Constant Sound Velocity Research Articles

Fast Ray-Tracing-Based Precise Localization for Internet of Underwater Things without Prior Acknowledgment of Target Depth

Underwater localization is one of the key techniques for positioning, navigation, timing (PNT) services that could be widely applied in disaster warning, underwater rescues and resource exploration. One of the reasons why it is difficult to achieve accurate positioning for underwater targets is due to the influence of uneven distribution of underwater sound velocity. The current sound-line correction positioning method mainly aims at scenarios with known target depth. However, for nodes that are non-cooperative nodes or lack depth information, sound-line tracking strategies cannot work well due to non-unique positional solutions. To solve this problem, we propose an iterative ray tracing 3D underwater localization (IRTUL) method for stratification compensation. To demonstrate the feasibility of fast stratification compensation, we first derive the signal path as a function of initial |grazing angle, and then prove that the signal propagation time and horizontal propagation distance are monotonic functions of the initial grazing angle, which guarantees the fast achievement of ray tracing. Simulation results indicate that IRTUL has the most significant correction effect in the depth direction, and the average accuracy has been improved by about 3 m compared to a localization model with constant sound velocity.

Exotic Cores with and without Dark-Matter Admixtures in Compact Stars

We parameterize the core of compact spherical star configurations by a mass (mx) and a radius (rx) and study the resulting admissible areas in the total-mass–total-radius plane. The employed fiducial equation-of-state models of the corona at radii r>rx and pressures p≤px with p(r=rx)=px are that of constant sound velocity and a proxy of DYΔ DD-ME2 provided by Buchdahl’s exactly solvable ansatz. The core (r<rx) may contain any type of material, e.g., Standard-Model matter with unspecified equation of state or/and an unspecified Dark-Matter admixture. Employing a toy model for the cool equation of state with first-order phase transition, we also discuss the mass-radius relation of compact stars with an admixture of Dark Matter in a Mirror-World scenario.

Processing of aerogels and their applications toward CO2 adsorption and electrochemical reduction: a review.

Aerogels are a unique class of nanoporous ultralight materials exhibiting wide range of textural characteristic properties and tunable porosities. Due to their remarkable features such as low density, high surface area, low refractive index, small thermal conductivity, low dielectric constant and low sound velocity, they exhibit a wide range of applications in different areas such as electronics, thermal and acoustic insulation, chemistry, biomedicine and optics. The special advantages of these materials are that they can be produced in different forms such as monoliths/granular, bead/microspheres, thin films or sheets and as blankets. Aerogels are found to be potential materials for the removal of CO2 through adsorption or electrochemical reduction. There is a plethora of research on different kinds of aerogels used for CO2 adsorption process. Research has been going on toward the development of aerogel-based electrocatalyst, which can be used for valorization of CO2 through electrochemical reduction methods. Although most of the review papers have covered applications of aerogels in CO2 capture, very few discuss the processing of aerogels, more so on their applications in CO2 valorization. In this review, we have collated literature of different forms of aerogels currently available and the steps involved in their fabrication process. In addition, we have covered applications of aerogels in CO2 capture. Furthermore, we focussed on the basic principles involved in the development of an aerogel electrocatalyst as well as recent developments of aerogels in electrochemical CO2 reduction.

A Chebyshev Spectral Method for Normal Mode and Parabolic Equation Models in Underwater Acoustics

In this paper, the Chebyshev spectral method is used to solve the normal mode and parabolic equation models of underwater acoustic propagation, and the results of the Chebyshev spectral method and the traditional finite difference method are compared for an ideal fluid waveguide with a constant sound velocity and an ideal fluid waveguide with a deep-sea Munk speed profile. The research shows that, compared with the finite difference method, the Chebyshev spectral method has the advantages of a high computational accuracy and short computational time in underwater acoustic propagation.

Effect of Ferroelectric Nanopowder on Electrical and Acoustical Properties of Cholesteric Liquid Crysta

Ferroelectric nano-materials are very sensitive to several external stimuli and have attracted great deal of attention due to their property of improving various properties such as photoluminescence, higher polarization, fast response time, low operating voltage and improved conductivity. For enhancing the physical properties, a proper selection of nano-materials for liquid crystals depends upon various factors such as size, shape, preparation methods, surfactant concentration and amount of doping materials. In the present study an attempt is made to study electrical and acoustical properties of cholesteric liquid crystal after dispersing ferroelectric nano-powder of Barium Titanate (BaTiO3). In addition with this particle size and surface area of pure and nono-particle dispersed liquid crystal were also measured. Our investigation shows increase in Rao’s constant or molar sound velocity, which indicates increase in molecular density indicating a close packing of the material. The measurement of dielectric relaxation at different frequencies gives information about the dynamics of polar groups and molecular motion.

