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On 13–15 January 2022, the Hunga Tonga–Hunga Ha'apai underwater volcano erupted. This powerful eruption generated infrasonic waves with amplitudes of thousands of Pascals in the near field. The ground infrasonic stations in China, located approximately 10 000 km from the Hunga volcano, also received waves with frequencies from 0.01 to 0.05 Hz. However, the amplitude reached 17 Pa, which is higher than the predicted amplitude using the absorption model without considering the dispersion effect in the thin thermosphere. At high altitudes, dispersion exists and the sound speed depends on the ratio of the molecular mean collision ratio to sound frequency, which is proportional to the ratio (frequency/pressure). And attenuation coefficients are complex to model. We simulate dispersive sound speeds and attenuation coefficients at different frequencies according to theory and our experimental data. In the thermosphere, the dispersion effect causes noticeable changes of sound speed and then affects wave propagation paths in the far field. The abnormal attenuation coefficient has a smaller impact on thermospheric returns than that of the dispersive sound speed, but it is also not negligible. It explains the large amplitude of thermospheric signals received in our infrasound stations. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'. [ABSTRACT FROM AUTHOR]

The orchestra pit is not a friendly workplace for musicians, and the need to ensure their interaction with the stage and audience places very high demands on this interior. The aim of the considerations presented in the article was to analyse the various possibilities of acoustic adaptation of the orchestra pit against the background of obtaining the expected effects. The conducted simulation studies and the use of the results of the experimental studies carried out in the well-equipped orchestra pit of the Krakow Opera allowed for the indication of representative acoustic parameters useful for the assessment of the results of acoustic treatment. Based on the comparison of the reverberation time T20 and EDT for various variants of the orchestra pit interior, the confirmation of the legitimacy of using sound diffusing and absorbing systems was obtained. They spread the energy of the first sound reflections in time and reduce the risk to the musicians' hearing. Reflective elements and sound dispersing within the orchestral will ensure adequate audibility between the musicians. [ABSTRACT FROM AUTHOR]

Monitoring pipeline thinning and material degeneration is becoming important for water-filled pipeline condition assessment. In this paper, an inverse method is proposed for estimating a pipeline's dimensional and material parameters using the dispersion characteristics of its modal wavenumbers. The inverse method is established by matching observed wavenumber dispersion characteristics of the water-filled pipeline with forward model predictions, where pipeline inner radius, thickness and density, and longitudinal and transverse wave speeds of the pipeline wall material are taken as unknown parameters. To account for the strong nonlinearity of the inverse problem and improve inversion efficiency, a Bayesian inversion scheme is formulated using a parallel-tempering Markov chain Monte Carlo approach. The characteristics and the performance of the proposed inverse method are investigated by systematic simulations which cover the impact of the number of modes utilized, dispersion frequency interval and observation errors. Laboratory experiments are utilized to validate the inversion method using wavenumber dispersion observations (below 50 kHz) from three metallic pipelines all with the same outer radius but different wall thicknesses and materials. The uncertainties of the estimated dimensional parameters are found to be lower than 0.2 mm and different materials are successfully distinguished and identified for the three pipelines. [ABSTRACT FROM AUTHOR]

With the proposal of the concept of parity-time-symmetry, more and more exotic properties are being unearthed in non-Hermitian systems. In this work, we investigate the related acoustic feature and phenomenon in two-dimensional phononic crystals with a complex modulus by introducing the balanced loss and gain. Different from Hermitian properties, partial band degeneracy has occurred in the dispersion diagram, leading to higher density of states, and self-collimated wave propagation has been predicted at the merged frequency. More importantly, the input locations of the point source have a significant impact not only on the direction of self-collimation waves but also on the intensity profiles of the pressure field, which is not achieved in self-collimation of Hermitian systems. We also propose a digital single-pole double-throw switch, which corresponds to two special self-collimating situations. Our finding provides an effective approach for controlling the wave direction and designing more digital acoustic functional devices in non-Hermitian systems. [ABSTRACT FROM AUTHOR]

