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The superhydrophobicity of submerged surfaces typically pertains to the trapped air film at the liquid–solid interface, subject to wettability transitions from a Cassie–Baxter state to more unstable states that gradually collapse to high retention regimes, which are energetically more favorable. In this work, the dynamic evolution of those transient metastable states is correlated to the underwater acoustic performance of laser textured superhydrophobic surfaces, resolving the dependence of the ultrasound spectral response with the immersion time to capture the genuine contribution of the hierarchical subwavelength morphology, regardless of the air layer effects. Acoustic wave attenuation of the incident ultrasound energy is extensively quantified in transmission, accounting for instantaneous broadband sound blocking (>30 dB) within the spectral range 0.5–1.5 MHz. As a result of the air layer detachment with the immersion time, transmission coefficients increase accordingly, while acoustic fields in reflection unexpectedly evolve toward stealthiness and naïve acoustic camouflage, mostly ascribable to dissipative mechanisms at air layer interfaces. The intrinsic decay of the air layer effect is tentatively determined at different frequencies, since quantitative understanding of the transient lifetime governing underwater surface wettability is critical to design stable superhydrophobic character of laser induced subwavelength metastructures on the most promising acoustic materials – from eco‐friendly natural to artificial. [ABSTRACT FROM AUTHOR]

The attenuation of slow shear acoustic waves in the (110) plane of tellurium dioxide (TeO2) crystals was investigated. The strong acoustic anisotropy of TeO2 crystals results in a non-uniform acoustic power distribution, which can introduce errors in conventional acousto-optic testing methods. In this study, we propose a general method to measure the acoustic power distribution along the propagation direction of acoustic waves in non-collinear acousto-optic tunable filters (AOTFs). Additionally, we analyze the errors introduced by the non-uniform acoustic field resulting from strong acoustic anisotropy in acousto-optic testing methods. The measurements were carried out for a crystal cutoff angle of 6.5° from the [110] axis, for the ultrasound frequency range from 40 to 140 MHz. The attenuation coefficients were determined and their quadratic dependence on ultrasound frequency was confirmed. [ABSTRACT FROM AUTHOR]

The insert-pipe double expansion chamber muffler has less passbands and higher sound attenuation in the plane wave range, which is widely used to reduce noise of industrial equipment. But there are many parameters that affect the transmission loss of the insert-pipe double expansion chamber muffler, and the sensitivity of each factor is not clear. Identifying the most influential factor among numerous parameters is an important challenge. It is necessary to investigate the sensitivity of all parameters and study parameter optimization algorithms. Based on the method of transfer matrix analysis, this paper studies the insert-pipe double expansion chamber muffler and its sound elimination mechanism. The global sensitivity analysis of the transmission loss of insert-pipe double expansion chamber muffler was conducted using Sobol's method, and the influence of each parameter sensitivity was examined. According to the noise characteristics of an air conditioner compressor, the external penalty function method (EPFM) algorithm is used to optimize the insert-pipe double expansion chamber muffler, and the maximum transmission loss is obtained in a specific frequency band. According to the optimized parameters, the muffler is designed, manufactured, and installed in a pipeline system to carry out a noise experiment to verify the effectiveness of the optimization scheme. [ABSTRACT FROM AUTHOR]

Leaky Rayleigh wave testing has the advantages of non-contact, high sensitivity, and good beam directivity. However, current leaky Rayleigh wave testing mostly uses monolithic transducers and is seriously affected by acoustic wave attenuation and diffusion, resulting in poor image quality, limited detection range and low imaging efficiency. In this paper, a novel leaky Rayleigh wave testing method using an ultrasonic phased array is developed, and the total focusing method (TFM) is used to achieve high-quality imaging of defects. To improve imaging efficiency, the root-mean-square velocity (RMSV) model is utilised to rapidly acquire an approximate analytical solution for the time of flight of the acoustic wave. To obtain more accurate defect images further, the amplitude of the total focusing imaging signal is calibrated. The experimental results demonstrate that the proposed method achieves a 48% improvement in the imaging accuracy of a 1 mm hole, compared to the traditional Synthetic Aperture Focusing Technique (SAFT) imaging method. The proposed method achieves an 92% reduction in imaging time compared to the conventional TFM. The proposed method provides a new non-contact approach for high-accuracy and high-efficiency defect imaging using leaky Rayleigh wave. [ABSTRACT FROM AUTHOR]

WE REVISIT THE DIFFUSIVE COMPRESSIBLE EULER (DCE) MODEL OF VISCOUS and heat conducting compressible fluid flow, proposed recently by M. Svärd to supersede the Navier--Stokes--Fourier (NSF) equations. Here, we use acoustic measurements in gases and liquids from the literature to demonstrate that the dcE model fails to describe sound wave attenuation in general fluids with physical accuracy. It is shown, for example, that the dcE model underestimates the sound attenuation coefficients of air and water at room temperature by about 13% and 51%, respectively. [ABSTRACT FROM AUTHOR]

This document is a reply to a letter to the editor regarding a study on the use of ultrasound-guided attenuation parameter (UGAP) to evaluate hepatic steatosis. The study used a General Electric Healthcare (GE) LOGIQ E10 ultrasound device with UGAP technology, which measures the attenuation of sound waves in real-time. The study found that increased probe force and altered body position improved the diagnostic accuracy for detecting hepatic steatosis. The authors address concerns raised in the letter and provide evidence to support their findings. The study suggests that increased probe force and altered body position can be used in UGAP technology for diagnosing hepatic steatosis. [Extracted from the article]

