ABSTRACT The performance degradation and safety risks of lithium-ion batteries under low-temperature conditions are closely related to changes in their electrochemical–mechanical behavior. In this study, acoustic emission (AE) techniques were used to investigate a distinctive class of dual-waveform AE signals observed during low-temperature discharge. Wavelet coherence analysis demonstrated strong time–frequency correlations between the paired waveforms, indicating a common or strongly coupled acoustic source. In addition, the time interval between the two waveforms gradually stabilized during discharge, suggesting systematic changes in the internal conditions affecting acoustic wave propagation. Time–frequency energy analysis revealed that decreasing temperature caused the peak frequency of AE signals to shift from the mid-frequency range (100–200 kHz) toward lower frequencies (<100 kHz). Simultaneously, AE activity evolved from multi-band, multi-scale behavior to a response dominated by low-frequency components associated with larger-scale structural processes. These results demonstrate that low-temperature conditions significantly affect AE characteristics and highlight the sensitivity of AE techniques to temperature-dependent internal responses, providing insights into battery condition assessment under low-temperature operation.

Abstract A simple and efficient one-dimensional discrete Boltzmann method is developed for compressible flows with tunable specific heat ratios by incorporating extra degrees of freedom. To guarantee Galilean invariance in numerical simulations, a discrete velocity set is constructed with high spatial symmetry. Furthermore, an operator-splitting scheme is proposed to extend the one-dimensional kinetic formulation to simulations of one-, two-, and three-dimensional flow systems within a unified framework. The proposed model and numerical method are verified and validated against several benchmark problems, including the Sod shock tube, Lax shock tube, 2D Riemann problem, uniform translational flow, and acoustic wave propagation. The results demonstrate the accuracy, robustness, and flexibility of the present approach for compressible flow simulations.

Bound states in the continuum (BICs), spatially localized states embedded in radiating continuum, have been extensively investigated in single-particle systems, also enabling high-performance applications in classical wave systems. Recently, exploring many-body BICs in correlated systems has emerged as a pivotal frontier with increasing research interest. However, the majority of previous studies focus largely on on-site interactions with tailored conditions, incurring significant implementation challenges. Whether off-site interactions, e.g., many-body hoppings, can generate many-body BICs and how to characterize them in experiments remains an open question. Here, we first predict and realize a boundary-localized two-body BIC, the bound pair state in the continuum (BPIC), arising from uniform two-body hoppings. Analytically, we demonstrate not just the formation of bound pairs, but the spectral coexistence of the boundary-localized BPIC and the scattering continuum. Experimentally, developing a phononic crystal platform as a classical-wave simulator, we map the correlated particle dynamics onto acoustic wave propagation and observe the acoustic BPIC. Our findings build a new bridge between the BIC research and many-body physics, and may promote the development of high-Q acoustic devices in applications.

In this study, we explore the formation of high-amplitude, low-spatial-frequency surface features - also commonly referred to as waviness - during laser deep engraving of dielectric materials. This unique phenomenon significantly increases surface roughness and undermines the ability to control it during deep engraving, limiting the technique's applicability. Here we establish the experimental conditions under which these features are formed and demonstrate how they depend on various processing conditions. Furthermore, we develop a numerical model that considers acoustic wave propagation in a confined cavity. Combining a numerical model with experimental results we demonstrate that the formation of low-spatial-frequency features is governed by the interaction between incident laser pulses and the residual effects of preceding pulses. Finally, building on this insight, we developed strategies to suppress the formation of such surface features, reducing the final surface roughness by a factor of ten.

In this work, the effect of liquids with different dielectric permittivities and acoustic impedances on the characteristics of the backward antisymmetric A1 Lamb wave propagating in a YX LiNbO3 plate was investigated theoretically, numerically and experimentally for the first time. It was found that the dielectric constant and acoustic impedance (density) of a liquid make independent and separable contributions to measured parameters of interdigital transducers, such as the resonant frequency and Q-factor. It was shown that the backward A1 Lamb wave in a YX LiNbO3 plate can be effectively used as a basis for multiparametric liquid sensors. The results obtained are both of fundamental importance for understanding the physics of propagation of backward acoustic waves in piezoelectric plates with a liquid load and of applied value for the development of a new generation of acousto-electronic sensors based on such waves.