The establishment of a soundscape baseline might allow the rapid detection of significant environmental changes and could potentially be a holistic instrument for biome conservation. Due to the diversity and variability of the sounds produced in an ecosystem, the establishment of a baseline is complex. Thus we evaluated if, in an ever-changing soundscape, it was possible to find dominant acoustic features on a monthly and hourly scale. Dominant power spectrums (PSs) in an area of premontane moist forest were identified by correlating PSs from February to December 2015, calculating the linear correlation between them, and defining a set of PSs that were strongly correlated (Pearson > = 0.95) to most of the obtained PSs. For any two PSs, the Pearson coefficient > = 0.95 accounted for an equivalence relation between the PSs; these relations allowed us to group spectrums into a few sets. In the daytime, 7.3% (eight out of 109 PSs) of the PSs (i.e., the dominant spectrums) were strongly correlated to 80.7% of the obtained PSs; the remaining 19.3% were singular PSs, weakly correlated to other diurnal PSs. During the night, 6.06% (four out of 66 PSs) of the PSs (i.e., the dominant spectrums) were strongly correlated to 53.0% of the obtained PSs; the remaining 47.0% were singular PSs, weakly correlated to other nocturnal PSs. Strong correlations between the PSs on an hourly scale and monthly scale could be used to denote features that prevail over time. The occurrence of strong associations between PSs (i.e., > 0.98) in spectrums from different months suggests that the generation of sound in the studied forest has a well-defined frequency distribution. [ABSTRACT FROM AUTHOR]

Reliable species identification is vital for survey and monitoring programs. Recently, the development of digital technology for recording and analyzing vocalizations has assisted in acoustic surveying for cryptic, rare, or elusive species. However, the quantitative tools that exist for species differentiation are still being refined. Using vocalizations recorded in the course of ecological studies of a King Rail (Rallus elegans) and a Clapper Rail (Rallus crepitans) population, we assessed the accuracy and effectiveness of three parametric (logistic regression, discriminant function analysis, quadratic discriminant function analysis) and six nonparametric (support vector machine, CART, Random Forest, k‐nearest neighbor, weighted k‐nearest neighbor, and neural networks) statistical classification methods for differentiating these species by their kek mating call. We identified 480 kek notes of each species and quantitatively characterized them with five standardized acoustic parameters. Overall, nonparametric classification methods outperformed parametric classification methods for species differentiation (nonparametric tools were between 57% and 81% accurate, parametric tools were between 57% and 60% accurate). Of the nine classification methods, Random Forest was the most accurate and precise, resulting in 81.1% correct classification of kek notes to species. This suggests that the mating calls of these sister species are likely difficult for human observers to tell apart. However, it also implies that appropriate statistical tools may allow reasonable species‐level classification accuracy of recorded calls and provide an alternative to species classification where other capture‐ or genotype‐based survey techniques are not possible. Reliable species identification is vital for survey and monitoring programs. We assessed the accuracy and effectiveness of three parametric and six nonparametric statistical classification methods for differentiating between the vocalizations of King and Clapper rails. This work suggests that appropriate statistical tools may allow reasonable species‐level classification accuracy of recorded calls and provide an alternative to species classification where other capture‐ or genotype‐based survey techniques are not possible. [ABSTRACT FROM AUTHOR]

Acoustic scattering by a single bubble is the most basic problem for bubble detection. For a two-phase flow, the subject of forced oscillations of a pair of bubbles is important because it controls how bubbles interact with each other. Attempts to incorporate interactions between bubbles have traditionally used a multiple scattering methodology. The proposed approach uses a bi-spherical coordinate system and is limited to a description of a sufficiently long-wave acoustical field, so that the bubbles are homobaric, and the medium in the vicinity of the bubbles can be considered incompressible. The choice of a specific coordinate system allows the authors to take into account the internal symmetry inherent in this problem and provides a partial summation on only the most important contributions to the multiple scattering series. A closed form solution was derived for the scattered acoustic field that determines its parametric dependence on bubbles sizes and the separation distance. [ABSTRACT FROM AUTHOR]

