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A modified Biot/Squirt flow model was developed. The difference between MBISQ and BISQ models is the expression for the porosity differential. Numerical analysis shows that the acoustic dispersion predicted by MBISQ is much higher than by BISQ. Investigations of the effects of permeability, viscosity, and squirt flow length on velocity and attenuation indicate that the behavior of MBISQ agrees with that of the BISQ model. The result of sediment acoustic inversion based on MBISQ was more reasonable than the result of BISQ model.

It has been previously shown using synthetic data that dispersion tracking with particle filtering can be used for sediment sound speed inversion. Here, dispersion tracking is performed with data collected in the Gulf of Mexico for sediment sound speed and thickness and water column depth estimation. In this experiment, sound that propagates a long distance from the source allows the identification of dispersion curves reflecting the different group velocities of modal frequencies within and across modes. Although the data are noisy, dispersion curves are tracked with sequential filtering and used for inversion. Probability density functions of the three unknown parameters are obtained. Water column depth is estimated with little uncertainty. The estimated sound speed is representative of sandy sediment and the sediment thickness matches to a large extent prior knowledge.

The acoustic modes of an idealized three-dimensional model for a curved shelf-slope ocean front [Lin and Lynch, J. Acoust. Soc. Am. 131, EL1-EL7 (2012)] is examined analytically and numerically. The goal is to quantify the influence of environmental and acoustic parameters on acoustic field metrics. This goal is achieved by using conserved quantities of the model, including the dispersion relation and a conservation of mode number. Analytic expressions for the horizontal wave numbers can be extracted by asymptotic approximations and perturbations, leading to accurate and convenient approximations for their parameter dependence. These equations provide the dependence on model parameter changes of both the real horizontal wavenumbers, leading to modal phase speeds and other metrics, and the imaginary parts, leading to modal attenuation coefficients. Further approximations for small parameter changes of these equations characterize the parameter sensitivities and produce assessments of environmental and acoustic influences.

This paper deals with wave characteristics of a cylinder with periodic ribs. The equation of motion of the stiffened cylinder is first derived using a receptance coupling method. The dispersion diagrams of the stiffened cylinder are then obtained in order to figure out the effects of the ribs on the wave propagation. Due to the effect of the ribs, the dispersion curves are found to be repeated along the axis of the wavenumber with the repetition period of 2π/d, where d is the rib distance. Also, dispersion curves are found to show a pass and stop band of the waves. The stop bands appeared at the wavenumber of half of the wavenumber periodicity. The stop band becomes wider as the increase of the circumferential order. The waves in the pass bands are propagating well through the ribs without decay. In contrast, the waves in the stop bands are not propagating, but decaying the magnitudes. The decay of the responses in the stop band increases as the circumferential order increase. The change of the rib stiffness causes the cut-on frequency to change and the modal order to jumble. The change of the rib stiffness also leads to generate a wave whose phase velocity is positive, while group velocity is negative.

An incremental wave superimposed on a pre-deformed hyper-elastic material perceives an elastic media with the instantaneous modulus of the current material. This offers a new route with a broadband feature to control elastic waves by purposely creating finite deformation field. This study proves that the governing equation of a semi-linear material under a symmetric pre-deformation condition maintains the form invariance for longitudinal wave, so the longitudinal wave control can be made by transformation method without the constraint condition on principle stretches, but this is not the case for shear waves. Therefore pre-deforming a semi-linear material provides a potential method for treating longitudinal and shear waves differently. Examples with elastic wave control and band structure shift through pre-deforming a semi-linear material are provided to illustrate this finding. Finally, a one-dimensional spring lattice is proposed to mimic a semi-linear material, and the dispersion relation for longitudinal waves in a sandwich structure with such spring lattice is shown to be invariant during elongation, confirming the result found based on a homogeneous semi-linear material. These results may stimulate researches on designing new hyper-elastic microstructures as well as designing new devices based on pre-deformed hyper-elastic materials., 7 pages, 4 figures