Estimation method for sound velocity distribution for high-resolution ultrasonic tomographic imaging.

With commercial ultrasonic equipment, the sound velocity is fixed to a constant value of 1530 or 1540m/s, which is used for beam formation. However, the assumption of a constant sound velocity is not optimal, as the sound velocity in a living body is heterogeneous. In this study, a novel method was proposed to estimate the distribution of the sound velocity in a region of interest. The sound velocity distribution was estimated by fitting the theoretical propagation time of the ultrasonic wave from the scatterer to each of the probe elements with measured values. In a phantom experiment, the sound velocity distribution was estimated by the proposed method with a maximum estimation error of 0.6%, and the resultant local sound velocity values successfully improved the quality of the ultrasonic image. The proposed method has the potential to improve ultrasonic image quality in in vivo experiments by estimating the sound velocity distribution.

Diffraction of the Field of a Point Source by a Compact Obstacle in a Continuously Layered Waveguide

Diffraction of the field of a point source by a body of revolution is considered for the case where the body is positioned in a plane layer bounded by a perfectly soft boundary on one side and by a fluid half-space on the other side. Characteristics of the medium inside the layer are assumed to depend in a continuous manner on the vertical coordinate. The proposed method is tested by comparing the results of wave field calculations for the case of a constant sound velocity profile within the plane layer and also by comparing the given method with the surface integral equation method. It is shown that the sound field is considerably affected by inhomogeneity of the medium within the layer.

Acoustic Pulse Diffraction by a Sphere in a Planar Layered Waveguide with a Gradient Layer

Consideration of the vertical sound velocity profile is highly important for solving problems of sound propagation in waveguides and scattering problems. A pulsed echo signal reflected from a spherical scatterer in a waveguide is modeled for the case of a waveguide characterized by sound velocity increasing with depth. The simplest model of the medium is considered in which the scatterer, the source, and the receiver are positioned in a layer with constant sound velocity. Below this layer, the sound velocity increases with depth so that the square of refractive index varies according to linear law. The scattering coefficients for the sphere are calculated using the normal wave method. The number of normal waves forming the echo signal is determined by the preset directionality of the source. Modeling is performed in a frequency band of 70−90 kHz for distances between the scatterer and the transmitter-receiver within 500−1000 m. The transmitted signal has the form of a pulse with cosine envelope and central frequency of 80 kHz.

Spectroscopic, elastic and dielectric properties of Ho3+ substituted Co-Zn ferrites synthesized by sol-gel method

Ho3+ substituted cobalt-zinc ferrite with a chemical formula Co0.7Zn0.3HoxFe2−xO4 (0≤x≤0.1) were synthesized by sol-gel method. Synthesized samples were characterized by using X-ray diffraction, field emission scanning electron microscopy, Raman, Infrared and UV–visible spectroscopy. X-ray diffraction patterns show the formation of cubic spinel structure of all the synthesized samples. The gradual changes in the Raman spectra of Co-Zn ferrite were observed at higher level of Ho3+ substitution. The Raman band shows that there is a strong electron-phonon coupling in the investigated ferrite system and is the highest for the T32g mode as compared to A1g and T22g modes. Fourier transform infrared spectroscopy shows that the vibration bands corresponding to Fe–O is slightly varied with the substitution of Ho3+ substitution. UV–visible spectra was studied by using the optical absorbance measurements in the wavelength ranges from 500 to 800nm. The estimated energy band gap is found to increase from 1.72 to 1.84eV with the increase in Ho3+ substitution. The elastic properties such as longitudinal velocity, shearing velocity, elastic constant, Poisson's ratio and Debye temperature were measured at room temperature by using the ultrasonic pulse transmission method. It is found that the elastic constant and Debye temperature decreased with the increase in Ho3+ substitution. A linear relationship between Debye temperature, elastic constant and average sound velocity with the Ho3+ substitution is observed and discussed in this manuscript. Dielectric properties have been investigated as function of frequency. The dispersion of the dielectric properties was discussed in the light of Koop's theory in accordance with the Maxwell-Wagner polarization.