Novel acoustic liner designs often incorporate new materials with non-uniform impedance distributions. Therefore, new methods are required for their modelling and analysis. In this paper, a theoretical model is developed to investigate the scattering of sound waves from an axially symmetrical flow duct with a semi-infinite, azimuthally non-uniform acoustic lining on the duct wall. More specifically, the incorporation of Fourier series expansions into the Wiener–Hopf method leads to an analytical model with a matrix kernel, which is further factorised by using the pole-removal method to obtain a closed-form solution. A new mathematical method is developed to solve the residues associated with the pole-removal technique. The proposed model has been verified and validated by comparing with corresponding computational results. In addition to shedding light on the possible physical effect of azimuthally non-uniform liners along with an axial hard–soft interface, the current model enhances the theoretical modelling capability for a complicated set-up of practical importance, and can be used to investigate new liner designs for passive noise control in flow ducts. [ABSTRACT FROM AUTHOR]

Dispersion Theory [DT; Liljencrants and Lindblom (1972). Language 12(1), 839-862] claims that acoustically dispersed vowel inventories should be typologically common. Dispersion is often quantified using triangle area between three mean vowel formant points. This approach is problematic; it ignores distributions, which affect speech perception [Clayards, Tanenhaus, Aslin, and Jacobs (2008). Cognition 108, 804-809]. This letter proposes a revised metric for calculating dispersion which incorporates covariance. As a test case, modeled vocal tract articulatory-acoustic data of stop consonants [Schwartz, Boe, Badin, and Sawallis (2012). J. Phonetics 40, 20-36] are examined. Although the revised metric does not recover DT predictions for stop inventories, it changes results, showing that dispersion results depend on metric choice, which is often overlooked. The metric can be used in any acoustic space to include information about within-category variation when calculating dispersion. [ABSTRACT FROM AUTHOR]

This work presents a novel multiscale semi-analytical technique for the acoustic plane wave analysis of (negative) dynamic mass density type local resonance metamaterials with complex micro-structural geometry. A two step solution strategy is adopted, in which the unit cell problem at the micro-scale is solved once numerically, whereas the macro-scale problem is solved using an analytical plane wave expansion. The macro-scale description uses an enriched continuum model described by a compact set of differential equations, in which the constitutive material parameters are obtained via homogenization of the discretized reduced order model of the unit cell. The approach presented here aims to simplify the analysis and characterization of the effective macro-scale acoustic dispersion properties and performance of local resonance metamaterials, with rich micro-dynamics resulting from complex metamaterial designs. First, the dispersion eigenvalue problem is obtained, which accurately captures the low frequency behavior including the local resonance bandgaps. Second, a modified transfer matrix method based on the enriched continuum is introduced for performing macro-scale acoustic transmission analyses on local resonance metamaterials. The results obtained at each step are illustrated using representative case studies and validated against direct numerical simulations. The methodology establishes the required scale bridging in multiscale modeling for dispersion and transmission analyses, enabling rapid design and prototyping of local resonance metamaterials. [ABSTRACT FROM AUTHOR]

The Fourier Pseudospectral time-domain (Fourier PSTD) method was shown to be an efficient way of modelling acoustic propagation problems as described by the linearized Euler equations (LEE), but is limited to real-valued frequency independent boundary conditions and predominantly staircase-like boundary shapes. This paper presents a hybrid approach to solve the LEE, coupling Fourier PSTD with a nodal Discontinuous Galerkin (DG) method. DG exhibits almost no restrictions with respect to geometrical complexity or boundary conditions. The aim of this novel method is to allow the computation of complex geometries and to be a step towards the implementation of frequency dependent boundary conditions by using the benefits of DG at the boundaries, while keeping the efficient Fourier PSTD in the bulk of the domain. The hybridization approach is based on conformal meshes to avoid spatial interpolation of the DG solutions when transferring values from DG to Fourier PSTD, while the data transfer from Fourier PSTD to DG is done utilizing spectral interpolation of the Fourier PSTD solutions. The accuracy of the hybrid approach is presented for one- and two-dimensional acoustic problems and the main sources of error are investigated. It is concluded that the hybrid methodology does not introduce significant errors compared to the Fourier PSTD stand-alone solver. An example of a cylinder scattering problem is presented and accurate results have been obtained when using the proposed approach. Finally, no instabilities were found during long-time calculation using the current hybrid methodology on a two-dimensional domain. [ABSTRACT FROM AUTHOR]