A predictive model for acoustic dispersion and attenuation in gassy sediments is proposed. The model combines the linear solution for gas-bubble pulsations in a viscoelastic medium with corrected Biot equations involving gas-bubble pulsations. Numerical results for sound speed and attenuation are compared with predictions from Anderson and Hampton's model to demonstrate the advantages of the proposed model. The most important advantage of the current model is that it combines the dispersion regimes associated with gas-bubble pulsations and relative motion between the pore water and solid framework. The reflection coefficient at the water/gassy-sediment interface is derived based on the current model, and numerical results show that gas-bubble resonance can lead to the highest reflection. This model can also be used with a full acoustic inversion to estimate gas-bubble size distributions.

This paper presents an application of the Waveform inversion approach to localization of objects in reverberant fields and with limited spatial measurements. Reverberant fields in enclosures can potentially carry useful information, however, in an incoherent way. Incoherency comes from the consecutive reflections of the wave energy several times in the domain. This, along with diffraction and dispersion effects, can ultimately lead to mixing of the wave energy in a seemingly random way. However, spreading of the wave energy can lead to multiple interrogations of each point in the enclosure. Hence, any substructural changes in the enclosure can be sensed with sufficient information carried by the wave energy flow. Furthermore, the temporal information buried in the data makes it feasible to conduct only a few spatial measurements. The authors present a localization scheme that benefits from the reverberant field and can reduce the required number of spatial measurements.

In the framework of the membrane theory of cylindrical shells, the localised vibration near the shell free edge is considered. This paper presents the development leading to the dispersion equations for finite length shells and for three types of boundary conditions. Based on the attained dispersion equations the localised membrane vibration conditions are analysed and remarks are offered.

In the present study, the structural, electronic, magnetic, anisotropic elastic and lattice dynamic properties of the ternary metal borides M2AlB2 (M=Mn, Fe and Co) known as MAB phases have been investigated by density functional theory. The obtained results from the structural optimizations show that all these compounds have negative formation enthalpy implying the thermodynamic stability and synthesizability. The spin effects on the M2AlB2 phases have been studied with the plotted energy-volume curves for different magnetic phases (antiferromagnetic (AFM), ferromagnetic (FM), and paramagnetic (PM)) of these compounds. The stable magnetic phase for the Mn2AlB2 compound is found to be AFM while the magnetic nature of Fe2AlB2 and Co2AlB2 compounds are FM. The calculated electronic band structures with the total and orbital projected partial density of electronic states imply that these ternary metal borides have metallic behavior. Also, the mentioned compounds have mechanical and dynamic stability due to the calculated elastic constants and the observed phonon dispersion curves. Some thermodynamic properties have been investigated by means of phonon dispersion curves. Furthermore, the anisotropic elastic properties have been visualized in three dimensions (3D) for Young's modulus, linear compressibility, shear modulus, Poisson's ratio, and sound wave velocities. © 2020 Elsevier B.V.

A laboratory experiment was conducted to measure the speed of sound in an artificial water-saturated granular sediment composed of cleaned and sorted medium-grained sand and degassed distilled water. The experiment was conducted within a range of frequencies where dispersion is predicted by a number of existing models. Between 2 and 4 kHz, the sound speed was inferred from measurements of the resonance frequencies of a thin-walled cylindrical container filled with the material. An elastic waveguide model was used to account for the effect of the finite impedance of the walls, although this effect was found to be small. From 20 to 300 kHz, the sound speed was obtained directly from time-of-flight measurements within the sediment. Dispersion in close agreement with the Williams effective density fluid model [K. L. Williams, J. Acoust. Soc. Am. 110, 2276-2281 (2001)] was observed.