To better combat the devastation of the protected forests in Côte d'Ivoire, a study was conducted to create a technique for detecting the acoustic signals produced by chainsaws deployed to fell trees in these areas. To improve the recognition rate of chainsaw sounds in a forest environment and increase the detection range of the recognition system, we are implementing the maximum ratio combining (MRC) technique on a microphone array. Therefore, the employment of an identification system is compared using one (01) microphone against the outcomes obtained by adopting system with three (03), six (06), and twelve (12) microphones. The use of MRC is then contrasted with an alternative recombining approach, designated as simple summation (SS). The SS is characterised by the simple addition of the signals acquired by the microphone array in the frequency domain. The MRC was employed on various microphone arrangements, accounting for varying degrees of attenuation experienced by chainsaw sounds. The K-Nearest Neighbors, in combination with Mel Frequency Cepstral Coefficients (MFCC), was employed to detect chainsaw sounds within the 16 kHz central frequency octave band. MRC applied to microphone arrays provided superior outcomes than simple summation. The enhancement in terms of classification rate ranged from [18; 51], favouring MRC. Moreover, it extended the chainsaw detection range from 520 m (using one microphone) to 1210 m (using a 12-microphone array). Taking into account the criteria for selecting an optimum microphone array, including classification rate, number of microphone nodes, information processing time and detection range, the six-microphone array was chosen as the best configuration. This configuration boasts a theoretical detection range of 1040 meters. [ABSTRACT FROM AUTHOR]

Acoustic attenuation coefficient per unit frequency square has been calculated for pure semiconductors like silicon and germanium, as well as for mixed semiconductors like gallium arsenide (III–V group) and tin telluride (IV–VI group), for longitudinal waves travelling along the crystallographic axis, within the temperature range 80–300 K. Second-order elastic constants of the materials are used to calculate the average Gruneisen parameter , using which the nonlinearity parameter DL has been calculated for different temperatures. The knowledge of DL is used for calculation of acoustic attenuation coefficient per unit frequency square due to phonon–phonon interaction and thermoelastic losses . Acoustic attenuations are found to be temperature dependent and increases with it. The magnitude of acoustic attenuation per unit frequency square due to phonon–phonon interactions is greater than that due to thermoelastic losses. The magnitude of acoustic attenuation for the investigated semiconductors is in the order Si < Ge < GaAs < SnTe. It is observed that anharmonicity of the solids which give rise to phonon–phonon interaction is the dominant cause of acoustic attenuation of propagating waves. The acoustic attenuation of propagating waves is found to be proportional to the molecular weights of semiconductors. [ABSTRACT FROM AUTHOR]

In this study, sound insulation materials with a high sound absorption coefficient were prepared. In this paper, using cellulose (CEL) and graphene oxide (GO) as the main raw materials and epichlorohydrin as the cross-linker, the CEL-GO composite aerogels were prepared via lyophilisation. The structure, molecular bonding, and acoustic absorption mechanisms of the composite aerogel were characterised and analysed using SEM, FTIR, XRD, BET, and Raman. In addition, corresponding molecular structure models were constructed. The acoustic attenuation of the CEL-GO composite aerogel was measured using a standing wave tube acoustic attenuation tester. The results show that the chemical bond between the GO and CEL composite is established, and the addition of graphene makes the pores of the composite more advanced, which is more favorable for sound absorption, and the acoustic absorption coefficient can reach up to 0.87. [ABSTRACT FROM AUTHOR]

Lamb waves propagating in the substrate of linear arrays integrated on a silicon (Si) chip may degrade the elementary performances of the imaging device. In fact, these waves are radiated in the imaging medium. Their superimposition with the relevant ultrasonic signals alters the image performances (i.e., lateral and axial resolutions). In this article, we investigate the interest of using a thin layer of porous silicon (PS) as an absorbing material, aiming to reduce the total device dimensions compared to more traditional backing materials and facilitate device integration with on-chip electronics. The proposed method was applied to Capacitive Micromachined Ultrasonic Transducers. To this purpose, a PS layer with a thickness of 60 μm and a porosity of 50% was etched on the rear side of a 256-elements linear array. The electroacoustic response of the elements integrated on the Si substrate was compared to those on the Si/PS substrate, showing no deterioration of the acoustic characteristics (i.e., center frequency and bandwidth) after PS layer fabrication. To assess the PS silicon layer influence on Lamb wave attenuation, acoustic cross-talks were measured for each array element. The radio-frequency dataset was used to determine the dispersion curves of Lamb waves in the substrate. The comparison between the two substrates showed a significant attenuation value (superior to 30 dB) of Lamb waves induced by the PS layer. [ABSTRACT FROM AUTHOR]

In this work, a field representation of the conservation law of linearmomentum is derived fromthe atomistic, using the theory of distributions as the mathematical tool, and expressed in terms of temperature field by defining temperature as a derived quantity as that in molecular kinetic theory and atomistic simulations. The formulation leads to a unified atomistic and continuum description of temperature and a new linear momentum equation that, supplemented by an interatomic potential, completely governs thermal and mechanical processes across scales from the atomic to the continuum. The conservation equation can be used to solve atomistic trajectories for systems at finite temperatures, as well as the evolution of field quantities in space and time, with atomic or multiscale resolution. Four sets of numerical examples are presented to demonstrate the efficacy of the formulation in capturing the effect of temperature and thermal fluctuations, including phonon density of states, thermally activated dislocation motion, dislocation formation during epitaxial processes, and attenuation of longitudinal acoustic waves as a result of their interaction with thermal phonons. [ABSTRACT FROM AUTHOR]