Modelling sound propagation in the Southern Ocean to estimate the acoustic impact of seismic research surveys on marine mammals

Modelling sound propagation in the ocean is an essential tool to assess the potential risk of air-gun shots on marine mammals. Based on a 2.5-D finite-difference code a full waveform modelling approach is presented, which determines both sound exposure levels of single shots and cumulative sound exposure levels of multiple shots fired along a seismic line. Band-limited point source approximations of compact air-gun clusters deployed by R/V Polarstern in polar regions are used as sound sources. Marine mammals are simulated as static receivers. Applications to deep and shallow water models including constant and depth-dependent sound velocity profiles of the Southern Ocean show dipole-like directivities in case of single shots and tubular cumulative sound exposure level fields beneath the seismic line in case of multiple shots. Compared to a semi-infinite model an incorporation of seafloor reflections enhances the seismically induced noise levels close to the sea surface. Refraction due to sound velocity gradients and sound channelling in near-surface ducts are evident, but affect only low to moderate levels. Hence, exposure zone radii derived for different hearing thresholds are almost independent of the sound velocity structure. With decreasing thresholds radii increase according to a spherical 20 log10 r law in case of single shots and according to a cylindrical 10 log10 r law in case of multiple shots. A doubling of the shot interval diminishes the cumulative sound exposure levels by −3 dB and halves the radii. The ocean bottom properties only slightly affect the radii in shallow waters, if the normal incidence reflection coefficient exceeds 0.2.

Correction of Thickness Measurement by Ultrasound for Articular Cartilage Using Sound Velocity Estimation

Background. Ultrasound measurement of osteoarthritis cartilage thickness is not sufficiently accurate if constant sound velocity is used. The aim of this study was to investigate the applicability of estimating the sound velocity using echo reflectance from the cartilage surface. Methods. Change in measurement error of cartilage thickness was evaluated using collagenase-treated osteochondral plugs. Sound velocity was calculated from the reflectance ratio, and the reference thickness was measured using a needle method. Findings. The relative errors of thickness measurements using constant sound velocity were increased after collagenase treatment. The errors using calculated sound velocity showed no significant difference between intact and degenerated samples. Interpretation. The collagenase treatment-induced error was reduced from 7% to 2% in absolute value, suggesting the applicability of the correction method for ultrasound measurement.

Water velocity estimation using inversion methods

It is known that the propagation velocity of sound waves in water can vary over time. For a 3D seismic survey, if data are acquired in adjacent lines but at different dates, this implies the same reflection point will be recorded at different times. To take this effect into account in seismic processing, it is necessary to measure the sound velocity in water. I have developed a 3D tomographic method that directly estimates it. It assumes a constant sound velocity for a group of shots belonging to a single sail line. Using a picked water-bottom reflection and an initial depth and velocity model, results good for use in subsequent processing can be obtained by estimating only two parameters: the variation of the propagation velocity and a constant vertical shift of the reflector depth in relation to the initial model. The method was tested with both synthetic and real data. The real data results were validated using two methods. First, I analyzed the histogram of the residuals of the final updated model. Second, I used a specially modified Kirchhoff migration algorithm to migrate the sea bottom. The main advantages of this method are that it takes into account the sea-bottom dips to estimate the velocities and it can be applied to each sail line separately. Also, the inversion is not ill-conditioned provided that data with large enough offsets are used. As a result, the method is simple to apply.

Low-frequency sound field focusing in shallow water.

Within the framework of numerical simulation a feasibility of low‐frequency (100–300 Hz) sound field focusing was studied in the waveguide common to shallow water. Two types of focusing are discussed: focusing of a wide‐band sound signal by time reversal mirror (TRM) and focusing of a monochromatic sound field by a phase conjugation mirror (PCM). It is demonstrated that if we use TRM we can focus sound field by both point source‐receiver (SR) element and a corresponding SR vertical array. At the same time, the focusing quality is somewhat improved with vertical array, and TRM provides focusing with approximately the same quality as PCM operating at the frequency equal to the carrier frequency of a wide‐band signal. It is shown that in a waveguide with a constant sound velocity at the range of 10 km wind waves deteriorate focusing quality at a wind speed greater than 12 m/s. [This research was supported by the Russian Foundation for Basic Research Project Nos. 08‐02‐00283 and 07‐02‐001205 and by the Civili...