The topology of the acoustic emission sensor array has an important effect on the performance of the leak localization technique. This paper compares the performances of different topologies of acoustic emission sensor arrays in the localization of gas leakage on a flat-surface structure. The principle of the leak localization is based on the near-field beamforming according to the spherical wave model and the narrowband filtering which can effectively avoid the influence of acoustic dispersion. The effect of different arrangements of the sensing elements in a sensor array on the localization accuracy is investigated and discussed. Eight typical topologies, including line, L-shaped, cross, triangle, star, circular, semi-circular and square shapes, are appraised through computer simulation. Simulation results suggest that all the arrays can perform leak localization but with different accuracies and that the L-shaped array outperforms all other topologies under the similar conditions. Furthermore, the optimal number of sensors in the L-shaped array which can maintain a reasonable accuracy of localization is analyzed. Experimental work was carried out on a laboratory scale test rig to verify and assess the effectiveness of the L-shaped array. The simulation and experimental results demonstrate that the L-shaped array is capable of identifying the location of a leak hole on a plate with a reasonably good accuracy.

This paper presents a wave-based numerical scheme based on a spectral element method, coupled with an implicit-explicit Runge-Kutta time stepping method, for simulating room acoustics in the time domain. The scheme has certain features which make it highly attractive for room acoustic simulations, namely (a) its low dispersion and dissipation properties due to a high-order spatio-temporal discretization; (b) a high degree of geometric flexibility, where adaptive, unstructured meshes with curvilinear mesh elements are supported; and (c) its suitability for parallel implementation on modern many-core computer hardware. A method for modelling locally reacting, frequency dependent impedance boundary conditions within the scheme is developed, in which the boundary impedance is mapped to a multipole rational function and formulated in differential form. Various numerical experiments are presented, which reveal the accuracy and cost-efficiency of the proposed numerical scheme.

Multilayered plate and shell structures play an important role in many engineering settings where, for instance, coated pipes are commonplace such as in the petrochemical, aerospace, and power generation industries. There are numerous demands, and indeed requirements, on nondestructive evaluation (NDE) to detect defects or to measure material properties using guided waves; to choose the most suitable inspection approach, it is essential to know the properties of the guided wave solutions for any given multilayered system and this requires dispersion curves computed reliably, robustly, and accurately. Here, the circumstances are elucidated, and possible layer combinations, under which guided wave solutions, in multilayered systems composed of generally anisotropic layers in flat and cylindrical geometries, have specific properties of coupling and parity; the partial wave decomposition of the wave field is utilised to unravel the behaviour. A classification into five families is introduced and the authors claim that this is the fundamental way to approach generally anisotropic waveguides. This coupling and parity provides information to be used in the design of more efficient and robust dispersion curve tracing algorithms. A critical benefit is that the analysis enables the separation of solutions into categories for which dispersion curves do not cross; this allows the curves to be calculated simply and without ambiguity.

The authors study the wave propagation in continuum acoustic metamaterials whose all or not all of the principal elements of the mass tensor or the scalar compressibility can be negative due to wave dispersion. Their time-domain wave characteristics are particularly investigated by the finite-difference time-domain (FDTD) method, in which algorithms for the Drude and Lorentz dispersion pertinent to acoustic metamaterials are provided necessarily. Wave propagation nature of anisotropic acoustic metamaterials with all admissible material parameters are analyzed in a general manner. It is found that anomalous negative refraction phenomena can appear in several dispersion regimes, and their unique time-domain signatures have been discovered by the FDTD modeling. It is further proposed that two different metamaterial layers with specially assigned dispersions could comprise a conjugate pair that permits wave propagation only at specific points in the wave vector space. The time-domain pulse simulation verifies that acoustic directive radiation capable of modulating radiation angle with the wave frequency can be realized with this conjugate pair. The study provides the detailed analysis of wave propagation in anisotropic and dispersive acoustic mediums, which makes a further step toward dispersion engineering and transient wave control through acoustic metamaterials.