This study aims at developing and experimentally validate a three-dimensional numerical model designed to predict acoustic propagation and thermal effects during High-Intensity Focused Ultrasound (HIFU) therapy, in order to support patient-specific treatment planning. A 3D time-domain simulation was implemented using the k-Wave toolbox, capturing both acoustic wave propagation and thermal effects. Validation was performed using five tissue-mimicking phantoms with distinct acoustic and thermal properties, each tested under three different HIFU exposure conditions. Lesion dimensions were assessed longitudinally and transversely in both simulations and experiments, and spatial concordance was quantified by evaluating the overlap between simulated and experimental lesions. Simulated longitudinal lesion extents fell within the interquartile range of experimental data in four out of nine phantom-condition combinations with a maximum absolute error of 3.14 mm while transverse dimensions in three out of nine with maximum absolute error equal to 4.16 mm. Spatial overlap between simulated and experimental lesions reached a maximum of 92%. Simulations on phantoms required approximately 4 min, while patient-specific cases completed in about 7 min, accurately capturing heterogeneous tissue characteristics and bone interfaces. In scenarios involving rib intersection, a 40% reduction in focal pressure was observed, leading to significant smaller lesions. The validated 3D simulation, integrated into a planning platform, reliably predicts HIFU ablation zones and focal shifts, showing strong agreement with experimental data and delivering results within clinically feasible times.

Nonlinear acoustic propagation plays a crucial role in many practical applications, such as medical therapy and underwater communications. However, the nonlinear propagation behavior of multi-frequency acoustic waves radiated from nonlinear vibrating structures remains insufficiently understood. This study develops a coupled structural-acoustic model to numerically investigate the nonlinear acoustic radiation and propagation from a hyperelastic structure undergoing nonlinear vibration. Both geometric and material nonlinearities are incorporated into the finite element model of the structure, while nonlinear acoustic propagation in the surrounding fluid is solved using a high-order finite-difference time-domain scheme. The fluid-structure coupling is achieved through an improved immersed boundary method, which ensures the implicit satisfaction of compatibility conditions at the interface. Based on this framework, the nonlinear transient acoustic responses of the hyperelastic structure are examined, and the effects of the fluid nonlinearity parameter B/A and excitation amplitude on multi-frequency wave interactions are systematically analyzed. The numerical results demonstrate that the multi-frequency response induced by structural nonlinearity provides the necessary foundation for nonlinear wave-wave interactions, which, in turn, redistribute energy in different frequency components and modify the spatial patterns of high-order acoustic harmonics.

The elastodynamic equations in multiphase porous media are fundamental to understanding acoustic wave propagation in subsurface environments. This paper presents a formulation of the dynamic equations of motion for partially saturated porous media derived directly from the principle of energy conservation. The model incorporates three distinct scales of wave-induced fluid flow-macroscopic, mesoscopic, and squirt flow-through explicit kinetic, potential, and dissipation energy density functions. Based on the derived equations, the paper analyzes the dispersion and attenuation characteristics of wave propagation under multiscale effects and examine the influence of key parameters. Numerical results reveal that mesoscopic flow introduces an additional dispersion band and attenuation peak for the P1 wave in the low-frequency range (seismic band), while squirt flow produces similar effects at high frequencies (ultrasonic band). The S1 wave exhibits a dispersion transition and attenuation peak only due to squirt flow at high frequencies. The slow waves (P2 and P3) show minimal attenuation at high frequencies and remain largely unaffected by squirt flow. The model predictions are validated against experimental data from partially saturated Berea sandstone, demonstrating good quantitative agreement. This energy-conservation-based framework provides a unified, physically grounded approach for modeling multiscale wave-induced fluid flow in partially saturated porous media.