Modal pulses are broadband contributions to an acoustic wave field with fixed mode number. Stable weakly dispersive modal pulses (SWDMPs) are special modal pulses that are characterized by weak dispersion and weak scattering-induced broadening and are thus suitable for communications applications. This paper investigates, using numerical simulations, receiver array requirements for recovering information carried by SWDMPs under various signal-to-noise ratio conditions without performing channel equalization. Two groups of weakly dispersive modal pulses are common in typical mid-latitude deep ocean environments: the lowest order modes (typically modes 1–3 at 75 Hz), and intermediate order modes whose waveguide invariant is near-zero (often around mode 20 at 75 Hz). Information loss is quantified by the bit error rate (BER) of a recovered binary phase-coded signal. With fixed receiver depths, low BERs (less than 1%) are achieved at ranges up to 400 km with three hydrophones for mode 1 with 90% probability and with 34 hydrophones for mode 20 with 80% probability. With optimal receiver depths, depending on propagation range, only a few, sometimes only two, hydrophones are often sufficient for low BERs, even with intermediate mode numbers. Full modal resolution is unnecessary to achieve low BERs. Thus, a flexible receiver array of autonomous vehicles can outperform a cabled array. [ABSTRACT FROM AUTHOR]

The aim of this work is to derive sharp quantitative estimates of the qualitative convergence results developed by Li et al. (2015) for regularized full- and partial-cloaks through transformation optics approach. LetΓ0be a compact set in ℝ3andΓδbe aδ-neighborhood ofΓ0forδ∈ℝ+.Γδrepresents the virtual domain used for the blow-up construction. By incorporating suitably designed lossy layers, it is shown that if the generating setΓ0is a generic curve, then one would have an approximate full-cloak withinδ2to the perfect full-cloak, whereas ifΓ0is the closure of an open subset on a flat surface, then one would have an approximate partial-cloak withinδto its perfect counterpart. The estimates derived are independent of the contents being cloaked; that is, the cloaking devices are capable of nearly cloaking an arbitrary content. Furthermore, as a significant by-product, our argument allows the relaxation of the convexity requirement onΓ0by Li et al. (2015), which is critical for the Mosco convergence argument therein. [ABSTRACT FROM PUBLISHER]

In this study, the acoustic dissipation is investigated experimentally in wooden pipes of different species commonly used in woodwind instrument making: maple (Acer pseudoplatanus), pear wood (Pyrus communis L.), boxwood (Buxus sempervirens), and African Blackwood (Dalbergia melanoxylon). The pipes are parallel to the grain, except one which forms an angle of 60° with the fiber direction. An experimental method, involving input impedance measurements with several lengths of air column, is introduced to estimate the characteristic impedance and the attenuation factor in the pipes. Their comparison reveals significant differences of acoustic dissipation among the species considered. The attenuation factors are ranked in the following order from largest to smallest: maple, boxwood, pear wood, and African Blackwood. This order is the same before and after polishing the bore, which is an essential step in the making process of wind instrument. For maple, changing the pipe direction of 60° considerably increases the attenuation factor, compared to those of the other pipes, parallel to the grain. Further, polishing tends to reduce the acoustic dissipation in the wooden pipes, especially for the most porous species. As a result, the influence of polishing in the making procedure depends on the selected wood species. [ABSTRACT FROM AUTHOR]

The relative importance of suspended particles and turbulence as backscattering mechanisms within a hydrothermal plume located on the Endeavour Segment of the Juan de Fuca Ridge is determined by comparing acoustic backscatter measured by the Cabled Observatory Vent Imaging Sonar (COVIS) with model calculations based on in situ samples of particles suspended within the plume. Analysis of plume samples yields estimates of the mass concentration and size distribution of particles, which are used to quantify their contribution to acoustic backscatter. The result shows negligible effects of plume particles on acoustic backscatter within the initial 10-m rise of the plume. This suggests turbulence-induced temperature fluctuations are the dominant backscattering mechanism within lower levels of the plume. Furthermore, inversion of the observed acoustic backscatter for the standard deviation of temperature within the plume yields a reasonable match with the in situ temperature measurements made by a conductivity-temperature-depth instrument. This finding shows that turbulence-induced temperature fluctuations are the dominant backscattering mechanism and demonstrates the potential of using acoustic backscatter as a remote-sensing tool to measure the temperature variability within a hydrothermal plume. [ABSTRACT FROM AUTHOR]

