bluuebunny/crossref_metadata_2025 huggingface dataset:
Design of inhomogeneous pentamode metamaterials for minimization of scattering
Authors: Cipolla Jeffrey , Gokhale Nachiket , Norris Andrew , Nagy Adam
Published: October 2011

Acoustic metamaterials use sub-wavelength, anisotropic, and inhomogeneous microstructures. Macroscopic properties can be related to the microstructure using homogenization theory Hassani and Hinton [Comput. Struct. 69, 719–738 (1998), which allows an analyst to confirm the extent to which a candidate metamaterial microstructure meets the requirements for a pentamode cloaking material. Norris [Proc. R. Soc. Ser. A, 464, 2411–2434 (2008)] presented a theory of transformation acoustics that enables the realization of inhomogeneous pentamode acoustic materials having anisotropic elastic tensors, isotropic density and finite mass. This theory describes the spatially varying material properties in terms of a mapping, which for separable geometries may be generated using a scalar function. This function, the constraints on its behavior implicit in the Norris theory, and the material equations constitute the defining relations for pentamode transformation acoustics. Previously, analytic work in transformation acoustics developed the material properties after having fixed a transformation. By reversing the process, we create a number of new families of pentamode cloaking materials. We validate the concept with three-dimensional explicit transient finite element simulations.
High frequency scattering properties of focused acoustic beams helpful for finite element research
Authors: Marston Philip L.
Published: October 2011

It can be helpful to understand apparent anomalies in the scattering when developing or testing finite element computations for the scattering of sound by objects in water with the wavenumber-radius product ka of approximately 10 or greater. In the case of focused acoustic beams incident on a sphere centered in the axis of the beam, analytical results are available using the method of superposition of Bessel beams [P. L. Marston, J. Acoust. Soc. Am. 129, 1773–1782 (2011)]. High ka analytical results can manifest an unusually large forward scattering lobe when the focal width of the beam becomes similar to the size of the sphere. (This is in contrast to the examples of scattering at relatively small ka illustrated in the aforementioned reference.) This new behavior can be understood by considering the definition of the total wave field as the superposition of the incident and the scattered fields combined with reasoning based on geometry, the localization principle, and other high frequency approximations. It is likely that high ka finite element computations of the scattering by narrowly focused beams for other shapes of objects will manifest a correspondingly enhanced scattering lobe. [Research supported by ONR.]
Case study: Multipurpose venue at Berklee College of Music
Authors: Mukherjee Raunak , Reuter Eric
Published: October 2011

This 90 m2 square meter multipurpose venue was intended to host a variety of programs, including musical performance, lecture, yoga, and dance instruction, etc. Most of these employ a built-in sound reinforcement systems. However, since its construction, the room has suffered from excessive reverberation, making it unsuitable for most of these intended uses, even with reinforcement. Venues of this type demand a balance between speech clarity and a level of reverberation sufficient to support musical performance. The authors will present a detailed acoustical analysis of the room at the start of the project, the goals of the design, and specific recommendations for acoustical treatment.
Active acoustics used in existing spaces for music practice
Authors: Freiheit Ron
Published: October 2011

For a number of years, modular practice rooms have offered the option of active acoustics technology as part of their integrated learning environment for music. This approach was effective, in part, because the interior acoustics of each room were well-defined with regard to reverberation time, background noise, and sound isolation. The desire to retrofit existing practice and music teaching spaces with active acoustics introduces numerous challenges. These include addressing environmental limitations (such as unencumbered physical locations for speakers and microphones) and the acoustic environment (issues of sound isolation, reverberation time, and background levels). This paper will explore how these challenges were addressed and how compromises were reached.
Temporal and spectral feature extraction from fish school using a broadband split-beam echosounder
Authors: Ito Masanori , Matsuo Ikuo , Imaizumi Tomohito , Akamatsu Tomonari , Wang Yong , Nishimori Yasushi
Published: October 2011

For sustainable fisheries, remote discrimination of fish species has been demanded recently. Features for fish discrimination should be extracted from schools. High spatial resolution of broadband split-beam echosounder system enabled tracking individual fish in a school. Temporal and spectral features such as waveform envelopes and target strength spectra were calculated individually. In addition, incident angles of sound were calculated by using positions and velocities of a fish, which were measured by the tracking process of the four channels split-beam system. Because the acoustical features from individual fish depended on the incident angles of sound, all feature parameters were sorted according to the angles to create temporal and spectral averaged pattern. In free ranging condition, echoes of Japanese jack mackerel (Trachurus japonicus), chub mackerel (Scomber japonicus), and red sea bream (Pagrus major) were measured. Results showed clear difference among temporal and spectral averaged patterns, which was consistent with test measurement obtained in an acoustic tank. Broadband split-beam system seemed to be appropriate to extract species specific feature in the ocean. [Work supported by the Research and Development Program for New Bio-industry Initiatives.]
Calculations of coherent backscatter enhancement from aggregations of underwater scatterers
Authors: Mookerjee Adaleena , Dowling David R.
Published: October 2011