We investigate the influence of realistic sheared grazing flow on acoustic propagation in three-dimensional rectangular ducts. We show that the conclusions reached in the literature about the effects of sheared grazing flow on acoustic propagation in lined ducts are dependent on the flow profiles used in those studies and that significantly different conclusions are reached once a realistic flow profile is used. We particularly focus on small ducts typical of most experimental impedance eduction facilities, for which velocity gradients are relevant in a significant fraction of the duct cross section. We assess the effect of simplifying the velocity distribution in the cross section to either a one-dimensional (two-dimensional, spanwise-infinite duct) or a uniform flow profile. Three flow profiles are considered, namely, i) the tensorized hyperbolic tangent, ii) the law of the wall, and iii) one obtained from a RANS simulation. These flow profiles are used as input in numerical simulations, based on the solution of the three-dimensional Pridmore–Brown equation, to perform in silico impedance eduction experiments. Results show that realistic flow profiles can be well approximated for acoustic wave propagation in ducts by uniform or one-dimensional flow profiles, provided that the bulk Mach number is correctly accounted for, which contrasts with previous findings considering more simplistic flow profiles. The key conclusion of this work is that if viscous effects are negligible and acoustic impedance is a good representation of a lined wall with grazing flow, then the simplification to a uniform flow is a reasonable approximation, and traditional eduction methods are sufficiently accurate.

Phononic crystal-based sensors have emerged as highly promising platforms for precise temperature monitoring due to their ability to manipulate acoustic wave propagation through engineered bandgaps. In this work, a 1D phononic crystal composed of alternating layers of tungsten and polycrystalline silicon is systematically investigated in both periodic and quasiperiodic configurations. The study aims to comparatively evaluate periodic and quasiperiodic architectures- including Fibonacci, Thue-Morse, double-periodic, and Cantor sequences, to identify an optimal structural arrangement that maximizes bandgap width and enhances sensing performance. The simulation upshots revealed that the Fibonacci quasiperiodic configuration exhibits the widest Phononic band gap, reaching 18 × 106 Hz at an operating temperature of 373 K. Meanwhile, the sensor performance is assessed in terms of temperature sensitivity, where the periodic structure demonstrates a stable and linear response over the investigated temperature range, with a maximum sensitivity of 62.5 Hz/K at 373 K. To evaluate practical feasibility, fabrication tolerances are incorporated by considering up to 5% deviations and material property disorders. Additionally, Monte Carlo simulations are employed to analyze the robustness of the transmission spectrum under such uncertainties. In this regard, the investigated results highlight the trade-off between enhanced bandgap characteristics in quasiperiodic structures and the superior stability of periodic configurations, providing valuable insights for the design of high-performance phononic crystal sensors.

• Comparative analysis of Finite Difference, Finite Volume, and Spectral Elements methods for full 3D time-domain acoustic wave propagation modelling. • Benchmarks ranging from simplified geometries that enable analytical comparison, to complex heterogeneous domains. • Implementation of a dedicated Finite volume-based acoustic solver in OpenFOAM with absorbing boundaries. • Comparison of omnidirectional and directional sources to analyze directivity effects on the resulting acoustic wave filed. • Best applicability range of each numerical method for near- and far- field acoustic prediction. A comparative study of three numerical methods - Finite Difference (FD), Finite Volume (FV), and Spectral Element Method (SEM) - for modeling underwater acoustic propagation is presented. The time-domain acoustic wave equation is solved using an in-house FD code, the open-source SPECFEM3D software for SEM, and a newly developed FV-based acoustic solver implemented and released within the OpenFOAM framework, extending a software environment traditionally used for computational fluid dynamics to underwater acoustics applications. The methods are systematically assessed through benchmark problems, ranging from homogeneous unbounded and semi-infinite domains to the Pekeris waveguide and a Gaussian canyon. Comparisons with analytical solutions demonstrate that all solvers accurately reproduce monopole and dipole radiation in simplified configurations. However, the analysis reveals that directional sources introduce non-trivial numerical sensitivities, even in simple environments. These effects manifest as spurious reflections and dispersion-related distortions, whose severity depends on the source implementation and the numerical scheme. The results show that SPECFEM3D generally provides the highest accuracy and robustness in heterogeneous and geometrically complex environments, while the in-house FD code and FV-based solver are more sensitive to dispersion but can recover accuracy through increased spatial resolution. Strategies to mitigate source-related artifacts, such as non-reflective hard sources and reduced source regions, are discussed. A preliminary investigation of moving sources highlights their straightforward implementation in FD and FV solvers, while requiring additional care within the SPECFEM3D framework. Overall, this work provides practical guidance on the accuracy, robustness, and applicability of different solvers for simulating underwater noise in near- and far-field conditions, while laying the ground for future source–propagation coupling within acoustic analogy frameworks in OpenFOAM.