Although transcranial photoacoustic imaging has been previously investigated by several groups, there are many unknowns about the distorting effects of the skull due to the impedance mismatch between the skull and underlying layers. The current computational methods based on finite-element modeling are slow, especially in the cases where fine grids are defined for a large 3-D volume. We develop a very fast modeling/simulation framework based on deterministic ray-tracing. The framework considers a multilayer model of the medium, taking into account the frequency-dependent attenuation and dispersion effects that occur in wave reflection, refraction, and mode conversion at the skull surface. The speed of the proposed framework is evaluated. We validate the accuracy of the framework using numerical phantoms and compare its results to k-Wave simulation results. Analytical validation is also performed based on the longitudinal and shear wave transmission coefficients. We then simulated, using our method, the major skull-distorting effects including amplitude attenuation, time-domain signal broadening, and time shift, and confirmed the findings by comparing them to several ex vivo experimental results. It is expected that the proposed method speeds up modeling and quantification of skull tissue and allows the development of transcranial photoacoustic brain imaging.

New insights on the applicability of Eringen’s nonlocal theory are introduced. Eringen’s nonlocal theory which assumes the same attenuation function for all material moduli is assessed according to its ability to reflect the exact dispersions of different materials. It is revealed that Eringen’s nonlocal theory fails to, simultaneously, fit both the longitudinal and transverse (shear) acoustic dispersion curves of some materials, such as Si, Au, and Pt. In order to extend the limits of applicability of Eringen’s nonlocal theory, a general form of the nonlocal theory is developed. This general nonlocal theory considers different attenuation functions for the distinct material moduli. This makes the proposed nonlocal theory easily fits both the longitudinal and transverse acoustic dispersion curves of different materials. Moreover, unlike Eringen’s conventional form of nonlocal theory, the general nonlocal theory can reflect both hardening and softening behaviors of the material. Another contribution in this study is the determination of the nonlocal parameters for various crystal materials by fitting their dispersions with the proposed nonlocal theory. Thus, the elastic moduli and the nonlocal parameters of metallic crystals (such as Au, Ag, Cu, and Pt), diamond structure crystals (such as diamond and Si), layered structure crystals (such as graphite), Metal Oxides and Alkali Halides with Rock-Salt structures (such as BaO, NiO, LiH and LiD), and Fluorite structure crystals (such as Mg2Sn and Mg2Pb) are reported. [ABSTRACT FROM AUTHOR]

Nonlinear phenomena of the planar and quasi-planar magnetoacoustic waves are considered. We focus on deriving of equations which govern nonlinear excitation of the non-wave motions by the intense sound in initially static gaseous plasma. The plasma is treated as an ideal gas with finite electrical conductivity permeated by a magnetic field orthogonal to the trajectories of gas particles. This introduces dispersion of a flow. Magnetoacoustic heating and streaming in the field of periodic and aperiodic magnetoacoustic perturbations are discussed, as well as generation of the magnetic perturbations by sound. Two cases, corresponding to magnetosound perturbations of low and high frequencies, are considered in detail. [ABSTRACT FROM AUTHOR]

Compact loudspeaker arrays have wide potential applications as portable personal audio systems that can project sound energy to specified regions. It is meaningful to investigate the scattering effects on the array performance since the scattering of the users' heads is inevitable in practice. A five-channel compact endfire array is established and the regularized acoustic contrast control method is evaluated for the scenarios of one moving listener and one listener fixed in the bright zone while another listener moves along the evaluation region. Both simulations and experiments verify that the scattering has limited influence on the directivity of the endfire array. [ABSTRACT FROM AUTHOR]

