Search

Ultralow-frequency band gaps are realized to suppress the ultralow-frequency flexural wave propagation in the beams with periodically attached quasi-zero-stiffness (QZS) resonators, which are designed by inserting quasi-zero-stiffness systems into the mass-in-mass structures. The band structure and the transfer matrix of the QZS locally resonant beam are derived by the transfer matrix method to quantify the wave attenuation performance of band gaps. Then, the effect of the stiffness ratio on the bandgap characteristic is studied. It is shown that, thanks to the introduction of the QZS system, the band gaps can be easily transferred to lower frequency or even ultralow frequency without weakening the static stiffness of the resonators. Finally, the flexural wave propagation in locally resonant beam consisting of multiple periodic arrays of QZS resonators is investigated. The result shows that differential design of the bandgap frequencies can be easily realized by adjusting the negative stiffness coefficient of the QZS resonators, so as to obtain broadband flexural wave suppression performance. [ABSTRACT FROM AUTHOR]

Stealth technology plays an important role in modern military conflicts, and because of its many practical advantages, plasma stealth technology has gained a lot of attention. In this paper, we describe our study of the reflection and absorption characteristics of waves propagating in multilayer plasma slabs. Our results show that positive effects on the attenuation can be obtained by optimizing the plasma parameters and by increasing the number of plasma layers. We also found that multilayer plasma slabs can effectively widen absorption bands, especially for low frequency bands. [ABSTRACT FROM AUTHOR]

Based on the same blasting vibration signal, the S transform and HHT method have been applied for the time and frequency analysis. Both the S transform and HHT method are analyzed and compared from the point of decomposition path and analysis function. It expressed that these two methods are complete and effective and have the characteristics of adaptive multiresolution. Especially, the time resolution of the HHT method is invariable and its accuracy is high. Moreover, its frequency resolution can be adaptively adjusted with the inherent characteristics of the signal. However, the time and frequency resolution of the S transform are limited by the uncertainty principle, and the resolution improvement is at the cost of reducing the resolution of another. Furthermore, the HHT method has much more adaptability and is superior to S transform in the analysis of nonstationary signals. In advance, the waveform curve and power spectral density of each detonator are extracted, which indicates that it is not scientific to use different segment detonators for the same circle of the auxiliary hole. Under the premise of the same charge quantity, the vibration energy peak value produced by the same segment detonator is less than that of several segments of detonators. Therefore, it is believed that the HHT method has broad application prospects in the study of wave propagation, attenuation law, structural dynamic response characteristics, and blasting vibration damage in rocks. [ABSTRACT FROM AUTHOR]

Young sea ice composed of grease and pancake ice (GPI), as well as thin floes, considered to be the most common form of sea ice fringing Antarctica, is now becoming the "new normal" also in the Arctic. A study of the rheological properties of GPI is carried out by comparing the predictions of two viscous wave propagation models: the Keller model and the close-packing (CP) model, with the observed wave attenuation obtained by SAR image techniques. In order to fit observations, it is shown that describing GPI as a viscous medium requires the adoption of an ice viscosity which increases with the ice thickness. The consequences regarding the possibility of ice thickness retrieval from remote sensing data of wave attenuation are discussed. We provide examples of GPI thickness retrievals from a Sentinel-1 C band SAR image taken in the Beaufort Sea on 1 November 2015, and three CosmoSkyMed X band SAR images taken in the Weddell Sea on March 2019. The estimated GPI thicknesses are consistent with concurrent SMOS measurements and available local samplings. [ABSTRACT FROM AUTHOR]

High-performance locally resonant metamaterials represent the next frontier in materials technology due to their extraordinary properties obtained through materials design, enabling a variety of potential applications. The most exceptional feature of locally resonant metamaterials is the subwavelength size of their unit cells, which allows to overcome the limits in wave focusing, imaging and sound/vibration isolation. To respond to the fast evolution of these artificial materials and the increasing need for advanced and exceptional properties, the emergence of a new mechanism for wave mitigation and control consisting in a nonlinear interaction between propagating and evanescent waves has recently been theoretically demonstrated. Here, we present the experimental proof of this phenomenon: the appearance of a subharmonic transmission attenuation zone due to energy exchange induced by autoparametric resonance. These results pave the path to a new generation of nonlinear locally resonant metamaterials. [ABSTRACT FROM AUTHOR]

The flow field around the RAMC‐II vehicle at differenrt angles of attack (AOA) was numerically solved using a computational fluid dynamics technique with a three‐dimensional model. Electron density, pressure, temperature, and collision frequency, which is given great importance in this paper. The maximal electron density, total pressure, and temperature at the same position decreases with the angle of attack on the leeward side and increases with the angle of attack on the windward side. The collision frequency is almost unaffected by AOA on the leeward side, but increases significantly with AOA on the windward side. Based on the realistic plasma sheaths obtained by hypersonic fluid model, a multilayer transmission model was used to study the electromagnetic (EM) wave propagation in plasma sheath at different AOA. The attenuation value of EM wave on the leeward (windward) side decreases (increases) with the AOA. So the influence of the AOA on the EM wave transmission must be considered in the communication of vehicle reentry. In addition, the EM wave attenuation on the leeward at the same angle of attack is always smaller than the windward side, especially at a large AOA. Therefore, the leeward side can be considered as one end of the communication to improve the communication quality. The conclusions of this paper have both theoretical and practical significance to better understand the communication"blackout" problem for the reentry of hypersonic vehicles. [ABSTRACT FROM AUTHOR]

The effect of spatial nonlocality on the decay of waves in a dissipative material is investigated. The propagation and decay of waves in a one-dimensional, viscoelastic peridynamic medium is analyzed. Both the elastic and damping terms in the material model are nonlocal. Waves produced by a source with constant amplitude applied at one end of a semi-infinite bar decay exponentially with distance from the source. The model predicts a cutoff frequency that is influenced by the nonlocal parameters. A method for computing the attenuation coefficient explicitly as a function of material properties and source frequency is presented. The theoretical results are compared with direct numerical simulations in the time domain. The relationship between the attenuation coefficient and the group velocity is derived. It is shown that in the limit of long waves (or small peridynamic horizon), Stokes' law of sound attenuation is recovered. [ABSTRACT FROM AUTHOR]