The dispersion curves of a cluster of closely spaced rods supported by a thin plate are characterised by subwavelength bandgaps and slow group velocities induced by local resonance effects. A recent analytical study [Williams, Roux, Rupin, and Kuperman (2015). Phys. Rev. B 91, 104307], has shown how the slow velocity branch depends, amongst other parameters, on the height of the rods that make up the cluster. Such metamaterial, offering easy-to-tune spatial velocity gradients, is a perfect candidate for building gradient index lenses such as Luneburg, Maxwell, and 90° rotating. Here theoretical results are combined with numerical simulations to design and test metalenses for flexural waves. The lenses are obtained by tuning the height of the cluster of rods such that they provide the required refractive index profile. Snapshots and videos from three-dimensional numerical simulations in a narrow band centered at ∼4 kHz are used to analyse the performances of three types of gradient index metalens (Luneburg, Maxwell, and 90° rotating).

Tight clastic reservoirs are an important aspect of hydrocarbon exploration and development worldwide. The Xu5 section of the Upper Triassic Xujiahe Formation in the western Sichuan Basin of central China contains tight terrestrial clastic reservoirs. The depth of these reservoirs is greater than 3000 m, while the thickness is generally greater than 500–580 m; the tight sandstone and shale reservoirs are frequently interbedded. Based on the proportion of sandstone and shale, the reservoirs can be divided into four types: sand-rich, interbedded Type I, interbedded Type II, and shale-rich. Horizontal wells and multistage fracturing are used due to the complexity and significant heterogeneity of the tight clastic reservoirs in the study area. The Young's modulus and Poisson's ratio of the reservoir rocks are the two most significant parameters that must be evaluated when implementing these key techniques. Based on mechanical and acoustic tests, a new assessment system for the Young's modulus and Poisson's ratio based on the Hoek-Brown criterion was proposed. This evaluation system does not require the shear wave time difference; thus, the increase in error caused by the unreliability of the shear wave time difference can be avoided when assessing strata with complex geological structures. This method can be directly applied in single wells using only conventional longitudinal wave time difference log data. After converting the wave velocity of the high experimental frequency into the wave velocity of the logging frequency using the acoustic dispersion technique, the analysis results were applicable to the log evaluations. The relationship between the Hoek-Brown criterion parameters and the mechanical and acoustic parameters of the rock were systematically analyzed. By distinguishing the lithologies, the equations for the Young's moduli of tight sandstones and shales were established based on the Hoek-Brown criterion. The Young's modulus can be expressed using only the longitudinal wave velocity (VP). A significant negative correlation exists between the Poisson's ratio and Young's modulus; therefore, the Poisson's ratio can be evaluated from the Young's modulus. The errors of the Young's modulus and Poisson's ratio predicted by the proposed assessment method are small, which meets engineering requirements and validates the accuracy of the method.

When using geoacoustic inversion methods, one objective function may not result in a unique solution of the inversion problem because of the ambiguity among the unknown parameters. This paper utilizes acoustic normal mode dispersion curves, mode shapes, and modal-based longitudinal horizontal coherence to define a three-objective optimization problem for geoacoustic parameter estimation. This inversion scheme is applied to long-range combustive sound source data obtained from L-shaped arrays deployed on the New Jersey continental shelf in the summer of 2006. Based on the sub-bottom layering structure from the Compressed High-Intensity Radiated Pulse reflection survey at the experimental site, a two-layer (sand ridge overlaying a half-space basement) range-independent sediment model is utilized. The ambiguities of the sound speed, density, and depth of the sand ridge layer are partially removed by minimizing these objective functions. The inverted seabed sound speed over a frequency range of 15–170 Hz is comparable to the ones from direct measurements and other inversion methods in the same general area. The inverted seabed attenuation shows a nonlinear frequency dependence expressed as αb=0.26f1.55(dB/m) from 50 to 500 Hz or αb=0.32f1.65(dB/m) from 50 to 250 Hz, where f is in kHz.

An empirical formula based on viscoelastic analysis techniques that employs concepts from the fractional calculus that was used to model the dielectric behavior of materials exposed to oscillating electromagnetic fields in the radiofrequency, terahertz, and infrared bands. This work adapts and applies the formula to model viscoelastic behavior of materials that show an apparent increase of phase velocity of vibration with an increase in frequency, otherwise known as anomalous dispersion. A fractional order wave equation is derived through the application of the classic elastic-viscoelastic correspondence principle whose analytical solution is used to describe absorption and dispersion of acoustic waves in the viscoelastic material displaying anomalous dispersion in a specific frequency range. A brief discussion and comparison of an alternative fractional order wave equation recently formulated is also included.