Coherent backscattering enhancement (CBE) is a multiple scattering phenomenon that occurs in optics and acoustics. For plane wave illumination of an aggregation of randomly placed omni-directional scatterers, CBE may lead to as much as a doubling of the scattered field intensity in the direction opposite that of the incident wave. This presentation compares different calculation techniques for CBE for plane-wave harmonic incident sound fields scattered from N underwater bubbles. The first calculation technique is based on the classical field theory of Foldy (1945) for point scatterers and is equivalent to a boundary-element calculation that uses one element per scatterer and requires inversion of a fully populated N × N matrix. The second calculation is based on the finite element method and involves inversion of a larger but more sparsely populated matrix, but it allows for the possibility of greater scatterer complexity. Results from both approaches are compared and contrasted for idealized underwater remote sensing scenarios involving different incident-wave frequencies, scatterer cross sections, and ranges from the scatterer aggregation to the receiving array. The eventual goal of this effort is to understand and predict CBE from fish schools. [Work supported by the Office of Naval Research.]
Techniques in coastal seismic oceanography: An example in the Adriatic Sea
Authors: Wood Warren , Hobbs Richard , Book Jeffrey , Carniel Sandro
Published: October 2011

The rapidly developing field of seismic oceanography (SO) uses frequencies far lower (10–200 Hz) than tradition acoustic oceanography, and is not a measure of particulate scattering strength, but rather a direct, quantitative measure of vertical temperature gradient. The temperature gradient is typically a very weak signal (reflection coefficients of 0.001 or less) in the presence of higher amplitude coherent noise, such as the direct wave, and the reverberation of the ship noise (in shallow water). Towing the system faster than 4–5 kts to cover greater distances quickly increases streamer noise. Our objective to develop SO into a useful oceanographic tool is, therefore, a signal-to-noise problem, with mostly coherent noise. Using data acquired in the first coastal application of SO (ADRIASEISMIC), we show the magnitude of the noise sources and how they have been mitigated to result in quantitative (albeit band-limited) measures of temperature gradient from a few tens of meters below the sea surface to just meters above the seafloor. The profiles allow the detailed tracking of very small ocean features, among them warm thermohaline intrusions and dense, cold, bottom water masses, both in places only 10 m thick.
Acoustic imaging and structural acoustic analysis of laboratory measurements of scattering from buried targets above critical grazing angles
Authors: Simpson Harry J. , Waters Zachary J. , Houston Brian H. , Jig Kyrie K. , Volk Roger R. , Yoder Timothy J. , Bucaro Joseph A.
Published: October 2011

A broadband (3-40 kHz) compact range measurement technique has been developed to obtain the acoustic scattering from buried unexploded ordnance and objects simulating natural and man-made clutter. The targets—two 5 in. rockets with 0, 30, and 60 deg pitch angles, a large rock, and cinder blocks with 0 and 45 deg roll—are buried 10 cm beneath the surface of a water-saturated sandy bottom with a mean grain size of 240 μm. A 2D synthetic array is generated at a height of 20 cm above the sediment–water interface with an element spacing of 3 cm (25 kHz Nyquist). Waveforms collected on the synthetic array are processed to extract the structural acoustic response of the buried targets. A Relevance Vector Machine algorithm applied to the scattered data for target identification, which shows that the target features separate even as the receiver array size is considerably decreased. Similar results are presented for numerical simulations of the bistatic returns for the buried 5 in. rocket and a rock of comparable size. [Work supported by ONR and SERDP.]
Buried object imaging using reflection tomography
Authors: Turgut Altan
Published: October 2011

A recent pioneering study of Steven Schock provided high-quality imaging of buried objects using the buried object sensing sonar (BOSS) system in a towed or AUV-mounted configuration [Schock, Proc. IEEE Oceans, 2005]. This paper numerically investigates the performance of compressive sensing (CS) and standard backprojection methods for imaging multiple buried objects using a sonar system similar to the BOSS. Synthetic data are generated using a 3-D pseudo-spectral model for different sediment types and geometries including synthetic aperture sonar. As compared to the standard backprojection methods, the CS-based reflection tomography provided sharper images of buried objects using smaller data sets. Based on the CS performance, several improvements on the BOSS system and data collection schemes are also presented. [Work supported by ONR.]
Explicit modeling of cubic stiffness in large amplitude dynamic response of metal plate under impingement of solid rocket motor plume
Authors: Vold Hvard , Blelloch Paul , Hutchings Allison , Yoder Nate
Published: October 2011