Image projection and composition with a front-scan sonar system: methods and experimental results

This paper describes a complete set of methods for arranging acoustic images of the sea floor by projecting and interpolating data gathered with a novel front-scan sonar system, developed in the context of the EC-COSMOS project. Traditional sonar imaging systems for sea-floor analysis generate acoustic images during the motion of a ship; on the contrary, the front-scan sonar system not only provides information unreachable by traditional devices (blind region), but also does not need the ship motion to compose a whole image of the sea floor. Two different projection methods have been devised: a simpler analytical solution and a more precise ray-tracing approach. The development of an analytical solution is possible under the classical assumptions about a flat sea floor and a constant sound velocity profile; when these hypotheses are not realistic or a more precise image is required, a numerical solution obtained by a ray-tracing approach can be applied, which is based on some ad hoc solutions worked out for the front-scan sonar system. To move from the projection results to an image defined over a dense matrix of pixels, an interpolation stage is needed. To this end, an algorithm based on the generation of virtual-beam signals (only where necessary) has been tested and compared with more-traditional techniques. The potentials of the proposed projection and interpolation methods have been evaluated and some comparisons have been made, using real data gathered with the COSMOS sonar prototype during trials at sea.

Snell’s law of refraction and sound rays for a moving medium

Fermat’s principle of least time is used to calculate a new law of refraction for a stratified medium moving horizontally with different temperatures in each layer. This new law of refraction includes velocity of sound, wind speed, and the angle between the vectorial sum of sound velocity and the wind speed. This new equation is compared with the usual approximations for the different refraction laws of a moving medium occasionally mentioned in literature as ‘‘Snell’s law for a moving media.’’ The sound rays in a moving thermically stratified medium are refracted more or less (dependent on either upwind or downwind sound propagation), then calculated according to the ‘‘Snell’s law.’’ For upward-oriented sound rays and a moderate thermical stratification with high wind speeds, the difference in the angle of refraction between the new refraction law and the usual approximation is roughly 3 degrees and neglectable for grazing incidence. The sound ray trajectories expressed as analytical equations for a constant sound velocity combined with a wind speed profile proportional to height; the results are shown in a figure. Furthermore, the sound ray trajectories for a linear sound speed profile combined with a linear height-dependent wind speed profile are calculated and shown graphically.

A model for the holographic reconstruction of sound fields disturbed with temperature gradients

Holographic techniques are used to detect noise sources in a wide range of devices. Some of these techniques are commonly employed in the reconstruction of sound fields produced by automobile or motorcycle engines. These sound fields are characterized by having several, rather than single, temperatures; in fact, temperature gradients are frequently found in regions near the engine. These gradients produce changes in every point of the sound wave velocity. The mathematical model presented in this paper was designed to minimize the effect of assuming a constant sound velocity in the entire space in reconstruction of the sound field. It is based on the space transformation of sound field (STSF) technique [Hald, Jo/rgen, B&K Tech. Rev. 1, 1–50 (1989)], which uses a microphone array to measure pressure or determine the principal component of cross spectra over a hologram plane. The model scans the sound field in two planes. The first plane is used as an STSF initial condition (hologram plane), and the second plane is considered the boundary condition to Rayleigh integral’s propagation.

Gauge-invariant cosmic structures-A dynamic systems approach.

Gravitational instability is expressed in terms of the dynamic systems theory. The gauge-invariant Ellis-Bruni equation and Bardeen's equation are discussed in detail. It is shown that in an open universe filled with matter of constant sound velocity the Jeans criterion does not adequately define the length scale of the gravitational structure.