Over-Tip-Rotor (OTR) liners have been investigated over the last decades as a technology to further reduce fan broadband noise in turbofan engines. The suppression of noise with OTR liners is attributed to conventional attenuation of acoustic waves and source modification effects. This paper describes a fundamental experiment to gain a better understanding of the source modification effects and establish whether they are purely due to acoustic back-reactions or also due to hydrodynamic changes on the source. The OTR liner configuration is approximated by a static airfoil with its tip located over a flat plate containing a flush-mounted liner insert and separated from the airfoil tip by a small gap. Synchronous measurements of the far-field noise and wall pressure fluctuations on the airfoil tip have shown that the reduction of wall pressure at the airfoil tip by the liner is the dominant mechanism of noise reduction. The reduction in unsteady pressure fluctuations on the airfoil tip by the liner is mainly caused by back-reaction effects at high frequencies and hydrodynamic modifications at low- and mid-frequencies. Over-tip liners are found to alter the unsteady flow field in the gap region and weaken the flow structures responsible for the generation of tip noise. This study has shown that far-field noise predictions based on analytical models are useful to estimate the performance of over-tip liners, but a complete assessment should also include the impact of the liner on the tip-leakage flow, and, consequently, the sources of tip noise. [ABSTRACT FROM AUTHOR]

The introduction of an acoustic field can change the hydrodynamic behavior of solid particles in gas-solid fluidized beds, and in turn, the quasi-fluid in fluidized beds affects the acoustic propagation characteristics, such as the attenuation of acoustic waves in the fluidized-bed particle swarm. In this work, we investigated the transmission and attenuation process of sound waves in the fluidized bed of group B particles in the acoustic frequency range from 50 to 500 Hz. The obtained results show that there are three different attenuation stages during the propagation of acoustic waves through the fluidized bed with group B magnetite powder particles. The sound pressure level distribution model is extended to the sound-assisted fluidization of group B particles, and the prediction of the transmission and attenuation process of sound waves in the fluidized bed is in good agreement with the experimental values with an error below 5%. [Display omitted] • The sound transmission and attenuation were investigated in gas-solid flows. • The mechanism of sound wave propagation at different H/D ratios was revealed. • Three different sound attenuation stages were found in a gas-solid fluidized bed. • The theoretical mathematical model predicts SPL distribution within an error of 5%. [ABSTRACT FROM AUTHOR]

Seismic wave propagation is affected by wave‐induced local fluid flow between stiff pores and multiscale fractures. To investigate this phenomenon, forced vibration (1–100 Hz) and ultrasonic (106 Hz) measurements are performed on two tight sandstones with complex pore geometry in dry and water‐saturated scenarios. Porosity, permeability, and ultrasonic velocities were also measured at different differential pressures. The results indicate that the nonlinear behavior of these properties is strongly influenced by the presence of cracks, and the correlations between the permeability/ultrasonic velocities and porosity are different. A wave propagation model is then developed in which penny‐shaped cracks are inclusions introduced stepwise into a porous medium to describe the wave anelasticity in a wide frequency band at different pressures. As a result, the model provides good agreement with the measured P‐wave velocity dispersion, and the pore aspect ratio spectrum and crack radii are determined. We then compare the estimated crack radii and pore size distributions from nuclear magnetic resonance spectroscopy. Published data of a tight sandstone and a low porosity sandstone in the frequency range (2–200, 106) and (1–3,000) Hz at different differential pressures are also analyzed to validate the model. The aspect ratios, volume fractions, and radii of pores/cracks are used to describe the measured permeability. The present work can provide new insights into the geophysical properties of reservoir rocks with complex pore geometry. Plain Language Summary: Multiscale cracks exert an important influence on the dispersion and attenuation of broadband acoustic waves. To better understand the underlying mechanism, we measured frequency‐dependent elastic moduli and pressure‐dependent porosity, permeability, and ultrasonic wave velocities of two tight sandstones. These data show that velocity dispersion is observable in water‐saturated rock and that permeability and ultrasonic velocities are differentially correlated with porosity. A proposed wave propagation model, in which penny‐shaped cracks are inclusions introduced stepwise into a porous medium, successfully describes the experimental data and other published data on tight sandstone and low‐porosity sandstone, and aspect ratios, volume fractions, and radii of pores/cracks are determined. Key Points: We propose a model to investigate the effect of multiscale cracks on seismic waves in tight sandstonesThe pore aspect ratio and crack‐radius spectra are estimated at different differential pressuresThe permeability is obtained according to the pore geometry [ABSTRACT FROM AUTHOR]

In this paper, the attenuation of sound waves in underground gas pipelines and the vibration characteristics of pipelines are studied, and the feasibility and effectiveness of acoustic measurement of PE pipelines are verified. In this paper, the attenuation equation of sound waves in an underground gas transmission pipeline is derived based on the propagation characteristics of gas and the vibration characteristics of the pipeline itself. In order to verify the experimental results, we conducted an experimental test on the air pipeline model and verified the feasibility and effectiveness of the acoustic measurement of the PE pipeline through the test under the action of pulse excitation. Then, we detect the background noise, design the test scheme according to the characteristics of the buried pipeline, and select the test site for field test. In the test process, we collected the test data and obtained the spectrum diagram of the test data by fast Fourier transform (FFT). By analyzing the results of the spectrogram, we find that the pulse signal can penetrate the medium composed of the pipe formation, but the amplitude of the sound will be sharply attenuated. At the same time, according to the size of the peak in the spectrum, we can determine the location of the pipe. In summary, the feasibility and effectiveness of acoustic measurement of the PE pipeline are verified through experimental tests, and attenuation equations based on acoustic wave propagation characteristics and pipeline vibration characteristics are proposed. It has important practical application value for the safety monitoring and positioning of the buried gas pipeline. [ABSTRACT FROM AUTHOR]