Porous materials are ubiquitous in the subsurface formations of the earth where acoustic and seismic waves are used for remote sensing. However, it is not well understood how the dissipation and the dispersion of poroelastic waves are caused by the viscoelastic and viscous properties of the constituents such as solid grains and pore fluid and by the viscoelastic dissipation of the solid frame, as well as the viscodynamic coupling of the pore fluid to the solid frame due to its global and local flows relative to the solid grains. Such attenuation mechanisms have seldom been incorporated in subsurface sensing simulations, although they can be very important to applications. In this paper, we propose a complete attenuation model, including both full stiffness and viscodynamic dissipation, for poroelastic media in seismic wave simulations. Completely based on a generalized Zener model, the effects associated with physical dissipation and frequency-dependent dispersion are accurately simulated by a finite-difference time-domain algorithm. Verifications with analytical solutions show the accuracy, efficiency, and flexibility of our method. Numerical results demonstrate that the attenuation of Biot’s model in the sediment of the seafloor has significant effects on acoustic wave scattering from complex geologic structures. [ABSTRACT FROM AUTHOR]

The influence of pipeline thickness and radius on the attenuation of guided waves in water-filled steel pipelines is investigated using theoretical analysis and experimental measurement. Attenuations of individual axisymmetric modes in unburied water-filled steel pipelines are predicted by an analytical model under different pipeline radius-thickness ratios. Model predictions indicate that attenuation of the fundamental mode increases as the ratio rises. This effect is investigated by finding the displacement variations under different ratios. Laboratory experiments were also carried out in four unburied steel pipelines with three distinctly different radius-thickness ratios using acoustic transducers to acquire signals uniformly spaced along the axis of the pipe. By applying the iterative quadratic maximum likelihood algorithm, the attenuations could be accurately estimated from the measurement data for individual modes. Experimental results show that attenuation of the fundamental axisymmetric mode is sensitive to radius-thickness ratio, but high-order modes are barely affected, agreeing with the model predictions mentioned in this paper. The characteristics of water-filled buried pipelines are also investigated using an analytical model to understand the relation between wave attenuation and the radius-thickness ratio. [ABSTRACT FROM AUTHOR]

Accurate and efficient forward modeling methods are important for simulation of seismic wave propagation in 3D realistic Earth models and crucial for high‐resolution full waveform inversion. In the presence of attenuation, wavefield simulation could be inaccurate or unstable over time if not well treated, indicating the importance of the implementation of a strong stability preserving time discretization scheme. In this study, to solve the anelastic wave equation, we choose the optimal strong stability preserving Runge‐Kutta (SSPRK) method for the temporal discretization and apply the fourth‐order MacCormack scheme for the spatial discretization. We approximate the rheological behaviors of the Earth by using the generalized Maxwell body model and use an optimization procedure to calculate the anelastic coefficients determined by the Q(ω) law. This optimization constrains positivity of the anelastic coefficients and ensures the decay of total energy with time, resulting in a stable viscoelastic system even in the presence of strong attenuation. Moreover, we perform theoretical and numerical analyses of the SSPRK method, including the stability criteria and the numerical dispersion. Compared with the traditional fourth‐order Runge‐Kutta method, the SSPRK has a larger stability condition number and can better suppress numerical dispersion. We use the complex‐frequency‐shifted perfectly matched layer for the absorbing boundary conditions based on the auxiliary difference equation and employ the traction image method for the free‐surface boundary condition on curvilinear grids representing the surface topography. Finally, we perform several numerical experiments to demonstrate the accuracy of our anelastic modeling in the presence of surface topography. Key Points: We develop a new optimal strong stability preserving Runge‐Kutta (SSPRK) method for solving the anelastic wave equationWe perform theoretical and numerical analyses of the SSPRK methodWe demonstrate the accuracy and efficiency of the SSPRK method in anelastic wavefield modeling in the presence of surface topography [ABSTRACT FROM AUTHOR]

The partial discharge (PD) in gas‐insulated switchgear (GIS) will stimulate the ultra‐high frequency (UHF) electromagnetic (EM) wave signal of 300 MHz to ∼3 GHz. The PD signal will appear in the resonance, distortion, and attenuation during propagating in GIS due to the complexity of the internal structure of GIS. Thus, it increases the intricacy of detection and evaluation. The Finite difference time domain (FDTD) algorithm has been used in this paper to investigate the influence of the insulator structure parameters on the propagation characteristics of the UHF signal. The results show that the dielectric loss of insulator causes the attenuation of the UHF signal inside and outside GIS, with an increase in the thickness and dielectric constant of the insulator. The shape of the insulator has little influence on the propagation characteristics of the UHF signal within GIS. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

The parabolic wave equation has been used extensively to model long-range paraxial wave propagation due to its high computational efficiency. Typical radar, sonar, and communication geometries are most naturally treated using cylindrical coordinates with the radial direction chosen as the paraxial direction. Here, a method is described for modeling waves with 3-D variation propagating through an inhomogeneous body of revolution with the paraxial direction chosen to be the axis of symmetry. The technique decomposes the initial field into independent modes that are propagated in parallel using 2-D solvers. The method is benchmarked by comparison to finite-element calculations. An example application—modeling the interaction of a radio communications system with a rocket exhaust plume—is then considered. [ABSTRACT FROM AUTHOR]

Ultrasonic interferometry is an indispensable tool in molecular chemistry and imaging, inclusive of liquid state studies where the standard theory is used to determine many physico-chemical parameters, such as the isentropic compressibility and adiabatic bulk modulus. The first principle analysis conducted here augments the standard model with potentially significant consequences in the interpretation of these parameters and the output spectrum. The effect of attenuation of a wave on the observed separation between peaks in acoustic interferometry is a focus of the investigation. Important aspects of the theory of Hubbard and others were collated to derive two mathematical models that were used to fit experimental spectra. The first model does not assume fictitious quantities found in Hubbard’s theory and fits the experimental data well. The second model includes the effects of the electronics of the measuring system and is in excellent agreement with the experimental data. Theoretical and numerical analyses were performed to validate the two models. Numerically, the attenuation of a wave is shown to cause the peaks to deviate either positively or negatively from the otherwise ideal half-wavelength of λ/2 and exact equations governing such deviations are derived that could have significant implications in theory and applications. [ABSTRACT FROM AUTHOR]