This Letter reports on the experimental validation of a two-dimensional acoustic hyperbolic metamaterial slab optimized to exhibit negative refractive behavior. The slab was designed using a topology optimization based systematic design method allowing for tailoring the refractive behavior. The experimental results confirm the predicted refractive capability as well as the predicted transmission at an interface. The study simultaneously provides an estimate of the attenuation inside the slab stemming from the boundary layer effects--insight which can be utilized in the further design of the metamaterial slabs. The capability of tailoring the refractive behavior opens possibilities for different applications. For instance, a slab exhibiting zero refraction across a wide angular range is capable of funneling acoustic energy through it, while a material exhibiting the negative refractive behavior across a wide angular range provides lensing and collimating capabilities. [ABSTRACT FROM AUTHOR]

Precise acoustical leak detection calls for robust time-delay estimates, which minimize the probability of false alarms in the face of dispersive propagation, multiple reflections, and uncorrelated background noise. Providing evidence that higher order modes and multi-reflected signals behave like sets of correlated noise, this work uses a regression model to optimize residual complexity in the presence of both correlated and uncorrelated noise. This optimized residual complexity (ORC) is highly robust since it takes into account both the level and complexity of noise. The lower complexity of the dispersive modes and multiple reflections, compared to the complexity of the plane mode, points to the robustness of ORC against multiple reflections and dispersion. Experimental investigations using recorded sounds of gas leaking from a pipe confirm the robustness of ORC against multiple reflections. Numerical simulations also show robustness against dispersive modes, even when they disturb the linearity of the cross-spectrum phase. Comparisons with other methods—mutual information, cross correlation, and residual complexity—underline the general advantages of ORC in terms of robustness in the presence of reflection and dispersion, against both correlated and uncorrelated noise, and to short signals. [ABSTRACT FROM AUTHOR]

Generic predictions for acoustic dispersion and absorption in the atmospheres of Mars and Venus are presented. For Mars, Pathfinder and Mars Express ambient data and averaged thermophysical parameters are used as inputs to a preliminary model based on the continuum approximation for Mars' thin atmosphere--the need for Boltzmann-based treatment is discussed in the context of Knudsen numbers. Strong absorption constrains acoustic sensing within the Martian planetary boundary layer. For the dense atmosphere of Venus, the van der Waals equation of state is used. The thermophysical and transport parameters were interpolated at the ambient conditions. Acoustic sensing is discussed at 50 km above Venus' surface, a level where aerostats (e.g., European Space Agency's EVE) and manned airships (e.g., NASA's HAVOC) may be deployed in the future. The salient atmospheric characteristics are described in terms of temperature, pressure, and convective stability profiles, followed by wavenumber predictions, and discussions of low- and high-frequency sensing applications. At low frequencies, emphasis is placed on infrasound. A simple generation mechanism by Martian dust devils is presented, yielding fundamental frequencies between 0.1 and 10 Hz. High-frequency sensing is exemplified by ultrasonic anemometry. Of the two environments, Venus is notably more dispersive in the ultrasonic range. [ABSTRACT FROM AUTHOR]

The extraordinary transmission in density-near-zero (DNZ) acoustic metamaterials (AMs) provides possibilities to manipulate acoustic signals with extremely large effective phase velocity and wavelength. Here, we report compact transformable acoustic logic gates with a subwavelength size as small as 0.82λ based on DNZ AMs. The basic acoustic logic gates, composed of a tri-port structure filled with space-coiling DNZ AMs, enable precise direct linear interference of input signals with considerably small phase lag and wavefront distortion. We demonstrate both theoretically and experimentally the basic Boolean logic operations such as OR, AND, XOR, and NOT with wide operational frequency ranges and controllability, by adjusting the phase difference between two input signals. More complex logic calculus, such as "I1+I2×I3," are also realized by cascading of the basic logic gates. Our proposal provides diverse routes to construct devices for acoustic signal computing and manipulations. [ABSTRACT FROM AUTHOR]