At present, whale optimization algorithm (WOA) is one of the hot issues in swarm intelligence algorithm. Since it was proposed, people have done a lot of improvement work for WOA algorithm. To address the shortcomings of WOA, an improved WOA combined with the equiangular spiral bubble net predation (named as IWOA) is proposed in this paper. In IWOA, search agent uses the equiangular spiral rather than 9-shaped path to mimics the foraging trajectory of humpback whale. This rule can increase the convergence speed and exploitation ability of the search agent to an extent. Additionally, with the guidance of the sound wave attenuation steering law, search agent in IWOA can switch back and forth between the actively swim (exploitation) and the randomly swim (exploration) to the goal, hence obtain a better tradeoff between the exploitation and exploration. Numerical experiments are conducted on a set of mathematical benchmark cases. The results show that IWOA has a better performance. [ABSTRACT FROM AUTHOR]

In this work, a phononic metamaterial that could be applied for both acoustic and elastic wave attenuation is numerically examined. The hollow yablonovite structure with the addition of hollow spheres on a face-centered cubic arrangement is examined numerically, and wide bandgaps are found for the propagation of acoustic and elastic waves. The calculations were performed by using the finite element method. The research includes an investigation of all the geometric parameters of the structure and how the bandgaps are being affected from the values of each parameter. The proposed structure is found to have wide bandgaps for both elastic and acoustic waves. These findings strongly indicate that the proposed acoustoelastic metamaterial is a powerful candidate for several applications such as sound and vibrations attenuation, noise reduction, and acoustic filtering. [ABSTRACT FROM AUTHOR]

The cold sintering process (CSP) is a low temperature processing technique that utilizes a transient phase to synthesize dense ceramics. However, some CSP parts contain microflaws that arise due to inhomogeneities in pressure, temperature, and transient phase. This work uses 20 MHz ultrasound to verify the presence of defects in CSP ZnO samples of varying densities (84%–97%). Acoustic metrics used in this work include wave speed, which is affected by differences in the effective elastic properties of the medium, and attenuation, which quantifies wave energy loss due to scattering from defects. Wave speed maps were inhomogeneous, suggesting density gradients which were verified with scanning electron microscopy. In addition, it was demonstrated that the pores produced by cold sintering are anisometric, which increases the anisotropy in the elastic properties. High attenuation regions (>300 Np/m) are present in all samples independent of relative density and correspond to defects identified in X‐ray computed tomography (XCT) which were as small as 38 µm in effective diameter. However, some high attenuation spots do not correspond to visible defects in XCT, which suggests the presence of features undetectable with XCT such as residual secondary phases at the grain boundaries. [ABSTRACT FROM AUTHOR]

Theoretical and experimental features of using an acoustic interferometer for determining the velocity and attenuation of an acoustic wave in liquids with different acoustic impedances are studied. It is shown for the first time that the indicated impedance determines the ratio of the resonance values of the maximum and minimum transmission coefficient S12 for the same transmitter–receiver pair on the dependence of the transmission coefficient on the distance between the transducers. A methodology has been developed for determining the wave attenuation of a liquid free from the influence of "apparent" attenuation associated with the loss of part of the acoustic power to the transducers. [ABSTRACT FROM AUTHOR]

Audible noise (AN) has been the main concern in recent years when considering electromagnetic environmental impact in designing overhead lines (OHLs). Driven by the increased demand of high voltage direct current (HVDC) transmission lines, a novel corona cage experiment is built in association with an acoustic simulation using the finite element method (FEM). The characteristics of the acoustic wave propagation within the testing hall are analyzed using FEM, and the optimized locations for AN detection are determined. On the basis of complying with measurement standards, the location of the measurement is selected to be closer to the sound source and further away from the reflecting surface, to generate more accurate measurement results. In the designated test hall for this paper, the influence of refraction and reflection of sound waves is not obvious. The attenuation of sound waves below 4 kHz is negligible, while for higher frequencies such as 4 kHz and 8 kHz it is significant. Finally, FEM simulation is used to optimize the location for measurement microphones, and further experiments are carried out to verify its accuracy. [ABSTRACT FROM AUTHOR]

Aiming at the problem of the unclear sound wave attenuation in the signal transmission of the load-bearing drill string, an experimental device for the sound wave propagation characteristics in the curved drill string was set up. The influence of the drill string structure and acoustic excitation parameters on the sound propagation characteristics in the drill string under different loads is different. The results show that the curvature of the drill string has an influence on the propagation of the sound wave in the drill string, and its rule is related to the curvature of the drill string and the frequency of the sound wave. The pulse repetition rate, excitation voltage, and pulse width only affect the passband amplitude, and the effect is significantly greater than the curvature of the drill string. The main influencing factor of its sound transmission characteristics is the degree of drill string curvature. The low-frequency signal should be preferentially selected as the carrier of downhole information transmission after considering the influence of drill string bending on acoustic transmission. [ABSTRACT FROM AUTHOR]