Most sonic logging data acquired in hydrate-bearing zones have shown increasing velocity of P and S waves accompanied by high attenuation of P and S waves. In the eastern Nankai Trough, the attenuation values of P and S wave estimated in the sonic logging frequency range (10–20 kHz for P waves and 0.5–1 kHz for S waves) also show a significant increase in hydrate-bearing sediments. However, P -wave attenuation values from zero-offset vertical seismic profiling (VSP) data (30–110 Hz) indicate a lower attenuation compared to the values in the sonic logging frequency range at the same well locations, which implies frequency-dependent P -wave attenuation in hydrate-bearing sediments. In order to elucidate the attenuation mechanisms responsible for this frequency-dependent P -wave attenuation in hydrate-bearing sediments, we apply two different rock physics models that have recently been developed to consider the squirt flow in porous/microporous hydrate and the interaction between sand and hydrate grains. The predicted attenuation values are compared with those derived from field sonic logging and VSP data. Finally, we infer that this frequency-dependent P -wave attenuation may be due to the squirt flow caused by the combined effect of the degree of hydrate saturation and two permeable systems (one is between sand grains and the other is between hydrate grains), and/or due to the squirt flow caused by fluid inclusions with different aspect ratios in a microporous hydrate. [ABSTRACT FROM AUTHOR]

A nonlinear thin layer interface model overcoming the long-wavelength assumption of displacement discontinuity model is developed to analyze the full-wavelength wave propagation across nonlinear parallel joints. In this study, the filling material is treated as a thin layer with reduced mechanical properties to reveal multiple reflections and time shifting within the filled joint. The nonlinearity of the filling material is considered, and a recursive matrix is derived in time domain. An experimental study on wave propagation across a filled joint was carried out by SHPB test. The quartz sand layer of different filling thicknesses sandwiched between Hopkinson bars was pressured by compressional waves to investigate the wave attenuation. Comparisons of the joint thickness, the wave frequency and the incident angle are carried out between the present model and the existing displacement discontinuity model. The results indicate that the thin layer interface model considering the thickness of the joint is capable of extending the long-wavelength assumption to full-wavelength research and it is more appropriate for filled joints with thick thickness that is comparable to wavelength. Then, this model is extended to parallel joints, and the properties of the filling material (i.e., initial elastic modulus and maximum closure), impact velocity, incident angle and wave frequency on wave attenuation are discussed for a joint set. The spacing dependency of the transmission coefficient for parallel joints is compared with displacement discontinuity model. [ABSTRACT FROM AUTHOR]

When ocean waves propagate over porous seabed, they cause variations of the pore pressure within seabed, leading to the possible wave attenuation and soil liquefaction. In order to advance and improve our understanding of the process of wave-induced seabed liquefaction and its impact on wave propagation, systematical experiments are carried out in a wave flume with a soil basin filled with silt. Both the pore pressures and water surface elevations are measured simultaneously, while the seabed liquefaction is videotaped using a high-speed camera. Laboratory measurements show that the pore pressure in surface layer mainly oscillates over time, while the wave period averaged pore pressure has little change. In the deep layer, however, the wave period averaged value of the pore pressure builds up dramatically. The results show that the wave height decreases rapidly along the direction of wave propagation when seabed liquefaction occurs. Such a wave attenuation is greatly enhanced when the liquefaction depth further increases. The experiments also demonstrate that the conditions (wave height and wave period) of incident waves have significant impacts on the wave-induced pore pressures, liquefaction depth and wave attenuation in a silt bed. [ABSTRACT FROM AUTHOR]

In this paper we characterize the high-frequency (1.0-10 Hz) seismic wave crustal attenuation and the source excitation in the Sicily Channel and surrounding regions using background seismicity from weak-motion database. The data set includes 15 995 waveforms related to earthquakes having local magnitude ranging from 2.0 to 4.5 recorded between 2006 and 2012. The observed and predicted ground motions form the weak-motion data are evaluated in several narrow frequency bands from 0.25 to 20.0 Hz. The filtered observed peaks are regressed to specify a proper functional form for the regional attenuation, excitation and site specific term separately. The results are then used to calibrate effective theoretical attenuation and source excitation models using the random vibration theory. In the log-log domain, the regional seismic wave attenuation and the geometrical spreading coefficient are modelled together. The geometrical spreading coefficient, g(r), modelled with a bilinear piecewise functional form and given as g(r) ∝ r-1.0 for the short distances (r < 50 km) and as g(r) ∝ r-0.8 for the larger distances (r<50 km). A frequency-dependent quality factor, inverse of the seismic attenuation parameter, Q(f) = 160f/fref 0. 35 (where fref = 1.0 Hz), is combined to the geometrical spreading. The source excitation terms are defined at a selected reference distance with a magnitude independent roll-off spectral parameter, κ 0.04 s and with a Brune stress drop parameter increasing with moment magnitude, from Δσ = 2 MPa for Mw = 2.0 to Δσ = 13 MPa for Mw = 4.5. For events M ≤ 4.5 (being Mwmax = 4.5 available in the data set) the stress parameters are obtained by correlating the empirical/excitation source spectra with the Brune spectral model as function of magnitude. For the larger magnitudes (Mw>4.5) outside the range available in the calibration data set where we do not have recorded data, we extrapolate our results through the calibration of the stress parameters of the Brune source spectrum over the Bindi et al. ground-motion prediction equation selected as a reference model (hereafter also ITA10). Finally, the weak-motion-based model parameters are used through a stochastic approach in order to predict a set of region specific spectral ground-motion parameters (peak ground acceleration, peak ground velocity, and 0.3 and 1.0 Hz spectral acceleration) relative to the generic rock site as a function of distance between 10 and 250 km and magnitude between M 2.0 and M 7.0. [ABSTRACT FROM AUTHOR]

