Dr. Edmund J. Sullivan developed synthetic aperture towed-array processing. Confronted by many nay-sayers Dr. Sullivan persisted with his work and developed the first approach as the signal processing group leader (SPGL) at the SACLANT ASW Center (now CMRE) in La Spezia, Italy. The first notional passive synthetic aperture processor using an overlapped correlation method was jointly developed [“Extended Towed Array Processing by Overlapped Correlator,” (JASA, 1989)]. This idea was to blossom even further in his collaborative works (Stergiopolous et al.) extending these ideas to a fully, recursive (in-time) model-based passive-synthetic aperture processor [“Space-time array processing: A model-based approach” (JASA, 1997)]. He also began collaborations with the Swedish Navy where he performed joint experiments in model-based passive synthetic aperture evaluating its performance in the ocean and demonstrating its effectiveness. Dr. Sullivan began mentoring more researchers and teaching at the University of ...

Acoustic detection of unexploded ordnance (UXO) that contaminate the world’s waterways is vital. Critical to the implementation of detection methods is the numerical simulation of the scattering response of these targets using finite element methods. We introduce a parametric approach based on the time-domain spectral finite element method (SEM). This technique allows for the computation of broadband acoustic response of complex heterogeneous 3-D fluid-solid objects, in particular a 5 inch rocket UXO used for this study. The scattered field is obtained at arbitrary distances using the Kirchhoff–Helmholtz integral. The main interest of the SEM is that it is particularly adapted to high-performance computing (CPU or GPU). Contrary to most of the other finite element methods it scales perfectly; using twice as many processors leads to a halving of computing time. In addition, working in the time domain is a direct simulation of common sonars and experiments. The method is first benchmarked against the commercial finite-element code COMSOL on the monostatic response of a rigid target over a full 180 deg. Finally, results obtained for the 5 inch rocket are compared to actual measurements obtained in the NRL underwater acoustics tank facility. [Work partially supported by the Office of Naval Research.]

Recently developed low-power Chip-Scale Atomic Clocks (CSACs) hold promise for underwater acoustics applications because they enable time-coherent processing, critical for estimating the directionality of the sound field, when acoustic array elements cannot share a timing reference. Controlled, tank-based experiments with a small acoustic array (N = 4) featuring CSAC-equipped elements show that optimal disciplining is important for continued array coherence. Clock drift equivalent to a 10% wavelength error at 0.3, 1, and 10 kHz was reached at approximately 25, 10, and 3 days, respectively. Within application-specific limits, this technology brings enhanced capabilities to acoustic thermometry, geoacoustic, biological, and under-ice acoustic oceanography.

It is predicted that Arctic Ocean acidity will increase during the next century as a result of carbon dioxide accumulation in the atmosphere and migration into ocean waters. This change has implications for sound transmission because low-pH seawater absorbs less sound than high-pH water. Altered pH will affect sound in the 0.3-10 kHz range if the criterion is met that absorption is the primary cause of attenuation, rather than the alternatives of loss in the ice or seabed. Recent work has exploited sound that meets the criterion, sound trapped in a Beaufort Sea duct composed of Pacific Winter Water underlying Pacific Summer Water. Arctic pH is expected to drop from 8.1 to 7.9 (approximately) over the next 30-50 yr, and effects of this chemical alteration on the intensity levels of this ducted sound, and on noise, are examined here. Sound near 900 Hz is predicted to undergo the greatest change, traveling up to 38% further. At ranges of 100-300 km, sound levels from a source in the duct may increase by 7 dB or more. Noise would also increase, but noise is ducted less efficiently, with the result that 1 kHz noise is predicted to rise approximately 0.5 dB.

On the basis of the principle of stationary phase, Chuprov [Ocean Acoustics: Current State (Nauka, Moscow, 1982)] proposed a scalar parameter-the waveguide invariant β, to interpret the dispersive properties of underwater acoustic waveguide. It has been found that β may be useful in many applications in underwater acoustics. A reasonable prediction of the value of β is often necessary in such applications. It was known that β has some "canonical" values for waveguides with simple sound speed profiles (SSP). When a thermocline exists, β for refractive modes (with a turning point) is no longer constant and can even change its sign. In this paper, by dividing the SSP into the non-refractive part and the refractive part, a clear explanation of how the value of β is affected by the thermocline is presented. The results show that β can be positive and increase continually from around 1 to +∞; or negative from -∞ to about -3. An analytic criterion of the sign of β is developed. This method can also be used to analyze the value of β for any other kind of SSP.

The Silver Medal is presented to individuals, without age limitation, for contributions to the advancement of science, engineering, or human welfare through the application of acoustic principles, or through research accomplishment in acoustics.

On April 9 and 13, 1999, two Arctic Climate Observation Using Underwater Sound tomography signals were transmitted from a 20.5-Hz acoustic source moored at the Franz Victoria Strait to an eight-element, 525-m vertical array at ice camp APLIS in the Chukchi Sea at a distance of ∼2720 km. The transmitted signal was a 20-min long, 255-digit m-sequence that can be treated as a binary-phase shift-keying communication signal with a data rate of 2 bits/s. The almost error-free performance using either spatial diversity (three elements) for a single transmission or temporal diversity (two transmissions) with a single element demonstrates the feasibility of ice-covered trans-Arctic acoustic communications.

The Pioneers of Underwater Acoustics Medal is presented to an individual irrespective of nationality, age, or society affiliation, who has made an outstanding contribution to the science of underwater acoustics, as evidenced by publication of research in professional journals or by other accomplishments in the field. The award was named in honor of five pioneers in the field: H. J. W. Fay, R. A. Fessenden. H. C. Hayes, G. W. Pierce, and P. Langevin.

The article of record as published may be found at 10.1121/1.4996458 The reciprocity equation commonly stated in underwater acoustics relates pressure fields and monopole sources. It is often used to predict the pressure measured by a hydrophone for multiple source locations by placing a source at the hydrophone location and calculating the field everywhere for that source. A similar equation that governs the orthogonal components of the particle velocity field is needed to enable this computational method to be used for acoustic vector sensors. This paper derives a general reciprocity equation that accounts for both monopole and dipole sources. This vector-scalar reciprocity equation can be used to calculate individual components of the received vector field by altering the source type used in the propagation calculation. This enables a propagation model to calculate the received vector field components for an arbitrary number of source locations with a single model run for each vector field component instead of requiring one model run for each source location. Application of the vector-scalar reciprocity principle is demonstrated with analytic solutions for a range-independent environment and with numerical solutions for a range-dependent environment using a parabolic equation model.

This paper considers extrapolation of the vertical coherence of surface-generated oceanic ambient noise to simulate measurements made on a longer sensor array. The extrapolation method consists of projecting the noise coherence measured with a limited aperture array into the domain spanned by prolate spheroidal wave functions, which are an orthogonal basis defined by array parameters and the noise frequency. Using simulated data corresponding to selected multi-layered seabeds as ground truth, the performance of the extrapolation method is explored. Application of the technique is also demonstrated on experimental data.