The constant development of the electrical engineering sector, especially in the transmission of electrical energy under high-voltage direct current (HVDC), requires research on new insulation materials and investigations of physical phenomena under ultrahigh electrical fields in solid dielectrics. One of the current problematic issues is the formation of space charge in HV insulation systems, which affects the operational electrical field distribution and can lead to faster insulation degradation. There are several problems that have to be considered before every space charge measurement, such as the attenuation and dispersion of sound waves in tested dielectric materials, reflections at the interfaces, and the spatial resolution of the measured charge profile. The spatial resolution is one of the most important technical factors of the PEA measurement stand. The spatial resolution, as it is assumed, depends on several factors, such as the width of the pulser and the pulse rise time, the thickness of the piezoelectric sensor, and the dispersion of the tested material. The article presents the laboratory measurement results of the impact of pulser parameters, such as pulse width and rise time, and sensor thickness on the equivalent thickness of the measured net charge layer corresponding to the resolution of the method. The dispersion in the tested LDPE material is also presented and analysed. The results show that with an increase in the pulser rise time, a higher resolution of the pea method can be achieved. [ABSTRACT FROM AUTHOR]

Thunder-induced seismic waves recorded at dense seismic stations in Seoul, South Korea are analysed for inversion of thunder source spectra. Thunder-induced seismic waves from four local thunder events are analysed. A theory is introduced for the inversion of acoustic source spectra from thunder-induced seismic waves. In the course of source-spectral inversion, the propagation and acoustic-to-seismic coupling effects are counted. The thunder-induced seismic signals were well identified at distances of <∼20 km. Direct acoustic-to-seismic coupled seismic waves present apparent phase velocities of sound speed in atmosphere (340 m s−1). Thunder-induced seismic waves are dominant at high frequencies (>20 Hz). Vertical peak ground accelerations of thunder-induced seismic waves in local regions (0.024–0.110 m s−2 at distances of 2.4–3.7 km) are equivalent to the ground motion levels induced by a moderate-size (∼ M 5) earthquake at regional distances. The thunder-induced acoustic waves in the atmosphere are obtained by removing the acoustic-to-seismic coupling effect and site-response effect from the observed thunder-induced seismic waves. The quality factors for acoustic wave attenuation in the atmosphere are determined. Urban landscapes and atmospheric effects cause strong acoustic attenuation over atmospheric absorption. Acoustic thunder source spectra are determined by stacking the inverted acoustic spectra at all stations. The peak frequencies of acoustic thunder source spectra are around 34–36 Hz, suggesting the acoustic energy per unit length in lightning strikes to be ∼4 × 106 J m−1. Local seismic records are applicable for the investigation of thunder and lightning properties. [ABSTRACT FROM AUTHOR]

A method for the determination of longitudinal and transverse bulk acoustic wave attenuation from measurements of the decay-rate of two independent zero-group-velocity resonances in a couple of matched plates is presented. A linear relation is derived, which links the bulk-wave attenuation coefficients to the decay-rate of plate-resonances. The relation is used to determine the acoustic loss of tungsten at GHz frequencies from noncontact laser-ultrasonic measurements in plates with thicknesses of about 1 µm. The longitudinal and transverse attenuation was found to amount to 1918 m−1 and 7828 m−1 at 2.16 GHz and 3265 m−1 and 12181 m−1 at 2.46 GHz. The presented approach is validated with calculated responses to a thermoelastic source, and the accuracy of the obtained attenuation values is estimated to be in the range of 10%. [ABSTRACT FROM AUTHOR]

The mapping of failure that uses acoustic emission (AE) is an advanced approach when tracking the location of cracks in rock materials under progressive stress. However, attenuation of the acoustic wave significantly affects the identification of the acoustic source. The isotropic attenuation model suggests a constant attenuation in all rock material directions, which provides unrealistic failure mapping for anisotropic rocks. This study investigated the impact of high attenuation on the mapping failure of a composite (sandstone–shale–sandstone), intact sandstone, and shale specimens that used AE under axile stress. The results proved that wave attenuation is the primary obstacle when obtaining a compatible failure map to the real specimen failure. Multiple setups of AE sensors were implemented to quantify the attenuation values for a wave that propagated across composite joints and shale bedding planes. The amplitude and energy attenuation (EA) values increased by 85% and 47%, respectively, when the bedding plane of the shale was from 0° to 90°, which reflected anisotropic behavior. The energy and amplitude reduction reached 99% and 39%, respectively, by propagating across a single joint. In addition, behind a double joint, no signal was received. However, the application of a load perpendicular to the joint interface improved the acoustical–wave characteristics. Therefore, when the rock material anisotropy was increased, there was lower accuracy when mapping specimen failure, because the attenuation varied at each angle between the source and sensor. In addition, the EA method suggested in this study presented hits location more precisely compared with the P-velocity (Vp) method. These findings could contribute to the development of an anisotropic attenuation model for field and laboratory application of AE. [ABSTRACT FROM AUTHOR]