An algorithm is introduced to solve the general multilateration (source localization) problem in a dispersive waveguide. The algorithm is designed with the intention of localizing impact forces in a dispersive floor, and can potentially be used to localize and track occupants in a building using vibration sensors connected to the lower surface of the walking floor. The lower the wave frequencies generated by the impact force, the more accurate the localization is expected to be. An impact force acting on a floor, generates a seismic wave that gets distorted as it travels away from the source. This distortion is noticeable even over relatively short traveled distances, and is mainly caused by the dispersion phenomenon among other reasons, therefore using conventional localization/multilateration methods will produce localization error values that are highly variable and occasionally large. The proposed localization approach is based on the fact that the wave’s energy, calculated over some time window, decays exponentially as the wave travels away from the source. Although localization methods that assume exponential decay exist in the literature (in the field of wireless communications), these methods have only been considered for wave propagation in non-dispersive media, in addition to the limiting assumption required by these methods that the source must not coincide with a sensor location. As a result, these methods cannot be applied to the indoor localization problem in their current form. We show how our proposed method is different from the other methods, and that it overcomes the source-sensor location coincidence limitation. Theoretical analysis and experimental data will be used to motivate and justify the pursuit of the proposed approach for localization in a dispersive medium. Additionally, hammer impacts on an instrumented floor section inside an operational building, as well as finite element model simulations, are used to evaluate the performance of the algorithm. It is shown that the algorithm produces promising results providing a foundation for further future development and optimization. [ABSTRACT FROM AUTHOR]

A series of experiments on damage detection of large-diameter (88 mm diameter) pipe with and without liquid by guided waves have been conducted. Two types of liquid as water and machine oil are chosen to fill in pipe for detecting the influence of filler. The influences of liquid types on guided wave propagation, dispersion and attenuation characteristics have also been studied. The mode conversion phenomenon of reflection waves exists regardless filling state of pipe, but it is most serious in water-filled pipe, and weakest in vacant pipe. The defect wave signal could be distinguished from converted wave mode by time-frequency analysis, and the time-frequency distributions of the two type signals are different. The relationship between defect length and defect reflect signal strength is linear in vacant pipe and nonlinear in liquid-filled pipe. With the defect length increasing, the defect signals of liquid-filled pipe increase too, but the rate of increment are reduced. [ABSTRACT FROM AUTHOR]

Average elastic properties of a fluid-saturated fractured rock are discussed in association with the extremely slow and dispersive Krauklis wave propagation within individual fractures. The presence of the Krauklis wave increases P-wave velocity dispersion and attenuation with decreasing frequency. Different laws (exponential, power, fractal, and gamma laws) of distribution of the fracture length within the rock show more velocity dispersion and attenuation of the P-wave for greater fracture density, particularly at low seismic frequencies. The results exhibit a remarkable difference in the P-wave reflection coefficient for frequency and angular dependency from the fractured layer in comparison with the homogeneous layer. The biggest variation in behaviour of the reflection coefficient versus incident angle is observed at low seismic frequencies. The proposed approach and results of calculations allow an interpretation of abnormal velocity dispersion, high attenuation, and special behaviour of reflection coefficients versus frequency and angle of incidence as the indicators of fractures. [ABSTRACT FROM AUTHOR]

Empirical and theoretical far-field attenuation relationships, which do not capture the near-field response, are most often used to predict the peak amplitude of blast wave. Jiang et al. (Vibration due to a buried explosive source. PhD Thesis, Curtin University, Western Australian School of Mines, 1993) present rigorous wave equations that simulates the near-field attenuation to a spherical blast source in damped and undamped media. However, the effect of loading frequency and velocity of the media have not yet been investigated. We perform a suite of axisymmetric, dynamic finite difference analyses to simulate the propagation of stress waves induced by spherical blast source and to quantify the near-field attenuation. A broad range of loading frequencies, wave velocities, and damping ratios are used in the simulations. The near-field effect is revealed to be proportional to the rise time of the impulse load and wave velocity. We propose an empirical additive function to the theoretical far-field attenuation curve to predict the near-field range and attenuation. The proposed curve is validated against measurements recorded in a test blast. [ABSTRACT FROM AUTHOR]

Attenuation has a large influence on ground motion intensity. Quality factors are used to measure wave attenuation in a medium and they are often difficult to estimate due to many factors such as the complex geology and underground mining environment. This study investigates the effect of attenuation on seismic wave propagation and ground motion using an advanced numerical tool-SPECFEM2D. A method, which uses numerical modeling and site-specific scaling laws, is proposed to estimate the shear quality factor of hard rocks in underground mines. In the numerical modeling, the seismic source is represented by a moment tensor model and the considered medium is isotropic and homogeneous. Peak particle velocities along the strongest wave motion direction are compared with that from a design scaling law. Based on the field data that were used to derive a semi-empirical design scaling law, it is demonstrated that a shear quality factor of 60 seems to be a representative for the hard rocks in deep mines to consider the attenuation effect of seismic wave propagation. Using the proposed method, reasonable shear quality factors of hard rocks can be obtained and this, in turn, will assist accurate ground motion determination for mine design. [ABSTRACT FROM AUTHOR]

We studied the transmission characteristics of hybrid modes in a corrugated circular waveguide above the Bragg frequency to develop a broad-band transmission line for millimeter waves. Millimeter waves at 294 GHz were transmitted into a straight waveguide. From observed power profiles in waveguide cross-sections, a high attenuation rate of 0.13 dB/m was obtained. To match a theoretical attenuation constant with the experimental one, we introduced an ad hoc coefficient of conventional surface reactance in the waveguide wall. This was necessary because the wall began to look like the surface with a decreasing anisotropic reactance owing to the frequency above the Bragg frequency. Using nonlinear optimization for mode content analysis, the observed power profiles in the waveguide cross-section were matched with theoretical profiles. There was good agreement between the calculated and observed centers of power profiles and attenuation rate along the waveguide. The theoretical analysis showed that the magnetic field at the waveguide wall increases and the substantial attenuation takes place. Above the Bragg frequency coupling to backwards propagating modes is a point of consideration. A combination of the backwards propagating EH and the forward propagating HE modes satisfied the Bragg condition at 294.7 GHz which was the nearest frequency of operating frequency. A strong attenuation of the incoming HE mode by Bragg resonance was not expected due to large difference of 0.7 GHz. It becomes clear that the observed high transmission loss outside of the Bragg resonance can be explained by a decrease in anisotropic surface reactance at the wall. [ABSTRACT FROM AUTHOR]