The dispersion pattern of a received signal is critical for understanding physical properties of the propagation medium. The objective of this work is to estimate accurately sediment sound speed using modal arrival times obtained from dispersion curves extracted via time-frequency analysis of acoustic signals. A particle filter is used that estimates probability density functions of modal frequencies arriving at specific times. Employing this information, probability density functions of arrival times for modal frequencies are constructed. Samples of arrival time differences are then obtained and are propagated backwards through an inverse acoustic model. As a result, probability density functions of sediment sound speed are estimated. Maximum a posteriori estimates indicate that inversion is successful. It is also demonstrated that multiple frequency processing offers an advantage over inversion at a single frequency, producing results with reduced variance.

Acoustic profiling methods are commonly used in the geosciences to obtain knowledge about the sedimentary structures of marine deposits. However, their interpretation and the correlation of acoustic reflectors with sedimentological boundaries remain difficult. The gradient in acoustic impedance determines the intensity of acoustic reflectors, which are not necessarily sedimentary boundaries. Suitable geo-acoustic models that cover a wide range of sediment types have not yet been developed for the Baltic Sea. Based on comprehensive investigations of sediment cores from Mecklenburg Bay and the Arkona Basin, we developed geo-acoustic models that enable the transfer of laboratory p-wave velocity measurements to in situ conditions while considering changes in pressure, salinity, temperature and the frequency dependence of the p-wave velocity (i.e. acoustic dispersion). This modelling allows the calculation of the wet bulk density and in situ p-wave velocity (from which the acoustic impedance is calculated) from core logging and selected sedimentological data, such as the water content, loss on ignition, and grain size. Precise plotting of core data into the acoustic profiles is now possible and represents a crucial step towards the interpretation of these profiles. Modelled p-wave velocities are used to convert two-way-travel-time to depth. The average sedimentological parameters for the different development phases of the Baltic Sea are calculated using geo-statistical methods and may allow for rough p-wave estimations if core logging data are not available.

The intersections between Lamb mode dispersion curves of free isotropic plates at real values of frequency and wave number are examined for the full allowed range of Poisson's ratio σ. The generic intersections between the dispersion curves for symmetric and anti-symmetric branches are classified into three types. Type F intersections are conditioned by the two additional real solutions of Rayleigh's cubic equation that occur for σ

The Navier-Stokes equations are solved using a finite-difference, time-domain (FDTD) approach for axi-symmetric environmental models, allowing three-dimensional acoustic propagation to be simulated using a two-dimensional Cylindrical coordinate system. A method to stabilize the FDTD algorithm in a viscous medium at atmospheric densities characteristic of the lower thermosphere is described. The stabilization scheme slightly alters the governing equations but results in quantifiable dispersion characteristics. It is shown that this method leaves sound speeds and attenuation unchanged at frequencies that are well resolved by the temporal sampling rate but strongly attenuates higher frequencies. Numerical experiments are performed to assess the effect of source strength on the amplitudes and spectral content of signals recorded at ground level at a range of distances from the source. It is shown that the source amplitudes have a stronger effect on a signal's dominant frequency than on its amplitude. Applying the stabilized code to infrasound propagation through realistic atmospheric profiles shows that nonlinear propagation alters the spectral content of low amplitude thermospheric signals, demonstrating that nonlinear effects are significant for all detectable thermospheric returns.