Determining the damping characteristics of launch vehicle structures has risen to a high priority issue, since dynamic response in modeling and simulations is directly related to assumed damping. An experiment was conducted wherein an instrumented flat plate was excited by hydrodynamic fringes of the plume of a solid rocket motor. Estimation of damping was performed by variations of maximum entropy parametric spectrum estimation. Equivalent damping ratios of 5% were observed, even though the damping was observed to be amplitude dependent, with lower damping ratios associated with lower amplitudes. Analytic simulations, with fixed damping ratios and large displacement and rotation geometric effects, showed similar characteristics to the measured test responses. The hypothesis was formed that the structure is dominated by cubic stiffness effects under the observed impingement conditions, and that the damping actually remains constant, effectively limiting the dynamic response and giving the appearance of an increased damping ratio. Since one can estimate velocity and displacement time histories by filtering measured acceleration time histories, a time domain maximum entropy model can be formulated. The cubic stiffness terms are linear parameters and all nonlinear terms being computed from the measured kinematic quantities. The estimated results will be compared with analytic simulations.
Experiences in performing a high-intensity, direct-field acoustic test on a contamination sensitive system
Authors: Stasiunas Eric C. , Babuska Vit , Skousen Troy J. , Gurule David J.
Published: October 2011

A direct-field acoustic test (DFAT) was performed on a Sandia system in order to verify survival due to an acoustic environment of 146.7 dB OASPL. The DFAT technique' performed by surrounding a test article with a wall of speakers and controlling the acoustic input with a closed-loop control system' was chosen as the test method in order to meet a critical schedule. In choosing this test method, other challenges became apparent, such as how to obtain the high-intensity acoustic levels and what occurs to that environment inside the bagged frame constructed to maintain a contamination-free system. In addition, the vast amounts of data measured during a single test necessitated a way for the test director to quickly visualize the acoustic environment, saving time and provide insight for input adjustments if necessary. Finally, even though the specified acoustic environment was successfully obtained, the results illustrated some drawbacks of the current DFAT method. This paper will detail the DFAT setup used to obtain the test specification, the effects of the contamination frame on the acoustic environment, the quick-look data program created for visual analysis of the acoustic field, and ideas for performing more diffuse DFAT tests in the future.
Angular dependence of high-frequency seafloor acoustic backscatter (200–400 kHz)
Authors: de Moustier Christian , Wendelboe Gorm , Maillard Eric , Kraft Barbara J.
Published: October 2011

Acoustic backscatter measurements were made with a RESON 7125-V2 multifrequency, multibeam sonar at 50 kHz increments between 150 and 450 kHz. The sonar was hull-mounted on a vessel held in a four-point moor in 17 m of water depth. The resulting constrained ship motion provided thousands of independent samples of the angular dependence of seafloor acoustic backscatter for grazing angles ranging from 90 deg to about 25 deg, over a well defined seafloor patch. Focused beamforming at all frequencies yielded fore-aft beam footprints with nearly constant width across the swath. Calibrated results are presented at 200 and 400 kHz, and relative results at all other frequencies. [Work funded by ONR-Ocean Acoustics (Code 32).]
Highly dynamic reverb—The recording studio always has what the performance hall sometimes wants
Authors: Case Alex
Published: October 2011

The orchestra wishes to perform in a space whose acoustics offers flattering reverberation. That reverb is generally unchanging, from the beginning to the end of the performance. And that reverb must be “one size fits all,” as every instrument in the band is offered the same acoustic treatment. Not so in recorded music. When reverb is generated acoustically, mechanically, and digitally and introduced into the music by way of a mixing console or digital audio workstation, under the watchful ear of the creative recording engineer, highly variable reverb becomes possible. Momentary reverb, dynamic reverb, multiple contrasting reverbs, and still more variations not easily realized in a physical space are found common in popular music recordings, demonstrated and discussed here.
Bearing estimation and 2D localization of bowhead whale calls using directional autonomous seafloor acoustic recorders
Authors: D'Spain Gerald L. , Batchelor Heidi A. , Freeman Simon E. , Kim Katherine H. , Greene, Charles R. , Blackwell Susanna B. , Macrander A. Michael
Published: October 2011

Passive acoustic monitoring of the summer/fall westerly bowhead whale migration in the Beaufort Sea has been conducted by Greeneridge Sciences, sponsored by SEPCO, every year since 2006. The directional autonomous seafloor acoustic recorder (DASAR) packages used in this effort each contain three acoustic sensors that simultaneously measure the two horizontal components of acoustic particle motion and acoustic pressure. A variety of data-adaptive beamforming methods have been applied to a selected subset of these data to examine direction-of-arrival estimation performance, including quantifying bearing bias and variance. A principal component analysis (PCA)-type eigenanalysis of the sensor data cross spectral matrix is used to decompose the received field into orthogonal components having different particle motion polarization and energy flux properties. Appropriate manipulation of these components provides high resolution directional estimates of the primary arriving energy while maintaining robustness to uncertainties in sensor calibration and acoustic propagation conditions. Representative results of beamforming as well as 2D localization performance will be presented. Application of data-adaptive beamforming techniques to bulk processing of these large monitoring data sets will be discussed. [Work supported by Shell Exploration and Production Company (SEPCO).]
Bat recording under controlled conditions: A replicable chamber for comparable results
Authors: Romero Vivas Eduardo , Cortés Calva Patricia , León López Braulio
Published: October 2011