This paper focuses on improving the noise attenuation performance of a ducted Helmholtz resonator (HR) system and fully utilizing an available space. The transmission loss achieved by a periodic ducted HR system is depended on the structure and the number of HRs. However, the number of HRs is restricted by the available space in longitudinal direction of the duct. Moreover, such system will occupy a large space and may have some spare space in the transverse direction of the duct. By adding HRs on the available space in the transverse direction, a modified ducted HR system is therefore proposed. The wave propagation in the periodic ducted HR system and the modified HR system are investigated theoretically and numerically. The transfer matrix method is developed to conduct the investigation. The predicted theoretical results fit well with the Finite Element Method (FEM) simulation results. The results indicate that both the noise attenuation band and peak amplitude are increased by adding HRs on arbitrary side of the cross-section of the duct. The proposed modified ducted HR system can improve the noise attenuation performance and fully utilize the available space, and it is practical to be used in an actual ventilation ductwork system. [ABSTRACT FROM AUTHOR]

A layered periodic structure is proposed to construct a periodic wave barrier, which is defined by extended typical cells consisting of two different components. The periodic structure with reasonable design has attenuation zones (AZs) which can block wave propagation. The surface-wave dispersion curves and material/geometric parameters of layered periodic structures were investigated using finite element method. A typical three-dimensional simulation model was applied to verify the efficiency of the proposed periodic wave barrier in terms of ground vibration isolation. The results showed that simulated frequency zones of vibration reduction are consistent with the theoretical surface-wave AZs, and that periodic wave barriers can greatly reduce train-induced vibrations. [ABSTRACT FROM AUTHOR]

A set of 14 teleseismic earthquakes was studied to determine how wave propagation was affected by a presumed magma body beneath Uturuncu volcano, Bolivia. Teleseisms are suitable for study because they are relatively long period, contain purely P waves, and have near-vertical incidence angles. The number of events is small but the events have good signal-to-noise ratios and very similar waveforms for each event so that reliable measurements could be made of arrival times and amplitudes. Attenuation of amplitudes occurs in a NW-SE trend beneath the volcano, 14 by 34 km (long axis NW-SE). Calculated values of the quality factor Qp are an average of 12.4, with extreme values as low as 1.8. These calculations are based on the assumption that the highest amplitude observed is the "true" amplitude, and all others have been attenuated. The average thickness of the anomaly is 10.2 km, and the center is ~20 km SE of the summit, within the area of surface uplift measured geodetically. Time delays of up to 0.8 s were also observed. The pattern of attenuation and relative time delays together showed four trends: fast and not attenuated (normal crust), slow and attenuated (partial melt), fast and attenuated (likely high fracture density), and slow but not attenuated (possible deep low Vp structure). Back azimuth differences of up to 60° were observed. In nearly all cases, azimuths were rotated into directions parallel to local rock fabric, suggesting that shallow crustal properties affected near-surface wave propagation. Overall results suggest partial melt as high as 10%-20% in a region of varying thickness, low Bouguer gravity and resistivity, high Vp/Vs, persistent seismicity, and overlapping a locus of recent uplift. [ABSTRACT FROM AUTHOR]

Ultrasound attenuation ( $$\alpha $$ ) and velocity ( V) at 9.6 MHz are measured in polycrystalline hcp $$^4\hbox {He}$$ . The ultrasound signal above 200 mK is linear and understood in terms of resonant vibration of dislocation segments pinned between network nodes with an average pinning length of 3.7 $$\mu \hbox {m}$$ , much shorter than 59 $$\mu \hbox {m}$$ estimated from a shear modulus measurement. Dramatic changes in $$\alpha $$ and V are observed below 200 mK. The changes are strongly dependent on temperature and are nonlinear and hysteretic. These effects result from pinning of dislocations by $$^3\hbox {He}$$ impurities (nominal concentration of 0.3 ppm). The dislocation damping constant due to thermal phonons, the binding energy between dislocation and $$^3\hbox {He}$$ , and the average network pinning length obtained from the ultrasound data are compared with those from the shear modulus experiments. [ABSTRACT FROM AUTHOR]

The acoustic performance of the ducted Helmholtz resonator (HR) system is analyzed theoretically and numerically. The periodic HR array could provide a wider noise attenuation band due to the coupling of the Bragg reflection and the HR's resonance. However, the transmission loss achieved by a periodic HR array is mainly dependent on the number of HRs, which restricted by the available space in the longitudinal direction of the duct. The full distance along the longitudinal direction of the duct for HR's installation is sometimes unavailable in practical applications. Only several pieces of the duct may be available for the installation. It is therefore that this paper concentrates on the acoustic performance of a HR array consisting of several periodic parts. The transfer matrix method and the Bragg theory are used to investigate wave propagation in the duct. The theoretical prediction results show good agreement with the Finite Element Method (FEM) simulation results. The present study provides a practical way in noise control application of ventilation ductwork system by utilizing the advantage of periodicity with the limitation of available completed installation length for HRs. [ABSTRACT FROM AUTHOR]

This study presents a novel method for determining the liquid level from the outside of a sealed container, which is based on the balance of echo energy received by two receiving sensors. The proposed method uses one transmitting transducer and two receiving sensors that are encapsulated in a coupling plane and arranged by certain rules. The calculation and comparison of echo energy are grounded on the difference ultrasonic impedance between gas and liquid media. First, by analyzing the propagation and attenuation characteristics of ultrasonic waves in a solid, an acoustic model for calculating the echo energy is established and simulated in MATLAB. Second, the proposed method is evaluated through a series of experiments. The difference and ratio of echo energy received by two receiving sensors are calculated and compared under two different coupling conditions. Two kinds of the sensors that are arranged by different rules are selected for measuring the liquid level, and the measurement are analyzed and discussed in detail. Finally, the experimental results indicate that the proposed method can meet the proposed accuracy requirements and can effectively solve the problems caused by some poor coupling conditions. [ABSTRACT FROM AUTHOR]