Acoustic rainbow trapping represents the phenomenon of strong acoustic dispersion similar to the optical “trapped rainbow,” which allows spatial-spectral modulation and broadband trapping of sound. It can be realized with metamaterials that provide the required strong dispersion absent in natural materials. However, as the group velocity cannot be reduced to exactly zero before the forward mode being coupled to the backward mode, such trapping is temporary and the local sound oscillation ultimately radiates backward. Here, we propose a gradient metasurface, a rigid surface structured with gradient perforation along the wave propagation direction, in which the inherent thermal and viscous losses inside the holes are considered. We show that the gradually diminished group velocity of the structure-induced surface acoustic waves (SSAWs) supported by the metasurface becomes anomalous at the trapping position, induced by the existence of the inherent losses, which implies that the system's absorption reaches its maximum. Together with the progressively increased attenuation of the SSAWs along the gradient direction, reflectionless spatial-spectral modulation and sound enhancement are achieved in simulation. Such phenomenon, which we call as absorptive trapped rainbow, results from the balanced interplay among the local resonance inside individual holes, the mutual coupling of adjacent unit cells, and the inherent losses due to thermal conductivity and viscosity. This study deepens the understanding of the SSAWs propagation at a lossy metasurface and may contribute to the practical design of acoustic devices for high performance sensing and filtering. [ABSTRACT FROM AUTHOR]

A method is proposed to evaluate in a non-contact way the phase velocity dispersion curves of circumferential waves around a shell of arbitrary shape immersed in a fluid. No assumptions are made about the thickness or the material of the shell. A geometrical model is derived to describe the shape of the radiated wavefronts in the surrounding fluid, and predict the positions of its centers of curvature. Then the time-reversal principle is applied to recover these positions and to calculate the phase velocity of the circumferential waves. Numerical finite-difference simulations are performed to evaluate the method on a circular and on an elliptic thin shell. Different dispersion curves can be recovered with an error of less than 10%. [ABSTRACT FROM AUTHOR]

A procedure and the experimental data of the dispersion dependences of the velocity and ultrasonic attenuation coefficient as measured by the resonance method in samples with a thickness below 1 mm at frequencies up to 100 MHz are presented. It is shown that structural inhomogeneities exert an influence on the acoustic properties of thin samples. [ABSTRACT FROM AUTHOR]

A model of the formation of a series of multiple reflections due to the propagation of acoustic waves that are radiated by an electromagnetic-acoustic trough transducer over the elliptical cross section of a bar is proposed. The impact of the deviation from a round cross section on the informative parameters of the obtained oscillogram is studied. [ABSTRACT FROM AUTHOR]

This paper presents experimental results for wave propagation in an anisotropic multilayered structure with linearly varying cross section. Knowing the dispersion and wave propagation properties in such a structure is of great importance for non-destructive material testing and structural health monitoring applications for accurate damage detection and localization. In the proposed study, the wavefield is generated by a circular piezoelectric wafer active sensor and measured by a scanning laser-Doppler-vibrometer. The measurements are compared with a theoretical group delay estimation and a signal prediction for the antisymmetric wave motion along the non-uniform propagation path. The required dispersion curves are derived from the well-known global matrix method for segments of constant thickness. A multidimensional frequency-wavenumber analysis of linescan data and the full wavefield provides further insight of the adiabatic wave motion because the wavenumber changes along the tapered geometry of the waveguide. In addition, it is demonstrated that a terahertz time-domain system can be used in glass-fiber reinforced plastic structures as a tool to estimate the thickness profile of thin structures by means of time-of-flight measurements. This information is particularly important for guided wave-based diagnostics of structures with unknown thickness. [ABSTRACT FROM AUTHOR]

This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section. [ABSTRACT FROM AUTHOR]

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

This paper presents a quantitative study of conversion of evanescent Lamb waves into propagating in isotropic plates. The conversion is substantiated by prescribing time-harmonic Lamb displacements/tractions through a narrow aperture at an edge of a semi-infinite plate. Complex-valued dispersion and group velocity curves are employed to characterize the conversion process. The amplitude coefficient of the propagating Lamb modes converted from evanescent is quantified based on the complex reciprocity theorem via a finite element analysis. The power flow generated into the plate can be separated into radiative and reactive parts made on the basis of propagating and evanescent Lamb waves, where propagating Lamb waves are theoretically proved to radiate pure real power flow, and evanescent Lamb waves carry reactive pure imaginary power flow. The propagating power conversion efficiency is then defined to quantitatively describe the conversion. The conversion efficiency is strongly frequency dependent and can be significant. With the converted propagating waves from evanescent, sensors at far-field can recapture some localized damage information that is generally possessed in evanescent waves and may have potential application in structural health monitoring. [ABSTRACT FROM AUTHOR]