In ecology, bats have become a major subject of study. Some pollinate and disperse the seeds of many tropical plants; some help to control insect populations; a few affect livestock by sucking blood, but in all cases bats are indispensable links in ecosystems. Bats use of sound waves make echolocation calls monitoring a powerful tool for distribution, census, and present studies. This technique depends on having a reliable database of sounds for species identification purposes. Although there are several databases of echolocation sounds; the signal emitted by a bat depends on environmental conditions (vegetation, weather conditions), biotic factors (prey size, movement, defensive measures), and the specific task (seek, flee, pursue, evade, wandering, obstacle avoiding). This variability makes comparisons among databases difficult. A portable chamber made of common materials for in situ recording under controlled conditions is presented. The use of the chamber is proposed as a tool which might help to generate databases that could be reliable compared. Nine species of desert bats from the northwest region of Mexico were recorder using the chamber under controlled conditions and compared with field recordings. The performance of the chamber and the utility of the database generated for filed identification are presented.
Computationally efficient finite element method for predicting wave propagation in periodic structures
Authors: Cotoni Vincent , Shorter Phil , Cordioli Julio
Published: October 2011

A generic numerical method for predicting the wave propagation in structures with two-dimensional periodicity is presented. The method is based on a combination of Finite Elements and periodic structure theory. A unit cell of the periodic structure is described with finite elements, and a Craig-Bampton reduction is applied to reduce the number of degrees of freedom. Periodic boundary conditions are then applied and the waves propagating in the structure are obtained by solving an algebraic eigenvalue problem. A number of analytical expressions are then used to derive the vibro-acoustic properties of the finite or infinite periodic structure. The method was recently extended to account for heavy fluid loading and material with frequency-dependent properties (typical of acoustic treatments). A number of examples are presented to validate the formulation and demonstrate the possible use of the method for design.
Examination of valued attributes of low sound level environments
Authors: Horonjeff Richard D. , Singleton Herb J.
Published: October 2011

The introduction of new noise sources into rural environments presents a number of unique community response challenges not typically encountered in urban and suburban environments. Low ambient sound levels combined with intermittently (rather than continuously) audible anthropogenic ambient contributors create unique soundscapes. Residents appear to value many attributes of these soundscapes and wish to preserve and protect them. Application of existing standards and guidelines (quantitative or nuisance-based) often does not meet resident expectations of protection. This paper examines a number of the valued attributes. It then asks questions about noise policy, resident expectations, adequacy of existing standards, source and ambient noise quantification, and the masking of valued attributes by a new noise source. It concludes with a list of issues that bear close examination in order to set future noise impact guidelines for low-ambient environments.
Case study: A quiet rural community in a commercial/industrial zone
Authors: Reuter Eric L.
Published: October 2011

A small New Hampshire town allowed a community to be built in a commercial zone, immediately adjacent to an industrial zone that is home to a long-dormant quarry. When the quarry owner applied for a permit to resume quarrying operations, the nearby residents objected on the grounds of noise impacts and other environmental concerns. A failure to consider noise as an element of community planning put the town in a difficult position. Should residents living in an commercial zone be afforded the same privilege of living in a quiet environment than those in residential zones? Should the industry be held to more stringent standards because of this non-conforming use? Both sides of this issue will be discussed.
Energy-based method for near real-time modeling of sound field in complex urban environments
Authors: Pasareanu Stephanie M. , Remillieux Marcel C. , Burdisso Ricardo A.
Published: October 2011

Prediction of the sound field in large urban environments has been limited thus far by the computational heaviness of standard numerical methods such as boundary element (BE) or finite-difference time-domain methods. Recently, a considerable amount of work has been devoted to developing energy-based methods for this application, and results have shown the potential to compete with conventional methods. However, these developments have been limited to 2-D models, and no real description of the phenomena at issue has been exposed. Here, the mathematical theory of diffusion is used to predict the sound field in complete 3-D complex urban environments. A 3-D diffusion equation is implemented by means of a simple finite-difference scheme and applied to two different types of urban configurations. This modeling approach is validated against BE and geometrical acoustic solutions, showing a good overall agreement. The role played by diffraction near the source is discussed, and suggestions are made on the possibility to accurately predict the sound field in complex urban environments in near real time simulations.
Acoustic velocity measurements on solids and liquids under extreme conditions of pressure and temperature
Authors: Li Baosheng , Liu Wei
Published: October 2011