The Jeans instability is a fundamental mechanism driving the gravitational collapse and subsequent structure formation in diverse self-gravitating astrophysical environments. We present comprehensive numerical fluid simulations of the Jeans instability in a three component dusty plasma system. The high-energetic nonthermal electrons and ions are considered to follow κ distribution in velocity space with inertial dust as cold fluid. A Gaussian-type of initial density perturbation is introduced in the equilibrium density to initiate the simulations. The effects of the self-gravity parameter (α_{G}) and nonthermal spectral index (κ) on the Jeans instability have been investigated by tracking the evolution of plasma parameters in the simulations. In the absence of gravity (α_{G}=0), the system exhibits stable dust acoustic wave propagation with no Jeans instability growth, confirming pressure-dominated dynamics. Whereas, for α_{G}>0, the density increases exponentially at center of plasma system manifesting localized collapse. The nonlinear growth rates estimated from our simulations increase with α_{G} and show good agreement with the linear theory, particularly for lower α_{G} values. Simulations further demonstrate that the nonlinear growth rate is weakly dependent on κ; however, lower κ values significantly decrease the characteristic collapse time (τ_{c}). These findings establish that nonthermal (non-Maxwellian) environments in protoplanetary disks and molecular clouds are more efficient at catalyzing rapid structure formation than previously predicted by traditional thermal (Maxwellian) models.

Abstract Thermoacoustic tomography (TAT) is an imaging technique based on the thermoacoustic effect, combining electromagnetic contrast and high resolution of ultrasound imaging. In TAT, a short micro- or radio wave pulse is directed to the imaged target. Energy of this pulse is absorbed depending on the dielectric parameters of the target, resulting in a spatially varying pressure distribution via the thermoacoustic effect. This pressure, known as the initial pressure distribution, propagates as acoustic waves that are measured on the boundary of the target using ultrasound sensors. In the inverse problem of TAT, the initial pressure is estimated from the measured ultrasound waves. TAT can further be extended to quantitative TAT (QTAT), where the aim is to estimate the dielectric parameters of the target from the measured ultrasound waves, utilizing a model for electromagnetic wave propagation. In this work, we study the inverse problem of QTAT, and propose an approach for simultaneous estimation of electrical conductivity and permittivity from the ultrasound waves. This problem is approached in the framework of Bayesian inverse problems, enabling incorporation of prior and noise models. The forward model describing electromagnetic and acoustic wave propagation is based on the Maxwell's equations and the acoustic wave-equation, respectively. The approach is evaluated with numerical simulations. The results show that the dielectric parameters can be estimated using the proposed approach with good precision. However, the ultrasound sensor geometry and the number of electromagnetic pulses have a significant effect on the accuracy of the estimated parameters.