This paper presents an empirical propagation path loss model for corridors in office buildings. The proposed model estimates changeable character of radio signal attenuation, based on a special approach as a combination of the simple free-space model with the author's model. The measurement stand and measurement scenario are described. The propagation path loss research have been made in corridor for different frequencies in range 30 MHz to 290 MHz. A significant number of measurement results were allowed an analysis of the radio wave propagation conditions in the environment. In general, the propagation path loss increases for each measurement frequencies with length of propagation route. Based on measurement data, the new empirical propagation path loss model was developed. For this purpose, the regression analysis was made. The novelty of this model is that it could be used for estimate propagation path loss in measured environment for different radio wave frequencies. At the end, in order to justification the practical usefulness of described method for estimate a radio wave attenuation, the statistical evaluation was made. Thus, the results of the statistical analysis ( ME, SEE and R2 values) are satisfactory for each measured radio wave frequency. [ABSTRACT FROM AUTHOR]

Blasting has been widely used in mining and construction industries for rock breaking. Ground vibration induced by blasting is an inevitable side effect that may cause damage to nearby structures, if not properly controlled. In this study, response and possible damage of rock-socketed pile near soil-rock interface subjected to ground shock excitations are investigated and quantified with coupled SPH-FEM method. Results indicate that the base of the pile is relatively vulnerable and that the soil properties significantly influence on response of pile subjected to a specific blast load. Furthermore, based on the numerical results, ground vibration attenuation equation is proposed. [ABSTRACT FROM AUTHOR]

The wave propagation in jointed rock mass depends on many factors, in particular the wave frequency, existing stress conditions and the induced strain levels. When the wavelength of the propagating wave is much longer than the spacing of the joints, it is referred to as long-wavelength condition. This is a dominant condition in seismological studies due to the presence of closely spaced joints in rocks. A comprehensive study on long-wavelength propagation of shear and compression waves in very weak rocks has been carried out with the help of a resonant column apparatus using a model material (plaster of Paris) at various strain levels. The working principle and suitability of the apparatus for testing stiff samples are discussed. The velocity reductions of shear and compression waves across joints are obtained. The influence of frictional and filled joints in attenuating stress waves under various strain levels is analysed. Wave velocities are found to be reduced with increasing strain levels and decreasing joint spacing. The joints with gouge material are more efficient at damping the waves. It is recognised that wave velocity reductions and damping across joints are functions of confining stress and strain levels induced in very weak rocks. [ABSTRACT FROM AUTHOR]

In this paper, a model is presented to simulate wave propagation in curved rectangular tunnels with imperfectly conducting walls. The model is based on treating the tunnel as a waveguide, which is an extension of previous proposed model by Mahmoud [3]. A new approach to calculate the total attenuation rate of the propagated wave inside tunnel is proposed. The approach is considering the effect of imperfect conductivity of the upper and lower walls of the tunnel. This approach is based on assuming that the boundaries of the tunnel section are constant impedance surfaces as the surface impedance of the wall is almost independent of the angle of the wave incidence onto the wall. A simple scenario is considered to check the accuracy of this model. This scenario is verified by comparing experimental and numerical simulation results. Good agreement between the proposed model and the experimental results is obtained. [ABSTRACT FROM AUTHOR]

We propose a novel Lg attenuation tomography model (Q Lg tomography) for the state of Gujarat, Western India, using earthquake data recorded by the Gujarat Seismic Network, operated by the Institute of Seismological Research in Gandhinagar. The waveform dataset consist of 400 3-component recordings, produced by 60 earthquakes with magnitude (M L ) spanning from 3.6 to 5.1, recorded at 60 seismic stations having epicentral distances spanning between 200 and 500 km. Spectral amplitude decays for Lg wave displacement were obtained by generalized inversion at 17 frequencies spanning between 0.9 and 9 Hz. Lg wave propagation efficiency was measured by Lg/Pn spectral ratio categorizing as efficient ratio ≥6 for 86%, intermediate ratio of 3–6 for 10% and inefficient ratio <3 for 4% paths of total 400 ray paths. The earthquake size and quality of waveform recorded at dense network found sufficient to resolve lateral variation of Q Lg in Gujarat. Average power-law attenuation relationship obtained for Gujarat as Q Lg (f) = 234f 0.64 , which corresponds to high attenuation in comparison to peninsular India shield region and other several regions around the world. Q Lg tomography resolves the highly attenuating crust of extremely fractured Saurashtra region and tectonically active Kachchh region. The Gujarat average attenuation is also lying in between them. The low attenuation in Cambay and Narmada rift basins and extremely low attenuation in patch of Surendranagar area is identified. This study is the first attempt and can be utilized as pivotal criteria for scenario hazard assessment, as maximum hazard has been reported in highly attenuating tectonically active Kachchh region and in low attenuating Cambay, Narmada and Surendranagar regions. The site and source terms are also obtained along with the Q Lg inversion. The estimated site responses are comparable with observed local geological condition and agree with the previously reported site amplifications at the same sites. The source terms are comparable with local magnitude estimated from Network. The M w (Lg) is nearly equivalent to M L (GSN) and the slight differences are noted for larger magnitude events. [ABSTRACT FROM AUTHOR]

The purpose of this study is to experimentally investigate the stress effect on normally incident P-wave attenuation through coal specimens. Laboratory tests were carried out using a Split Hopkinson pressure bar (SHPB) system, and a modified method was proposed to determine the quality factor ( Q ) of P-waves through coal specimens. Larger quality factor denotes less energy attenuated during P-wave propagating through coal. Experimental results indicate that the quality factor and stress ( σ ) within coal specimens are positively correlated. The P-wave propagation through coal specimens causes crack closure at the beginning of the coal fracture process in SHPB tests, an innovative model was thus proposed to describe the relationship between the crack closure length and the dynamic stress induced by P-wave. Finally, the stress effect on P-wave attenuation through coal was quantitatively represented by a power function Q = a ( c – bσ ) − 6 , and the material constants a , b , and c were determined as 1.227, 1.314, and 0.005, respectively. The results obtained in this study would be helpful for engineers to estimate seismic energy attenuation and coal mass instability in coal mines. [ABSTRACT FROM AUTHOR]