Elastic properties of materials under extreme conditions of pressure and temperature are of great interests to researchers in many disciplines. In the last decade, acoustic velocity measurements using an improved ultrasonic interferometry method have been developed in both MA-6 and MA-8 types of multi-anvil high pressure apparatus. By placing the piezoelectric transducer (lithium niobate, 10 deg Y-cut) in a stress-free location and using extended delayline, high S/N acoustic signal can be maintained, while sample is under high pressure andtemperature. By combining with synchrotron X-radiation, measurements of sound velocities using ultrasonic interferometry, crystalstructure and unit cell parameters using X-ray diffraction, and sample strain (length) using X-radiographic imaging, can be made simultaneously, all in-situ at a high pressure and temperature, enabling a pressure-standard-free characterization of solid and liquid materials to 25 GPa and 1800 K. Results on ceramic and metallic materials from recent experiments will be presented to show velocities as a function of pressure and temperature, absolute pressure determination, equation of state for glass, and the application to liquids. Other new developments, such as controlling sample stress state at high P and T, the study of composites, and materials undergoing phase transformations will also be reviewed [Work sponsored by NSF and DOE/NNSA.]
Weakly nonlinear simple waves in Hertzian chains
Authors: McDonald B. Edward , Calvo David
Published: October 2011

The discrete system of equations for a granular chain consisting of a large number of spheres interacting via the hertz force is cast as an effective medium. In the long wavelength limit, the second order equation of motion for the effective medium possess a subset of simple waves obeying a first order equation of reduced nonlinear index. Simple waves are those in which knowledge of one dependent variable determines all the rest. For a given initial strain, the simple wave solution prescribes initial mass velocity. Strain and velocity profiles from the first order equation are used as initial conditions in simulations for the second order discrete system. Results for viscous and inviscid shock formation compare very well between the second order system and the reduced first order equation. Second order simulation of colliding waves reveals the ability of waves to pass through each other, with a phase advance accruing during the collision process. Results may be related to explosions in granular media. [Work supported by the Office of Naval Research.]
Cylindrical bubble dynamics
Authors: Ilinskii Yurii A. , Hay Todd A. , Zabolotskaya Evgenia A. , Hamilton Mark F.
Published: October 2011

Bubbles generated between closely spaced parallel surfaces are approximately cylindrical. Here, cylindrical bubble dynamics are compared with those of spherical bubbles. In the linear approximation, the product of bubble radius and natural frequency of the monopole mode in water is ∼1m/s for a cylindrical bubble versus ∼3m/s for a spherical bubble. Radiation damping of a cylindrical bubble is an order of magnitude greater than for a spherical bubble, and for cylindrical bubbles larger than ∼1 μm it is the dominant loss mechanism, whereas spherical bubbles must be larger than ∼1 mm before radiation losses dominate. Also in contrast with spherical bubbles, the inertial load on a cylindrical bubble in an unbounded incompressible liquid is infinite and prohibits the existence of a monopole mode. Derivation of a dynamical equation for cylindrical bubbles in Rayleigh-Plesset form therefore requires the incompressible liquid to be of finite extent. The radial extent needed to impose the appropriate inertial load is found by comparison with results for a compressible liquid to be approximately 200 bubble radii. The resulting dynamical equation was further augmented to account for bubble deformation due to coupling of surface modes through nonlinearity. [Work supported by NIH DK070618 and EB011603.]
Acoustic oscillations in the early universe
Authors: Knobles David
Published: October 2011

Acoustic oscillations in the baryonic-photon plasma leave an imprint on both the cosmic microwave background radiation (CMBR) and the power spectrum of the mass distribution in the universe. This effect is well known and is currently applied to determine the value for quantities such as the percentage of dark matter and dark energy in the universe. This study examines the contributions of higher order terms in the equations for the radiation-baryonic acoustic oscillations prior to radiation decoupling from matter. The energy and momentum conservation equations are combined with the Einstein field equations for a Robertson Walker metric containing the perturbations. An attempt is made to consider the effect of the higher order perturbations on the observed power spectrum and on the anisotropy of the CMBR. The sensitivity of these higher order terms to adiabatic, instead of non-adiabatic, boundary conditions on the fluctuations outside the particle horizon is considered. [Work supported by KSA.]
Extracting the earth response from noise and complex earthquake data
Authors: Snieder Roel , Nakata Nori , Wapenaar Kees , Slob Evert
Published: October 2011

Theory shows that when random noise is generated in proportion to the local dissipation rate, one can extract the response of a system from correlations of field measurements of such noise. Such a condition of equilibrium is rarely satisfied for elastic waves of the earth. Yet the earth response can be extracted from measured noise. We show examples of the retrieval of P-waves and of S-waves. The latter type of waves are retrieved from traffic noise in an urban environment. Since the traffic noise is excited by localized sources (road and railroads), it displays strong amplitude variations. In order to compensate for such amplitude variations, we use cross-coherence rather than cross-correlations for the data processing. We also analyze complicated waveforms excited by earthquakes to create maps of the shear-wave velocity in Japan and show that the shear wave velocity changes with time; this velocity drops throughout northeastern Japan with about 5% after the recent Tohoku-Oki earthquake. Our measurements show that the shallow subsurface in Japan weakens after the earthquake over a region about 1 200 km wide.
Using cross-correlations of ambient vibrations for passive structural health monitoring of a high-speed naval ship
Authors: Sabra Karim G.
Published: October 2011