The nonlinear progressive wave equation (NPE) is a time-domain formulation of the Euler fluid equations designed to model low-angle wave propagation using a wave-following computational domain. The wave-following frame of reference permits the simulation of long-range propagation and is useful in modeling blast wave effects in the ocean waveguide. Existing models do not take into account frequency-dependent sediment attenuation, a feature necessary for accurately describing sound propagation over, into, and out of the ocean sediment. Sediment attenuation is addressed in this work by applying lossy operators to the governing equation that are based on a fractional Laplacian. These operators accurately describe frequency-dependent attenuation and dispersion in typical ocean sediments. However, dispersion within the sediment is found to be a secondary process to absorption and effectively negligible for ranges of interest. The resulting fractional NPE is benchmarked against a Fourier-transformed parabolic equation solution for a linear case, and against the analytical Mendousse solution to Burgers' equation for the nonlinear case. The fractional NPE is then used to investigate the effects of attenuation on shock wave propagation.

While usually negligible in standard optical fibers, the group velocity dispersion of acoustic waves may in some cases play a significant role in the dynamics of stimulated Brillouin scattering (SBS) in propagation media with more complex structures, such as microstructured fibers. The usual three-wave coherent model of SBS can be adapted to take perturbative acoustic dispersion into account, but the slowly varying envelope approximation does not hold for stronger values of the acoustic dispersion, which call for a more sophisticated inertial model of SBS. A new regime of SBS mirrors with a spatially inverted acoustic profile is predicted in this limit. In the presence of strong acoustic dispersion, this regime exhibits a higher conversion efficiency than the usual mirror in the dispersionless case, as well as nonlinear self-stabilization of the phase of the acoustic wave when the pump is strongly depleted. Formal calculations allow the identification of regions of strong dynamic dispersion.

Acoustic mode decomposition is used for evaluating the damping of aircraft liners and, in general, to investigate acoustic scattering in flow ducts. Classical methods rely on analytical solutions of the wave properties and accept uncertainties due to simplified descriptions of the duct flow. In contrast, the current study provides a wave decomposition method that does not require explicit analytical knowledge of the wave properties and registers a wide range of flow-related acoustic phenomena. A multilayer perceptron artificial neural network is trained to learn acoustic wave decomposition for plane-wave-like duct modes. Training data are the numerical solutions of the Linearized Navier-Stokes Equations, from which the network not only learns the wave motion properties but also the dispersion of sound into the fluid flow. The network can account for flow-related effects, such as turbulent attenuation, refraction, convection, and thermo-viscous dissipation, which are only included in the classical models based on simplifications. The new method is validated for plane-waves against analytical data and experiments. It is demonstrated that the network can mimic the classical solutions accurately when trained under the same flow simplifications. In addition, it can cope with complex flow effects, such as turbulent attenuation, by including them in the training data. Therefore, the proposed wave decomposition complements the classical plane-wave decomposition when investigating in-duct sound with complex flow conditions. An important continuation of this work is to extend the new wave decomposition method to multi-modal sound fields. As the first step in this direction, it is demonstrated that the proposed training scheme also works for higher-order modes.

Acoustic propagation in seagrass meadows is highly sensitive to oxygen production through photosynthesis. In addition to gas volumes encapsulated within the seagrass, free bubbles are released into the water column as oxygen diffuses through the plant tissue, affecting acoustic dispersion, absorption, and scattering. Because the oxygen production cycle is largely driven by sunlight, these effects exhibit a diurnal dependence. Previous work explored using acoustics to monitor seagrass photosynthetic activity, but this study presents new results that span both diurnal and seasonal time scales. Acoustic propagation experiments were conducted in a seagrass meadow in a shallow bay on the Texas Gulf Coast. A piezoelectric sound source transmitted frequency-modulated chirps (0.1–100 kHz) over several diurnal cycles, and the received acoustic signals were match-filtered to obtain band-limited impulse responses. Water temperature, salinity, depth, dissolved oxygen, and solar irradiance were concurrently measured with oceanographic probes. Measurements were taken both in winter and summer to examine the seasonal dependence of seagrass photosynthesis and its effect on the acoustic propagation environment. Dependence of the received acoustic signal on various environmental parameters will be discussed with the goal of using acoustics to study seagrass photosynthesis and productivity. [Work supported by ARL:UT IR&D and ONR.]