Ultrasound image degradation in the human body arises from the propagation and reflection of acoustic waves in a complex acoustic environment. The heterogeneous distribution of soft tissue and the variation in acoustic properties distort the ultrasonic beam causing deterioration in image quality, including loss of resolution and contrast. Here, we establish a framework to construct images based on a separable (additive or multiplicative) representation of aberration, multiple reverberation, and trailing clutter. A separable approach enables high modularity and flexibility when generating quantitatively degraded image datasets. This framework provides the capability to generate images with quantitative levels of image degradation related directly to imaging physics, thus allowing for a flexible approach to augmentation techniques in ultrasound imaging datasets, as demonstrated in the included repository code. Experimentally calibrated abdominal simulations were performed in Fullwave2 by matching relevant imaging metrics, such as phase aberration, reverberation strength, speckle brightness, and coherence length, to experimental measurements. Then, simulations were performed to separate and characterize the different components of image degradation. Finally, these components were scaled and combined to construct quantitatively degraded image datasets. Reverberation is shown to be depth and brightness dependent, while aberration and trailing clutter are not. This general framework was tested for values in acoustic ranges that significantly, synthetically, and independently enhance or reduce these effects compared to the levels naturally occurring in the body. Identifying, quantifying, and modeling these differing and complex mechanisms of degradation can be used to develop and test rational approaches to overcome these degradation mechanisms to improve image quality, particularly for traditionally harder to image patients. Additionally, the framework to synthetically modify the effects of aberration, multiple reverberation, and trailing clutter is provided, allowing for the generation of augmented datasets with a wide range of degradation effects, based on imaging physics, to improve machine learning models.

This work is devoted to theoretical and experimental studies of acoustic waves in air-saturated fractal porous media. The theoretical model of oscillatory flow through a porous medium is based on the concept of a fractal continuum in a non-integer dimensional space. This model allows us to obtain analytical solutions for the velocity field and frequency-dependent permeability. Accordingly, static and dynamic permeability are expressed as functions of fractal dimension numbers and characteristic length scales. To validate theoretical findings, we conducted two independent acoustic experiments with polyurethane open-cell foams having a fractal pore structure. High-frequency ultrasound measurements (in the range of tens to hundreds of kHz) were used to evaluate the fractal properties of the studied foams. The fractal parameters were determined by fitting the signals transmitted from the foam to theoretical output signals. These parameters were then used to predict the static permeability k0. We found that the predicted values of k0 are in good agreement with the experimental values obtained in low-frequency long acoustic waveguide (a few hundred Hz range) experiments. Thus, this work provides a robust and physically meaningful framework for describing acoustic wave propagation in fractal porous materials.

High-fidelity three-dimensional (3D) wave solvers accurately simulate acoustic wave propagation in complex vocal tract geometries but are computationally demanding, limiting their usage in real-time applications. In contrast, low-dimensional models are efficient but limited to cylindrical tracts, neglecting higher-order modes in their frequency responses. This paper introduces a lightweight lumped two-dimensional (2.5D) solver that combines the efficiency of low-dimensional models with the accuracy of 3D approaches to model straight tracts constrained to mid-sagittal symmetry. Like 3D, the 2.5D model captures transverse wave propagation and accounts for higher-order modes. We validate the model by comparing its transfer functions and pressure distributions against those of a conventional two-dimensional (2D) solver and a high-fidelity 3D finite element model for six straight tract geometries of varying complexity. This analysis demonstrates the abilities and limitations of the proposed method. The results show that the 2.5D solver closely matches the 3D model's transfer functions up to 12 kHz, with correlation coefficients exceeding 0.8 for symmetric tracts. For asymmetric geometries, it still performs significantly better than the 2D model. Additionally, the 2.5D solver achieves over two orders of magnitude computational speed-up compared to the 3D model, offering a better trade-off between accuracy and efficiency for vocal tract acoustic modeling.