Previous studies have used the cross-correlation of ambient vibrations (CAVs) technique to estimate the impulse response (or Green's function) between passive sensors for passive imaging purposes in various engineering applications. The technique (CAV) relies on extracting deterministic coherent time signatures from the noise cross-correlation function computed between passive sensors, without the use of controlled active sources. Provided that the ambient structure-borne noise field remains stable, these resulting coherent waveforms obtained from CAV can then be used for structural monitoring even if they differ from the actual impulse response between the passive sensors. This article presents experimental CAV results using low-frequency random vibration data (>50 Hz) collected on an all-aluminum naval vessel (the HSV-2 Swift) operating at high speed (up to 40 kn) during high sea states. The primary excitation sources were strong wave impact loadings and rotating machinery vibrations. The consistency of the CAV results is established by extracting similar coherent arrivals from ambient vibrations between the pairs of strain gages, symmetrically located across the ship's centerline. The influence of the ship's operating conditions on the stability of the peak coherent arrival time, during the 7 days trial, is also discussed. [Sponsored by ONR, N00014-09-1-0440.]
Extracting information from an array of sensors in a diffuse noise field using random matrix theory
Authors: Menon Ravi , Gerstoft Peter , Hodgkiss William
Published: October 2011

Isotropic noise fields are often used to model several practical diffuse noise fields. For an array of equidistant sensors in such a noise field, the cross-spectral density matrix (CSDM) of the array is a Toeplitz sinc matrix. Here, the eigenvalues of the CSDM for ideal isotropic noise fields are first derived for infinite arrays. The eigenvalues have close connections with classical array processing concepts such as the invisible region in frequency-wavenumber space (region where there is no propagating energy, but a spectrum can be calculated). Random matrix theory deals with eigenvalue distributions of random matrices and its concepts are applied here by modeling the array snapshot vectors as zero-mean, unit variance Gaussian random variables, with a sinc covariance matrix. Using the Stieltjes transform, the eigenvalues of the ideal CSDM are related to those of the sample CSDM, and an analytical solution for the distribution of the eigenvalues of the sample CSDM is obtained. At frequencies where the array is spatially undersampled, increasing the number of observations results in the noise masquerading as a signal, which could lead to erroneous signal detections. We demonstrate how knowing and understanding the eigenvalue distribution helps improve the extraction of information from ocean ambient noise.
A bottom-sediments province database and derived products, Naval Oceanographic Office
Authors: Fleischer Peter , Becker William M. , Baas Peter D.
Published: October 2011

The Naval Oceanographic Office maintains a bottom-sediment province database at three levels of resolution: (1) a low-resolution legacy data set derived from secondary sources for the low- and mid-latitude ocean basins; (2) medium-resolution, actively maintained ``Regional Sediments'' data sets covering most of the continental margins of Eurasia and North America; and (3) high-resolution, limited-extent data sets derived from acoustic imagery. Sediment provinces are categorized via an ``Enhanced'' sediment classification consisting of seven locational, ten compositional, and 97 textural components. The resultant large number of categories allows for retention of source-data nomenclature but can be unwieldy and redundant. To provide accessibility and consistency to a variety of users, the Enhanced categories are reclassified into simplified or specialized sediment category sets. These include the High Frequency Environmental Acoustics categories for performance prediction, a mean grain-size set, a non-technical littoral set, as well as other bottom characteristics such as burial potential and bottom type. The Enhanced categories have also been assigned a limited number of geoacoustic and physical properties. Although the province approach and the reclassification schemes produce useful inputs for models and other predictions, users must recognize that they are subject to inherent limitations and ambiguities.
Direct measurements of sediment sound speed at mid- to high-frequency in a sand sediment
Authors: Yang Jie , Hefner Brian T. , Tang Dajun
Published: October 2011

Direct measurements of sediment sound speed were made near Panama City, Florida in April 2011. Considerable effort is being made to provide detailed environmental characterization for an upcoming mid-frequency sound propagation and reverberation experiment in 2013 and the measurements presented here are part of that effort. Two direct measurement systems are shown: one is called the Sediment Acoustic-speed Measurement System (SAMS) and the other is the attenuation array which was deployed in the Sediment Acoustics Experiment 2004. SAMS consists of ten fixed sources and one receiver. The receiver is driven into the seabed by a motor, which allows precise penetration depth up to 3 m with arbitrary step size. Measurements were made in the frequency range of 1 – 50 kHz. The attenuation array consists of two transmitters and two receivers mounted on a diver-deployable frame and can provide surficial sediment sound speed and attenuation to a depth of about 10 cm between 40 and 260 kHz. Sediment sound speeds obtained using the two systems can be compared in the overlapping frequency region for consistency. Initial analysis of sediment sound speed will be shown. [Work supported by ONR.]
RPG Diffusor Systems: Overview of nearly 30 years of research and development
Authors: D'Antonio Peter
Published: October 2011