Introduction: Gradient-based sonic crystals (GBSCs) based on rigid resonators that improve the acoustic attenuation over the traditional sonic crystals (SCs) were recently studied numerically. The present work is a numerical and experimental study of the GBSCs based on rigid and cladded resonators and aims to further improve the attenuation of the GBSCs over the traditional SCs. Methods: Extensive finite element (FE) and experimental studies were conducted on the GBSCs and the traditional uniform SCs using jute, a natural fibrous material as claddings over rigid resonators in the form of felts. The multiple linear regression (MLR) technique was used to find the influence of the airflow resistivity and thickness of the claddings on the attention spectrum of the GBSC. The effect of the angle of incidence of the sound waves on the attenuation by the GBSC and traditional SC was also studied experimentally. Results: Though the claddings were found to improve the overall attenuation by the GBSC, they do not improve the attenuation at all frequencies. Similarly, the claddings were found to be more effective on the GBSC than the traditional Helmholtz resonator-based SC (HRSC). However, the GBSC was found to be less effective than the traditional HRSC if the angle of incidence of the sound waves changes. Conclusions: For a fixed filling ratio of the GBSC resonators, the Bragg scattering and local resonance of a GBSC based on rigid resonators are more effective attenuation mechanisms in the resonance and first Bragg gap regimes than the combined effects of absorption by the fibrous cladding, Bragg scattering and local resonance of a GBSC based on cladded resonators. However, the claddings significantly improve the attenuation post the resonance and first Bragg gaps. Therefore, the claddings should only be used when the targeted frequency regime lies above the resonance and first Bragg gaps. [ABSTRACT FROM AUTHOR]

This paper presents new experimental and numerical evidence that perforations in a pipe wall result in a low-frequency bandgap within which sound waves rapidly attenuate. These perforations are modelled as an acoustically soft boundary condition on the pipe wall and show that a low frequency bandgap is created from 0 Hz. The upper bound of this bandgap is determined by the dimensions and separation of the perforations. An analytical model based on the transfer matrix method is proposed. This model is validated against numerical predictions for the pipe with varying perforation geometries. A numerical study is undertaken to model the effect of perforations with ideal acoustically soft boundary conditions and surrounded with an air gap. Close agreement is found between the numerical and analytical models. Experimental evidence shows that the width of the bandgap is accurately predicted with the numerical and analytical models. [ABSTRACT FROM AUTHOR]

The possibility to control supersonic acoustic wave propagation is intriguing, but when modeling phononic crystal devices, supersonic surface acoustic waves are mired by radiative attenuation and, hence, eschewed in many device designs. In this paper, we study supersonic surface acoustic wave modes in shallow hole phononic crystals computationally with respect to the three bulk wave sound barriers of cubic (001) GaAs. From a first principles modeling approach of linear elasticity, the finite element method, and with the aid of characterization parameters for systematic modal categorization, detailed nuances are observed for supersonic surface waves propagating along the [110]-direction of GaAs with a periodically patterned surface. Modes of interest are distinguished by possessing both strain energy and squared polarization ratios above defined thresholds. The square array of shallow inclusions imparts a metamaterial surface layer effect that results in marked changes in the dispersion, the bulk wave hybridization, and the modal interactions of the surface modes in the Γ-X direction of the phononic crystal, which are characterized by their modal profiles and attenuation via bulk wave radiation. From these findings, we propose an extended sound cone concept to accommodate supersonic surface acoustic waves with low attenuation. Furthermore, at frequencies above the shear vertical bulk dispersion line, well-bounded surface acoustic wave modes are revealed, and the phenomenon of these supersonic modes with limited bulk wave coupling is explored. From these detailed band structures, the systematic method of mode characterization reveals deeper insights into modes that exist in shallow phononic crystals on cubic GaAs. [ABSTRACT FROM AUTHOR]

P(VDF-TrFE) is a promising material for flexible acoustic devices owing to its good piezoelectric performance and excellent stretchability. However, the high density of internal pores and large surface roughness of the conventional P(VDF-TrFE) results in a high propagation attenuation for acoustic waves, which limits its use in flexible acoustic devices. In this paper, a novel method based on two-step annealing is proposed to effectively remove the pores inside the P(VDF-TrFE) film and reduce its surface roughness. The obtained P(VDF-TrFE) film possesses excellent characteristics, including a high breakdown strength of >300 kV/mm, a high-purity β-phase content of more than 80%, and high piezoelectric coefficients (d33) of 42 pm/V. Based on the low-porosity β-phase P(VDF-TrFE) film, we fabricated flexible film bulk acoustic resonators (FBARs) which exhibit high sharp resonance peaks. The pressure sensor was made by sandwiching the FBARs with two PDMS microneedle patches. Heartbeat and respiration rate monitoring were achieved using the pressure sensor. This work demonstrates the feasibility of high-performance flexible piezoelectric acoustic resonators based on low-porosity P(VDF-TrFE) films, which could see wider applications in the wearable sensors for both physical and chemical sensing. [ABSTRACT FROM AUTHOR]

In view of the shortage of structural defect monitoring methods for deep submersibles, numerical simulation and experimental research on underwater SAW propagation based on interdigital transducers are carried out in this paper. PVDF interdigital transducer (PVDF-IDT) has shown considerable potential in the application of structural health monitoring because of its micro size, soft material characteristics, and the characteristics of long-term bonding on the surface of the tested structure. In order to realize the application of IDT on submersible or underwater structures, it is necessary to understand the influence of underwater environment on IDTs with different structures. The underwater attenuation of IDT with 2–5 mm wavelength and the underwater attenuation of Lamb (A0 mode) wave on a 4 mm thick titanium alloy plate is obtained through COMSOL software simulation. The experimental verification shows that the simulation results match with the actual situation, which proves that COMSOL software can accurately calculate the acoustic attenuation of surface waves at the solid–liquid interface. At the same time, the underwater attenuation of IDT with different structures is very different, providing important design parameters for the underwater interdigital transducer. In this paper, it is found that the Lamb wave has significant advantages over the Rayleigh wave in the health monitoring of underwater thin plate structures. [ABSTRACT FROM AUTHOR]