This paper presents a numerical method for reconstructing the spatial distribution of sound speed in inhomogeneous media based on the inverse analysis of acoustic wave propagation. The mathematical model relies on the second-order wave equation with variable coefficients. The inverse problem is formulated as an optimization task to minimize the residual functional between simulated and observed pressure data at the domain boundaries. To efficiently calculate the gradient of the functional, an adjoint (auxiliary) problem method is employed, derived via variational calculus. The numerical implementation is performed using an explicit finite-difference scheme. Computational experiments on a one-dimensional model of a heterogeneous medium (soil-metal-soil) demonstrate that the proposed algorithm allows for reliable reconstruction of the velocity profile, particularly in zones of sharp contrast. The study analyzes the sensitivity of the solution and the convergence rate, showing that 500 iterations provide an optimal balance between accuracy and computational cost.

Noise pollution has become a significant issue in modern society, with long-term exposure to high-volume environments considerably increasing the risk of hearing damage. Based on Coupled-Mode Theory, this paper proposes an active noise cancellation (ANC) headphone system that integrates acousto-electric coupling modeling and artificial intelligence-based voice enhancement.By establishing a coupled model of acoustic wave propagation and electronic signal processing, and employing the Finite Difference Method and Runge—Kutta algorithm for system simulation, precise noise cancellation is achieved. In terms of hardware, the system uses an ESP32-S3 as the main control unit, integrated with feedforward and feedback microphone structures and an A-29P intelligent voice processing module to achieve active suppression of ambient noise and real-time extraction of human voice. Experimental results demonstrate that the system effectively reduces the risk of hearing impairment even in high-noise environments and significantly improves speech communication clarity.

Using time-resolved optical spectroscopy, we investigate the acoustic dynamics of polycrystalline germanium telluride (GeTe) nanoscale thin films, a multifunctional material combining phase-change behavior for nonvolatile memory and reconfigurable photonics with promising thermoelectric performance. We probe the generation and propagation of longitudinal acoustic waves with submicrometer wavelengths and frequencies in the range of tens of gigahertz. Our experimental approach integrates phonon echo measurements in thicker layers (∼360 nm) with the analysis of fundamental thickness-breathing modes in thinner layers (∼80 nm), enabling separation of intrinsic and interface contributions to attenuation. This allows the direct extraction of both intrinsic acoustic attenuation and longitudinal sound velocity in polycrystalline GeTe thin films. We determine mean free paths of approximately 0.7-7 μm for 10-20 GHz acoustic waves. These findings clarify hypersonic phonon dynamics in GeTe, establish a benchmark for high-frequency acoustic dissipation in chalcogenide thin films, and support the development of devices ranging from phase-change memory to thermoelectrics.

The fabrication of lightweight structures increasingly depends on adhesively bonded joints because they permit flexible designs and distribute loads more uniformly. However, attaining a perfect defect-free bond is very important, as any mismatches during fabrication or improper adhesives curing can cause adhesive deficiencies in the interfaces. Such adhesive deficiencies may adversely affect the reliability of adhesive-bonded lap joints. This study investigates the influence of adhesive deficiencies in single-lap joints made of single and dual-adhesives between different substrates were investigated. Araldite-2015 was utilized as the exclusive bonding agent in single-adhesively bonded joints, while in dual-adhesives configuration, Araldite-2015 was applied at the ends and AV138 was located in the middle segment of the bonded region. Adhesive deficiencies of different sizes were introduced by inserting Poly Tetra Fluoro Ethylene (PTFE) films at the central interface of the aluminum substrate and adhesive. X-ray radiographic and acoustic wave propagation methods were utilized to detect and characterize the adhesive deficiency portions. The defect-free single-adhesive joint exhibited 8.01% lower strength than the dual-adhesive joint. With increasing adhesive deficiencies (20% and 40%), strength reductions of 14.89% and 33.06% were observed for single-adhesive joints compared to 10.47% and 29.78% for dual-adhesive joints. These results clearly demonstrate the progressive degradation in joint strength with increasing defect size and dual-adhesives configurations exhibited reduced sensitivity to the effects of adhesive deficiencies compared to single-adhesively bonded joints indicating improved damage tolerance and reliability in dual-adhesives systems.