Each company is defined by its mission. RPG Diffusor System mission is to provide continuing innovation and expands the acoustical palette through a commitment to pioneering fundamental acoustics research. Rather than focus on any particular products, this presentation will provide an overview of nearly 30 years of research and development of a unique range of absorptive, reflective, and diffusive acoustic tools. With respect to absorption, the presentation will describe the progression from traditional fabric covered porous absorbers, to binary amplitude diffsorbers (attenuating the unneeded high frequency absorption), to absorptive wood Helmholtz absorbers, to transparent microperforated and microslit absorbers, to absorptive/diffusive CMU, membrane and plate resonator absorbers. With respect to reflection, we will describe the advancement from flat reflectors, to combined reflectors, to one and two-dimensional curved shapes in wood and glass reinforced gypsum. Finally, the evolution of diffusive surfaces from the venerable QRD in the early 1980s to modulated optimized diffusors, which provide unlimited bandwidth and freedom from grating lobes and flat plate limitations, to shape-optimized wood and glass reinforced gypsum spline, bicubic and other curvilinear architectural shapes. As part of the historical development, theoretical explanations, proof of performance metrics, and installation photos of completed projects will be provided.
Modeling treatments to reduce sound transmission through an open window into a room: Effect of window thickness
Authors: Sieck Caleb F. , Lau Siu-Kit
Published: October 2011

Sound transmission loss through an exterior wall is limited by its weakest structure, generally a window, especially if it is open. Considering the acoustic modes within a window and a room are important because much of the acoustic energy from noise sources such as traffic and large wind turbines is in the low to middle frequency range. Previous models of open windows have either neglected the thickness of the window or the influence of room modes on the sound transmission. The present investigation considered a baffled rectangular aperture of finite thickness backed by a rigid walled cavity. An impedance/mobility approach was used to study the effect of the thickness of an open window on the insertion loss and sound pressure levels inside the cavity. The insertion loss study was confirmed using FEM modeling, and the difference in sound pressure levels was compared to experimental results. Increasing window thickness decreases the amount of sound transmitted at frequencies below the second order modes of the cavity for both window sizes under investigation. Using the impedance/mobility approach was effective in this study and allows the model to be easily extended.
Bayesian inversion of seabed interface-wave dispersion from ambient noise
Authors: Li Cuilin , Dosso Stan E. , Dong Hefeng , Liu Lanbo , Yu Dingyong
Published: October 2011

This paper applies Bayesian inversion to estimate seabed shear-wave speed profiles and their uncertainties using interface-wave dispersion curves extracted from ocean ambient noise, and compares the resolution of seabed structure for fundamental-mode and multi-mode data. Ambient-noise recordings were collected for 2.15 h at hydrophones of an entrenched ocean-bottom cable in the North Sea. Scholte-wave dispersion curves for the fundamental mode and several higher-order modes within the frequency range 0.26–3.8 Hz are extracted from cross-correlations of noise recordings at sensor pairs via slowness-frequency transform. The Bayesian information criterion is used to determine the preferred model parameterizations in terms of the number of sediment layers supported by the data for inversions based on the fundamental mode alone and on the first three modes. Adaptive-hybrid optimization and Metropolis-Hastings sampling are applied to estimate the most-probable shear-wave speed models and to compute marginal posterior probability profiles. The results show quantitatively that multi-mode inversion provides higher-resolution of shear-wave speed structure at shallow depths and smaller uncertainties at all depths than inversion of the fundamental mode alone. [We thank StatOil for providing data.]
Coherent averaging of the passive fathometer response using short correlation time
Authors: Traer James , Gerstoft Peter
Published: October 2011

The passive fathometer algorithm was applied to data from two drifting array experiments in the Mediterranean, Boundary 2003 and 2004. The passive fathometer response was computed with correlation times from 0.34 to 90 s and, for correlation times less than a few seconds, the observed signal-to-noise ratio (SNR) agrees with a 1 D model of SNR of the passive fathometer response in an ideal waveguide. In the 2004 experiment, the fathometer response showed the array depth varied periodically with an amplitude of 1 m and a period of 7 s consistent with wave driven motion of the array. This introduced a destructive interference, which prevents the SNR growing with increasing correlation time. A peak-tracking algorithm applied to the fathometer response of experimental data was used to remove this motion allowing the coherent passive fathometer response to be averaged over several minutes without destructive interference. Multirate adaptive beamforming, using 90 s correlation time to form adaptive steer vectors which were applied to 0.34 s data snapshots, increases the SNR of the passive fathometer response.
Performance bounds on geoacoustic inversions
Authors: Baggeroer Arthur B.
Published: October 2011