The use of Global Navigation Satellite System (GNSS)-tracked Lagrangian drifters allows more realistic quantification of fluid motion and dispersion coefficients than Eulerian techniques because such drifters are analogs of particles that are relevant to flow field characterization and pollutant dispersion. Using the fast-growing real-time kinematic (RTK) positioning technique derived from GNSS, drifters are developed for high-frequency (10 Hz) sampling with position estimates with centimeter accuracy. The drifters are designed with small size and less direct wind drag to follow the subsurface flow that characterizes dispersion in shallow waters. An analysis of position error from stationary observation indicates that the drifter can efficiently resolve motion up to 1 Hz. The result of the field deployments of the drifter in conjunction with acoustic Eulerian devices shows a higher estimate of the drifter streamwise velocities. Single particle statistical analysis of field deployments in a shallow estuarine zone yielded estimates of dispersion coefficients comparable to those of dye tracer studies. The drifters capture the tidal elevation during field studies in a tidal estuary. [ABSTRACT FROM AUTHOR]

We describe a way to generate and detect arbitrary frequency components in time-resolved surface acoustic wave imaging based on optical pumping and probing with a periodic light source. The detailed theory of the technique, based on beam modulation and Fourier analysis, for a variety of possible experimental configurations is presented, followed by experimental data for a glass substrate covered with a thin gold film. We show how the acoustic dispersion relation can be obtained to arbitrary frequency resolution, not limited by the laser pulse repetition rate. [ABSTRACT FROM AUTHOR]

A bubble metascreen, i.e., a single layer of gas inclusions in a soft solid, can be modeled as an acoustic open resonator, whose behavior is well captured by a simple analytical expression. We show that by tuning the parameters of the metascreen, acoustic superabsorption can be achieved over a broad frequency range, which is confirmed by finite element simulations and experiments. Bubble metascreens can thus be used as ultrathin coatings for turning acoustic reflectors into perfect absorbers. [ABSTRACT FROM AUTHOR]

The estimation of slowness (the reciprocal of velocity) and attenuation dispersion of borehole acoustic and surface seismic data is key to a variety of applications. The emerging broadband approach for dispersion extraction of multiple modes has shown significant advantages over traditional narrowband approaches. In this letter, Cramér-Rao bounds (CRBs) are established to characterize the best achievable performance of any unbiased broadband estimator of the dispersion. One noteworthy observation from the derived CRBs is that the same estimation accuracy can be achieved between the angular wavenumber and the attenuation and, between the group slowness and the attenuation rate. The broadband CRB is also shown to include the narrowband CRB as a special case and it quantifies the performance benefit of the broadband approach. [ABSTRACT FROM AUTHOR]

Abstract: We study the effects of random perturbations on collective dynamics of a large ensemble of interacting cells in a model of the cell division cycle. We consider a parameter region for which the unperturbed model possesses asymptotically stable two-cluster periodic solutions. Two biologically motivated forms of random perturbations are considered: bounded variations in growth rate and asymmetric division. We compare the effects of these two dispersive mechanisms with additive Gaussian white noise perturbations. We observe three distinct phases of the response to noise in the model. First, for weak noise there is a linear relationship between the applied noise strength and the dispersion of the clusters. Second, for moderate noise strengths the clusters begin to mix, i.e. individual cells move between clusters, yet the population distribution clearly continues to maintain a two-cluster structure. Third, for strong noise the clusters are destroyed and the population is characterized by a uniform distribution. The second and third phases are separated by an order–disorder phase transition that has the characteristics of a Hopf bifurcation. Furthermore, we show that for the cell cycle model studied, the effects of bounded random perturbations are virtually indistinguishable from those induced by additive Gaussian noise, after appropriate scaling of the variance of noise strength. We then use the model to predict the strength of coupling among the cells from experimental data. In particular, we show that coupling must be rather strong to account for the observed clustering of cells given experimentally estimated noise variance. [Copyright &y& Elsevier]