A NEUTRON SCATTERING STUDY OF THE DENSITY OF VIBRATIONAL STATES OF FRACTAL SILICA AEROGELS

The various methods to measure the density of states by neutron inelastic scattering are discussed. Purely incoherent scattering data on silica aerogels have been obtained by taking the difference spectra between protonated and identical deuterated samples. The results so far cover the particles modes and the upper part of the fracton regime. Complemented with Brillouin scattering information, these measurements provide an overall picture of the absolute density of states, from which low temperature thermal properties can be calculated, in remarkable agreement with experiments. 1 INTRODUCTION The theory of vibrational excitations in fractal structures predicts the existence of 111. Those vibrational modes are localized. Also, in contrast to long-wavelength acoustic phonons, which obey a linear dispersion law, o = v q , where w is the frequency, q is the wavevector, -and v is the constant sound velocity, fractons are expected to exhibit dispersion according to o C C ~ ~ I ~ 11.21. Here D is the fractal (Hausdorff) dimension, and k is a characteristic length for the fracton. Finally, the density of states (DOS) of fractons Z(w) was predicted to be proportional to wa -1 /I/. The dimensions obey the inequality d 2 D > z, with d the Euclidean dimension. As an example, for 3-d percolation clusters and scalar waves the values are D = 2.5 and a = 413. The above predictions are at strong variance with the well-known behavior of acoustic phonons, a@ the differences should be observable experimentally. Silica aerogels are ideally suited for the experimental study of fracton dynamics. They are porous solid materials. Large pieces can be prepated, so that their elastic properties can be measured by static as well as by ultrasonic techniques. The optical transparency of aerogels in a broad range of densities and preparation conditions allows the study of their vibrational dynamics by Brillouin and Raman light scattering. (I) Supported by Neutron Scattering Project IIKW, Natuurkunde, University of Antwerp, 8-2610 Wilrijk, Belgium (2) Laboratoire mixte CNRS-Commissariat a I'Energie Atomique. (3) Laboratory associated to the Centre National de la Recherche Scientifique (CNRS). No 1129. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989424 Structural studies /3,4/ have demonstrated that silica aerogels are made of elementary particles, of typical size a, with more or less defined surfaces. Those particles assemble to form fractal networks, up to an average cluster size 5. At length scales above (, the material is an homogeneous assembly of such clusters. Corresponding to these different length scales, three distinct vibrational regimes are expected, namely, with increasing frequency : phonons, fractons, and particle modes. A crossover from phonons to fractons at oco,12n -1 GHz has been demonstrated by Brillouin scattering experiments 151. This technique also allowed the determination of the dispersion curve for fractons 16,7/, and gave indication for the occurrence of localization at the phonon-fracton crossover. Evidence of an extended fracton domain, ranging approximately from 40 peV to 1 meV, was obtained from very-low frequency Raman scattering IS/. It is obviously of interest to measure directly the DOS of these excitations. 2 EXPERIMENTAL APPROACHES There are several ways to measure a vibrational DOS by inelastic neutron scattering. One basic idea is that the DOS is reflected in the incoherently scattered neutrons. Taking only the one phonon terms in a multiphonon expansion, the expression for the double differential incoherent cross section of a Bravais lattice

Hydrodynamics of ultra-relativistic heavy ion collisions

The central rapidity regime in a central collision of two heavy ions at ultra-relativistic energies is expected, according to Bjorken's recent space-time picture of this event, to undergo a longitudinal hydrodynamic expansion with accompanying transverse rarefaction. This paper describes in detail the hydrodynamic equations governing this behavior, the reduction of their solutions to that of the central rapidity slice, certain analytic solutions, and for the case of constant sound velocity, numerical and approximate analytic solutions for the full expansion. We compare the numerical results with previous calculations within the Landau hydrodynamic model of a spherically expanding collision volume, and find that matter in the present cylindrical geometry undergoes a more rapid cooling in the interior; a quark-gluon plasma formed in the collision will thus rapidly hadronize. Effects of the hadronization transition on the hydrodynamic evolution and the possibility of shock formation, as well as effects of transverse expansion on expected transverse momentum distributions, are briefly discussed.

Experiment on Sound Propagation in Shallow Water with Velocity Structure

An investigation into the propagation of sound in shallow, isovelocity water was carried out previously using an extensive area of the continental shelf off the west coast of New Zealand. This area has been re-examined when a negative velocity gradient was present in the water layer. The results obtained show that this area provides a classic example of normal-mode propagation in shallow water under conditions of both constant sound velocity and a negative gradient. The values of attenuation obtained are on the whole greater when a negative gradient occurs, but their general behavior with frequency is similar in both cases. In addition, the attenuations are comparable with those obtained in other shallow-water areas and are consistent with the predictions of normal-mode theory.