Lower bounds on the mean square error for parameter estimates using geoinversion methods are developed in terms of the tomographic version of the Cramer Rao bound. Two approaches are considered: (i) an active, coherent source with known waveform and time synchronization as in ocean acoustic tomography (OAT) and (ii) a passive, broadband source with known spectrum but with no time synchronization as in matched field tomography (MFT). Both approaches assume the receiver to be an array, e.g., vertical line array or towed horizontal line array (HLA). The formulation requires specifying the Green's functions connecting source and the sensors of the receiving array. The bounds demonstrate (i) OAT performance depends upon the mean square group speed spread of modes and (ii) MFT performance depends upon the phase speed spread where both are integrated across the band of the respective sources. The bound also indicates the coupling among parameter estimates. Uncorrelated estimates are usually desirable for efficient parameterization formulations. An example using a Pekeris waveguide with three unknown parameters: the ocean and bottom sound speeds and the depth. The extension to Bayesian problems where a joint a priori probability is given. This permits the evaluation enabled by the observed data.
Stabilizing reverberation inversion by regression relations involving a grain size parameter
Authors: Abrahamsson L. , Andersson B. L. , Ivansson S. , Pihl J. , Ainslie M. , Benders F. , Colin M. , van Vossen R. , Chalindar B. , Cristol X. , Juhel B. , Dybedal J. , Olsen G. K. , Eidem E.
Published: October 2011

Within the European Defence Agency (EDA) project Rumble-2, an operational low-frequency active sonar system has been used to collect reverberation data at several sea trials in the North Sea. A global optimization method is used to determine the bottom parameters that provide the best match between measured and modeled time traces. A fast ray model is used for the forward computations. The bottom parameters are the Lambert back-scattering parameter and the sound speed c, density ρ, absorption α, and thickness of the sediment. The reverberation data do not constrain all these parameters to unique values, however, and different approaches have been tried in the project to reduce the ambiguity problems. The approach reported here is to use the mean grain size Mz as a common descriptive parameter. From regression relations by Hamilton and Bachman, c, ρ, and α can be set as functions of Mz. More ambitiously, the regression relations could be applied as a priori constraints, with uncertainties, in a Bayesian framework. The obtained inversion results are consistent with ground truth for the grain size, as measured from bottom samples. Moreover, similar results are obtained for trials in the same area with quite different environmental conditions.
Geoacoustic inversion in shallow water using broadband synthetic aperture and single hydrophone acoustic data
Authors: Tan Bien Aik , Gerstoft Peter , Yardim Caglar , Hodgkiss William
Published: October 2011

A majority of geoacoustic inversion experiments sample the acoustic field on long arrays. This paper uses a moving single hydrophone to create a large synthetic aperture array for geoacoustic inversion. Practically, this is operationally attractive compared to using long arrays. For example, one possible application is the use of autonomous underwater vehicles (AUVs) to perform acoustic field sampling and pre-processing for geoacoustic inversion. The approach comprises synthetic aperture formation and a directed Monte Carlo Bayesian broadband frequency coherent geoacoustic inversion. This is demonstrated with simulated and real data from the MAPEX2000 experiment conducted by the NATO Undersea Research Center, using only one hydrophone of a towed array and a moored source in the Mediterranean Sea. The method yielded similar results compared to an equivalent physical array.
Interferometry for three-dimensional acoustics in shallow water
Authors: Badiey Mohsen , Katsnelson Boris G. , Lynch James F.
Published: October 2011

Interferometry in optics has been known for decades. It requires identification of specific refracted rays via some reflecting front and existence of a mechanism for interference between rays to occur (i.e., constructive and destructive amplitude and phase information). In underwater acoustics, the principal phenomenon is the same. However, identification of the refracted (or reflected) rays requires a well defined geometry between the acoustic source and receiver path and the reflecting (or refracting) front. The interference between the direct and reflected wave fronts from sea surface has been shown for some time. Recent experimental observations have reported the identification of the interference in the horizontal plane [J. Acoust. Soc. Am. 129(4), EL141 (2011)]. Theoretical description of the horizontal interference phenomenon as well as the follow up work for potential use of interferometry techniques for three dimensional acoustic wave propagation in shallow water is presented. Theoretical and experimental results are shown. [Work supported by ONR.]
Automatic detection of the number of raypaths in colored noise using short-length samples
Authors: Jiang Longyu , Mars Jérôme
Published: October 2011

In ocean acoustic tomography (OAT) (especially in shallow water where raypaths are mixed), knowledge of the number of raypath is crucial for inversion algorithm. In this paper, a noise-whitening exponential fitting test (NWEFT) is presented in this context for detecting the number of raypaths. Classically, two suggested approaches are the Akaike information criterion (AIC) and the minimum description length (MDL). Based on ideal assumption of ergodic Gaussian random processes and white Gaussian noise, MDL is shown to be asymptotically consistent, whereas the AIC tends to overestimate the order of model. However, these assumptions could not be fulfilled in practical case of OAT. In order to be adapted for real case of OAT, noise-whitening processing is applied as first step. Then, NWEFT bases on the fact that the profile of the ordered eigenvalues fits an exponential law for short-length samples of white Gaussian noise. The number of raypaths could be detected when a mismatch occurs between observed profile and exponential model. The fact that NWFET works on short-length samples is very important as a long duration of the received signal in OAT is unavailable. Its performance is studied with synthetic and real data set and compared with classical algorithms.