The impact of methane explosions on mining operations can never be over-emphasized. The safety of miners could be threatened and local ventilation facilities are likely to be damaged by the flame and overpressure induced by a methane explosion event, making it essential to understand the destructiveness and influence range of a specific explosion. In this paper, the attenuation effect of geometric changes, most commonly bends, obstacles, and branches, present in the way of blast-wave propagation and the capability of the selected numerical model were studied. Although some relevant experimental research has been provided, quantitative analysis is insufficient. This paper investigates the attenuation factors of seven bends, three obstacles, and two T-branch scenarios to ascertain a better insight of this potentially devastating event quantitatively. The results suggest that (1) the numerical model used is capable of predicting four of the seven validated scenarios with a relative error less than 12%; (2) the maximum peak overpressure is obtained when the angle equals 50° for bend cases; and (3) the selected numerical scheme would overestimate the obstacle cases by around 15%. [ABSTRACT FROM AUTHOR]

Attenuation characteristics of the crust in the eastern Himalaya and the southern Tibetan Plateau are investigated using high quality data recorded by Himalayan Nepal Tibet Seismic Experiment (HIMNT) during 2001–2003. The present study aims to provide an attenuation model that can address the physical mechanism governing the attenuation characteristics in the underlying medium. We have studied the Coda wave attenuation ( Q c ) in the single isotropic scattering model hypothesis, S wave attenuation ( Q s ) by using the coda normalization method and intrinsic ( Q i - 1 ) and scattering ( Q sc - 1 ) quality factors by the multiple Lapse Time Window Analysis (MLTWA) method under the assumption of multiple isotropic scattering in a 3-D half space within the frequency range 2–12 Hz. All the values of Q exhibit frequency dependent nature for a seismically active area. At all the frequencies intrinsic absorption is predominant compared to scattering attenuation and seismic albedo ( B 0 ) are found to be lower than 0.5. The observed discrepancies between the observed and theoretical models can be corroborated by the depth-dependent velocity and attenuation structure as well as the assumption of a uniform distribution of scatterers. Our results correlate well with the existing geo-tectonic model of the area, which may suggest the possible existence of trapped fluids in the crust or its thermal nature. Surprisingly the underlying cause of high attenuation in the crust of eastern Himalaya and southern Tibet makes this region distinct from its adjacent western Himalayan segment. The results are comparable with the other regions reported globally. [ABSTRACT FROM AUTHOR]

Waves propagating over broad, gently-sloped shallows decrease in height due to frictional dissipation at the bed. We quantified wave-height evolution across 7 km of mudflat in San Pablo Bay (northern San Francisco Bay), an environment where tidal mixing prevents the formation of fluid mud. Wave height was measured along a cross-shore transect (elevation range − 2 m to + 0.45 m MLLW) in winter 2011 and summer 2012. Wave height decreased more than 50% across the transect. The exponential decay coefficient λ was inversely related to depth squared ( λ = 6 × 10 − 4 h − 2 ). The physical roughness length scale k b , estimated from near-bed turbulence measurements, was 3.5 × 10 − 3 m in winter and 1.1 × 10 − 2 m in summer. Estimated wave friction factor f ̂ w determined from wave-height data suggests that bottom friction dominates dissipation at high Re w but not at low Re w . Predictions of near-shore wave height based on offshore wave height and a rough formulation for f w were quite accurate, with errors about half as great as those based on the smooth formulation for f w . Researchers often assume that the wave boundary layer is smooth for settings with fine-grained sediments. At this site, use of a smooth f w results in an underestimate of wave shear stress by a factor of 2 for typical waves and as much as 5 for more energetic waves. It also inadequately captures the effectiveness of the mudflats in protecting the shoreline through wave attenuation. [ABSTRACT FROM AUTHOR]

Waves that propagate at low frequencies in buried pipes are of considerable interest in a variety of practical scenarios, for example leak detection, remote pipe detection, and pipeline condition assessment and monitoring. Whilst there has been considerable research and commercial attention on the accurate location of pipe leakage for many years, the various causes of pipe failures and their identification, have not been well documented; moreover, there are still a number of gaps in the existing knowledge. Previous work has focused on two of the three axisymmetric wavetypes that can propagate: the s =1, fluid-dominated wave; and the s =2, shell-dominated wave. In this paper, the third axisymmetric wavetype, the s =0 torsional wave, is investigated. The effects of the surrounding soil on the characteristics of wave propagation and attenuation are analysed for a compact pipe/soil interface for which there is no relative motion between the pipe wall and the surrounding soil. An analytical dispersion relationship is derived for the torsional wavenumber from which both the wavespeed and wave attenuation can be obtained. How torsional waves can subsequently radiate to the ground surface is then investigated. Analytical expressions are derived for the ground surface displacement above the pipe resulting from torsional wave motion within the pipe wall. A numerical model is also included, primarily in order to validate some of the assumptions made whilst developing the analytical solutions, but also so that some comparison in the results may be made. Example results are presented for both a cast iron pipe and an MDPE pipe buried in two typical soil types. [ABSTRACT FROM AUTHOR]

Strong shock waves can be generated by pulse discharge in water. Study of the pressure characteristics and attenuation law of these waves is highly significant to industrial production and national defense construction. In this research, the shock-wave pressures at several sites were measured by experiment under different conditions of hydrostatic pressure, discharge energy, and propagation distance. Moreover, the shock-wave pressure characteristics were analyzed by combining them with the discharge characteristics in water. An attenuation equation for a shock wave as a function of discharge energy, hydrostatic pressure, and propagation distance was fitted. The experimental results indicated that (1) an increase in hydrostatic pressure had an inhibiting effect on discharge breakdown; (2) the shock-wave peak pressure increased with increasing discharge voltage at 0.5 m from the electrode; it increased rapidly at first and then decreased slowly with increasing hydrostatic pressure; and (3) shock-wave attenuation slowed down with increasing breakdown energy and hydrostatic pressure during shock-wave transfer. These experimental results were discussed based on the mechanism described. [ABSTRACT FROM AUTHOR]

The broadband microwave propagating characteristics of a novel, large volume, coaxial gridded hollow cathode helium plasma is reported in this paper. The basic plasma parameters were determined using an Impedans Ltd. Langmuir probe under a variety of conditions. The transmission attenuation was recorded by using Scattering Parameters (S-parameters) of a vector network analyzer with the frequency range from 2 GHz to 18 GHz and a propagation model was established using the Z transform finite-difference time-domain method for simulating the transmission of microwave. The effects of both the gas pressure and the input power on the electromagnetic wave propagation are analyzed. The results showed that the computational and experimental results of transmission attenuation were in good agreements. Moreover, the electron density ne and the effective collision rate c were found to play important roles in the propagation of microwave. [ABSTRACT FROM AUTHOR]