To control the physical properties of polar and nonpolar liquid media, the parameters of model systems based on paraffin and silicone oils, as well as glycerin, were measured using electrophysical and acoustoelectric methods. Electrophysical studies were performed with an Agilent E4980A LCR meter and a measuring cell consisting of an Eppendorf tube and two coaxial nickel electrodes forming a cylindrical capacitor. The permittivity of the liquid was determined from the formula for the capacitor. For the acoustic part of the problem, ST,X-quartz was used as the piezoelectric plate, on which a fluoroplastic cell for liquid was placed. The measurements were carried out in three stages: measurement of the phase and amplitude of the acoustic wave (i) without contact with the liquid, (ii) in contact with a pure test liquid, and (iii) in contact with the test liquid with a filler. Microparticles of pharmaceutical activated carbon and the surfactant sorbitan monooleate were used as fillers. The viscosity of the suspensions was determined from the difference between the attenuation of an acoustic wave in the presence of the pure liquid and liquid with filler. [ABSTRACT FROM AUTHOR]

Trailing-edge noise (TE noise) is an aeroacoustic sound radiated from an isolated airfoil in the specific ranges of low-speed flow. We used a pulsed laser as an actuator to reduce the TE noise without modifying the airfoil's surface. The wind tunnel test was conducted to verify the capability of an Nd:YAG laser as the actuator. The laser beam was focused into the air just outside the velocity boundary layer on the lower side of an NACA0012 airfoil. The experimental result shows that the TE noise is suppressed for a certain period after beam irradiations. We then analyzed the physical mechanism of the noise reduction with the laser actuation by the implicit large eddy simulation (ILES), a high-fidelity numerical method for computational fluid dynamics (CFD). The numerical investigations indicate that the pulsed energy deposition changes the unstable velocity amplification mode of the boundary layer, the source of an acoustic feedback loop radiating the TE noise, to another mode that does not generate the TE noise. The sound wave attenuation is observed once the induced velocity fluctuations and consequently generated vortices sweep out the flow structure of the unstable mode. We also examined the effect of the laser irradiation zone's shape by numerical simulations. The results show that the larger irradiation zone, which introduces the disturbances over a wider range in the span direction, is more effective in reducing the TE noise than the shorter focusing length with the same energies. [ABSTRACT FROM AUTHOR]

Bottom formations known as pockmarks basically arise due to extensive gas emission. Active pockmarks are characterized by exceptionally high gas saturation and substantially reduced sound speed. The latter circumstance leads to strong attenuation of sound waves contacting with a pockmark. In the present paper, we study low-frequency sound propagation in a 10-km long waveguide crossing a giant pockmark. A new method of acoustic waveguide scanning based on measurement of the wavefield propagator is represented. This method allows one to explore attenuation anomalies associated with the presence of the gas-saturated bottom region. In particular, one can find out which beams fall into a pockmark area and therefore experience strong losses. Identifying such beams, as well as beams which avoid pockmark-assisted losses, one can estimate probable locations of the pockmark segment in the waveguide, provided information about the background medium is sufficient. [ABSTRACT FROM AUTHOR]

Low-intensity ultrasound (LIUS) enhances the proliferation rate of various mammalian stem cells through mechanical stimulation. This study quantitively finds suitable LIUS stimulation parameters for increasing the proliferation rate of human adipose-derived mesenchymal stem cells (hAdMSCs) for mass production. Various stimulation conditions of LIUS were assessed based on the beam pattern of the ultrasonic transducer and the attenuation of the sound waves. Using optimal LIUS stimulation parameters for enhancing proliferation of hAdMSCs taken from bromodeoxyuridine (BrdU) incorporation assay, long-term culture of hAdMSCs was performed for 16 days. The resultant hAdMSCs were characterized for various biomarkers such as CD34−, CD45−, CD73+, CD95+, CD105+ and cytological staining and a cytokine array assay. LIUS stimulation parameters found for enhancing the hAdMSCs proliferation were the frequency of 5 MHz, an intensity of 300 mWcm−2, a duration of 10 min per day, and continuous waves with a 100% duty cycle. The LIUS stimulated hAdMSCs group showed a 3.25-fold increase in the cell number compared to the control group after 16 days of culture. By confirming the effects of quantitatively measured LIUS stimulation on the enhancement of hAdMSCs proliferation, this study may be a foundation for the applications of LIUS stimulation in the industrial-scale production of hAdMSCs. [ABSTRACT FROM AUTHOR]

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that provides an extremely large AO interaction length for achieving extremely high spectral resolution of AO tunable filters (AOTFs). Large AO interaction length also makes it possible to implement multifrequency diffraction in quasi-collinear AOTFs, which has found multiple applications in modern optoelectronics. The most widespread of them being ultrashort laser pulse shaping when the pulse shape and spectral composition is controlled by the spectral composition of the ultrasound pulse aroused in the AO crystal. The operation of quasi-collinear AOTFs is accompanied by the appearance of the temperature gradients in the AO device mainly due to the acoustic power absorption by the material. We experimentally assessed the influence of these gradients on the AOTF spectral characteristics. A theoretical model based on the Raman–Nath equations was proposed, which allows to consider the influence of ultrasound attenuation and temperature gradients on the AOTF transmission. This model is valid for transmission simulations both in single- and modulated-frequency AOTF operation modes. The study includes the effects of acoustic wave attenuation and AO phase matching shift caused by inhomogeneous crystal temperature along the optical beam path. The compensation strategy based on ultrasound frequency and magnitude adjustment is proposed for minimizing the effect of temperature gradients and acoustic field attenuation on AOTF spectral transmission for broadband operation in ultrashort laser pulse shaping. [ABSTRACT FROM AUTHOR]

Coal spontaneous combustion is a major threat for safe production. In order to achieve the application of sound wave technology to the monitoring and early warning of coal spontaneous combustion, clarify the propagation characteristics of sound waves in loose coal heating process, and illustrate the temperature sensing mechanism of the sound wave detection and early warning of coal spontaneous combustion, the present study built a test system of sound wave kinematic parameter for loose coals to survey the attenuation characteristics of sound wave propagation during loose coal heating. The elastic wave CT algorithm was used to reconstruct the sound wave velocity field, clarify the propagation path, and identify the temperature anomaly area. The results show that with the increase of temperature, the time for sound wave propagation reduces rapidly and then slowly, the velocity increases gradually, and the internal structure and pores of coals become enlarged. In the sound velocity field reconstructed on the basis of SIRT algorithm, the area with high value corresponds to the high temperature area of the coal, which better reflects the changes of the temperature anomaly area during coal heating. • The propagation characteristics of acoustic waves in loose coal during the warming process were investigated. • Acoustic velocity field inversion to identify temperature anomaly zones. • The sound wave propagation path and temperature sensing mechanism were clarified. [ABSTRACT FROM AUTHOR]

The acoustic attenuation inside the bandgaps is, together with the bandgap width, a fundamental design parameter for phononic-crystal-based systems. We discuss approximate expressions for the maximum attenuation inside the bandgaps of one-dimensional longitudinal phononic crystals and its dependence on the acoustic contrast and the fractional bandwidth. We provide different approximations at small and large fractional bandwidths, computed from the trace of the transmission matrix of the crystal elementary cell. We show that, for relatively small gaps, the attenuation is roughly proportional to the fractional bandwidth, in analogy with the flexural case. For larger gaps, a large attenuation can be obtained only for high (and possibly impractical) acoustic contrasts. Approximate expressions are validated through comparison with FEM results. We also derive asymptotic upper limits for the bandgap borders and show that high contrasts do not necessarily lead to wide bandgaps, a fact connected to geometrical phase inversion for the acoustic wave in the crystal. We finally compare the attenuation of flexural and longitudinal waves at a fixed fractional bandwidth and derive regions of optimum attenuation for the two propagation modes. [ABSTRACT FROM AUTHOR]

To explore acoustic performance and flame retardancy of expandable graphite (EG) and molybdenum trioxide (MoO3) in rigid polyurethane foam (RPUF) with nitrogen/phosphorus synergistic system, ammonium polyphosphate (APP), diethyl ethylphosphonate (DEEP), EG, and MoO3 were employed in RPUF. Compared with flame‐retarded RPUFs, neat‐RPUF attained an open porosity value of 45.3%, which provided good tortuosity for wave propagation and acoustic attenuation, and therefore had the best performance in acoustic absorption. Although the addition of flame retardant sacrificed sound‐absorbing properties, the flame retardancy performance of foams were indeed significantly improved. The Limited Oxygen Index (LOI) value of RPUF with 70 php EG alone increased from 19.4% of Neat‐RPUF to 35.6%. Regarding to CO and CO2 emission, RPUF with 10 php MoO3, 30 php APP, and 30 php DEEP gained minimum value of average CO yield and average CO2 yield, which were 0.06 kg/kg and 0.78 kg/kg, respectively, ascribed to the accelerated formation of the compact carbon layers by MoO3. [ABSTRACT FROM AUTHOR]

A method for solving an inverse problem, which consists in the simultaneous assessment of the bottom topography and lake bed effective acoustic parameters for a shallow-water area covered with ice, is developed. This area is a plane waveguide of variable cross section. The method is based on the reception and processing of broadband (pulse) signals excited by a single sound source and recorded by a remote vertical chain of hydrophones (array). The bottom topography measurements are carried out by comparing the propagation times of signals reflected from the lower boundary of the waveguide and arriving at different array hydrophones. The effective parameters (sound speed, density, attenuation coefficient of acoustic waves) are determined by analyzing the angular dependence of the reflection coefficient, calculated as the ratio of the amplitude of the pulse reflected from the bottom to the amplitude of the direct signal, taking into account the spherical spreading of the wave front. Experiments to test the proposed approach were carried out in the coastal zone of Lake Baikal in the winter–spring period. [ABSTRACT FROM AUTHOR]

The acoustic black hole (ABH) effect in waveguides is studied using frequency-domain finite element simulations of a cylindrical waveguide with an embedded ABH termination composed of retarding rings. This design is adopted from an experimental study in the literature, which surprisingly showed, contrary to the structural counterpart, that the addition of damping material to the end of the waveguide does not significantly reduce the reflection coefficient any further. To investigate this unexpected behavior, we model different damping mechanisms involved in the attenuation of sound waves in this setup. A sequence of computed pressure distributions indicates occurrences of frequency-dependent resonances in the device. The axial position of the cavity where the resonance occurs can be predicted by a more elaborate wall admittance model than the one that was initially used to study and design ABHs. The results of our simulations show that at higher frequencies, the visco-thermal losses and the damping material added to the end of the setup do not contribute significantly to the performance of the device. Our results suggest that the primary source of damping, responsible for the low reflection coefficients at higher frequencies, is local absorption effects at the outer surface of the cylinder. [ABSTRACT FROM AUTHOR]