This paper presents an exploration of noncollinear Timoshenko beam structures for attenuating the stress waves generated from impulsive transient loadings. Due to the existence of the noncollinear segments, flexural waves are generated in addition to the longitudinal waves. Furthermore, the resulting waves are dispersive and thus provide higher potential for the attenuation of the stress waves. Symmetric patterns are found to be more appropriate for increasing the stress-wave attenuation capacity, as these patterns can eliminate bending stress effects at the boundary of the structures. More generally, an adaptive optimization methodology is developed to find the most efficient layout of the noncollinear stress-wave attenuators (NSWAs). This methodology uses genetic algorithms (GA), coupled with an explicit finiteelement (FE) method for analyzing the wave-propagation behavior of the structures. The developed methodology is capable of updating the FE model of each trial solution, as the geometric characteristics of the solutions are evolving during the optimization procedure. Through this evolutionary design process, the attenuation capacity of the noncollinear structures is observed to be a complex function of the number of noncollinear segments and the relative wavelength associated with the transient loading. In addition, the results show that the single-material noncollinear structures are not effective for attenuating the amplitudes of transient pulse loadings with longer durations, such as blast waves. Most interestingly, however, by incorporating geometric and material discontinuities, bimaterial NSWAs are found to provide impressive levels of attenuation within compact geometries. Although much analytical and experimental validation remains, designs along these lines could help to initiate a paradigm shift from hardening of structures for blast protection towards a protective systems design approach featuring creative management of stress wave propagation. [ABSTRACT FROM AUTHOR]

On 2013 April 13, an inland earthquake of Mw 5.8 occurred in Awaji Island, which forms the western boundary of the Osaka sedimentary basin in western Japan. The strong ground motion data were collected from more than 100 stations within the basin and it was found that in the Osaka Plain, the pseudo velocity response spectra at a period of around 6.5 s were significantly larger than at other stations of similar epicentral distance outside the basin. The ground motion lasted longer than 3 min in the Osaka Plain where its bedrock depth spatially varies from approximately 1 to 2 km. We modelled long-period (higher than 2 s) ground motions excited by this earthquake, using the finite difference method assuming a point source, to validate the present velocity structure model and to obtain better constraint of the attenuation factor of the sedimentary part of the basin. The effect of attenuation in the simulation was included in the form of Q(f) = Q0(f/f0), where Q0 at a reference frequency f0 was given by a function of the S-wave velocity, Q0 = αVS. We searched for appropriate Q0 values by changing α for a fixed value of f0 = 0.2 Hz. It was found that values of α from 0.2 to 0.5 fitted the observations reasonably well, but that the value of α = 0.3 performed best. Good agreement between the observed and simulated velocity waveforms was obtained for most stations within the Osaka Basin in terms of both amplitude and ground motion duration. However, underestimation of the pseudo velocity response spectra in the period range of 5-7 s was recognized in the central part of the Osaka Plain, which was caused by the inadequate modelling of later phases or wave packets in this period range observed approximately 2 min after the direct S-wave arrival.We analysed this observed later phase and concluded that it was a Love wave originating from the direction of the east coast of Awaji Island. [ABSTRACT FROM AUTHOR]

This study considers the propagation of harmonic plane waves in a double porosity solid saturated with a non-viscous fluid. Existence of three longitudinal waves and a transverse wave is explained through the Christoffel equations, which define the phase velocities and the polarizations of constituent particles. Reflection of plane waves is studied at the stress-free plane surface of the composite medium. A numerical example is solved to calculate the partition of incident energy among the reflected waves. Conservation of the incident energy could be achieved only through the share of interaction energy. The presence of interaction energy is something unexpected, when the medium behaves non-dissipative to the propagation of elastic waves. The reason lies in the constitutive relations being used for double porosity medium, which are not symmetric in elastic coupling, as required by the Betti’s reciprocal theorem. Corrections to the constitutive relations are proposed and the correct relations are used to study the reflection and refraction phenomena at the boundaries of double porosity solids. Dispersion in velocity and attenuation is studied for the four attenuated waves in double porosity solid saturated with viscous fluid. [ABSTRACT FROM AUTHOR]

Frequency-dependent attenuation ( Q − 1 ) in the crust of northern Morocco is estimated from Lg -wave spectral amplitude measurements every quarter octave in the frequency band 0.8 to 8 Hz. This study takes advantage of the improved broadband data coverage in the region provided by the deployment of the IberArray seismic network. Earthquake data consist of 71 crustal events with magnitudes 4 ≤ m b ≤ 5.5 recorded on 110 permanent and temporary seismic stations between January 2008 and December 2013 with hypocentral distances between 100 and 900 km. 1274 high-quality Lg waveforms provide dense path coverage of northern Morocco, crossing a region with a complex structure and heterogeneous tectonic setting as a result of continuous interactions between the African and Eurasian plates. We use two different methods: the coda normalization (CN) analysis, that allows removal of the source and site effects from the Lg spectra, and the spectral amplitude decay (SAD) method, that simultaneously inverts for source, site, and path attenuation terms. The CN and SAD methods return similar results, indicating that the Lg Q models are robust to differences in the methodologies. Larger errors and no significant frequency dependence are observed for frequencies lower than 1.5 Hz. For distances up to 400 km and the frequency band 1.5 ≤ ƒ (Hz) ≤ 4.5, the model functions Q ( f ) = (529 − 22 + 23 )( f /1.5) 0.23 ± 0.06 and Q ( f ) = (457 − 7 + 7 )( f /1.5) 0.44 ± 0.02 are obtained using the CN and SAD methods, respectively. A change in the frequency dependence is observed above 4.5 Hz for both methods which may be related to the influence of the Sn energy on the Lg window. The frequency-dependent Q − 1 estimates represent an average attenuation beneath a broad region including the Rif and Tell mountains, the Moroccan and Algerian mesetas, the Atlas Mountains and the Sahara Platform structural domains, and correlate well with areas of moderate seismicity where intermediate Q values have been obtained. [ABSTRACT FROM AUTHOR]