Sound propagation over layered poro-elastic ground using a finite-difference model

This article presents an axisymmetric pressure-velocity finite-difference formulation (PV-FD) based on Biot's poro-elastic theory for modeling sound propagation in a homogeneous atmosphere over layered poro-elastic ground. The formulation is coded in a computer program and a simulation of actual measurements from airblast tests is carried out. The article presents typical results of simulation comprising synthetic time histories of overpressure in the atmosphere and ground vibration as well as snapshots of the response of the atmosphere-ground system at selected times. Comparisons with the measurements during airblast tests performed in Haslemoen, Norway, as well as the simulations by a frequency-wave number FFP formulation are presented to confirm the soundness of the proposed model. In particular, the generation of Mach surfaces in the ground motion, which is the result of the sound speed being greater than the Rayleigh wave velocity in the ground, is demonstrated with the help of snapshot plots.     

Coherent waves in a multiply scattering poro-elastic medium obeying Biot's theory

Twersky's theory is generalized to multiple scattering by a uniform random distribution of cylinders in a poro-elastic medium. The high-frequency regime only, where no dispersion effects occur in the absence of scatterers, is investigated in the frame of Biot's theory. The scatterers lie within a slab of the host medium, and an incident wave gives rise to a fast longitudinal coherent wave, a slow longitudinal one, as well as a shear one in the slab. The dispersion equations of those three coherent waves are derived. The shear coherent wave propagates independently of the other two, while the longitudinal coherent waves obey a coupled dispersion equation involving conversion terms. Numerically speaking, coupling effects are significant only when forward scattering by a single cylinder of the fast wave into the slow one (or the slow wave into the fast) is larger than forward scattering with no conversion.     

Phase shift and attenuation characteristics of acoustic waves in a flowing gas confined by cylindrical walls

Wave propagation in flowing ideal gases confined by cylindrical waveguides is described in the low-frequency range using an iterative Frobenius series expansion method. The primary concern is to present a mathematical model enabling any radial-dependent flow profile to be analyzed. In contrast to previous analytical results, the present model is applicable in the general case where cubic and higher order terms in the axial acoustic velocity become important and to examine the influence of a non-vanishing radial velocity term. As a numerical test case, it is found that a gas flow velocity w(r)—for simplifying reasons assumed to be a linear combination of a flat flow profile and a parabolic flow profile corresponding to a mean flow equal to —is well approximated by a flat flow profile of the same mean flow value at low shear wavenumbers and at higher shear wavenumbers (calculations were done for shear wavenumbers up to 8). In actual fact, the error introduced by making this mean flow approximation is smaller than the error introduced by neglecting the radial velocity term.     

Coherent waves in a multiply scattering poro-elastic medium obeying Biot's theory

Twersky's theory is generalized to multiple scattering by a uniform random distribution of cylinders in a poro-elastic medium. The high-frequency regime only, where no dispersion effects occur in the absence of scatterers, is investigated in the frame of Biot's theory. The scatterers lie within a slab of the host medium, and an incident wave gives rise to a fast longitudinal coherent wave, a slow longitudinal one, as well as a shear one in the slab. The dispersion equations of those three coherent waves are derived. The shear coherent wave propagates independently of the other two, while the longitudinal coherent waves obey a coupled dispersion equation involving conversion terms. Numerically speaking, coupling effects are significant only when forward scattering by a single cylinder of the fast wave into the slow one (or the slow wave into the fast) is larger than forward scattering with no conversion.     

Interference acoustical method for measuring velocity,attenuation and reflection of acoustic surface waves

An interference acoustical method for measuring velocity, attenuation and reflection of acoustic surface waves was developed. Applications to diagnostics and nondestructive testing of acoustic surface wave devices are discussed.     

Ultrasonic attenuation of surface acoustic waves in a thin film of superconducting Nb3Sn

The attenuation of 660 MHz surface acoustic waves propagating in a thin film of Nb₃Sn 5000 Å thick has been measured as a function of temperature from 4.2 K to 16 K. The A 15 Nb₃Sn, electron-beam codeposited on YZ lithium niobate and annealed at 700°C, was studied using 5.1 μm wavelength interdigital electrodes. The film revealed a transition temperature of 14.2 ± 0.1 K and using the BCS theory, an energy gap 2Δ(0) = 3.5 k_BT_c.     

Influence of a structurally damaged surface layer of the hexagonal crystal on the dispersion and attenuation of Rayleigh surface acoustic waves

Analytical expressions for the dispersion of the phase velocity and the inverse attenuation length of Rayleigh waves are derived with allowance made for a thin (as compared to the length of the surface wave) isotropic damaged surface layer that is contiguous with vacuum and located on the surface of a hexagonal crystal with the sixfold axis perpendicular to the surface. It is demonstrated that, in the limit of long wavelengths (as compared to the characteristic inhomogeneity size), which is of greatest interest for experimenters, the change in the dispersion of the phase velocity of Rayleigh waves is proportional to the second power of the frequency, whereas the inverse attenuation length of Rayleigh waves is proportional to the fifth power of the frequency. The inverse attenuation length of the Rayleigh wave is calculated numerically. The calculation method previously proposed by one of the authors (Kosachev, 1998) is generalized to the case of an isotropic damaged layer on an anisotropic (hexagonal) substrate.     

插管结构对膨胀腔消声器声学性能的影响

将数值模态匹配法(NMM)拓展应用于计算和分析外插管膨胀腔消声器的声学性能,推导了相应的理论公式并编写了计算程序。使用二维有限元法提取横向波数和本征向量,应用模态匹配法计算消声器的传递损失。使用数值模态匹配法和三维有限元法(FEM)研究了插管长度和进出口位置对带有外插进出口管椭圆形非同轴膨胀腔消声器声学性能的影响,两种方法计算结果吻合良好,从而验证了本文数值模态匹配法的正确性。研究结果表明,设置特定的插管长度和进出口位置可以消除消声器的通过频率,进而改善消声器中低频的消声性能。     

Temperature-dependent attenuation of surface acoustic waves in proton-exchanged 128°-rotated Y-cut LiNbO3

3 has been experimentally studied in the frequency range 100 to 460 MHz and in the temperature range 90 to 300 K. At room temperature, the proton exchange leads to the considerable enhancement of acoustic attenuation as compared to the pure samples. Annealing in air, in general, reduces the attenuation. However, an anomalous enhancement of the attenuation at several frequencies for particular conditions of annealing is observed. When the temperature is reduced, the attenuation decreases practically to zero in as-exchanged samples. In the annealed ones, the attenuation attains a minimum in the vicinity of 160 K, and begins to grow at lower temperatures. A sharp peak in the attenuation is observed at 210 K. Several different physical mechanisms including acousto-protonic interaction and structural phase transition seem to be responsible for the observed acoustic attenuation behaviour. Received: 17 July 1996/Accepted: 2 December 1996     

Manipulation with sound and vibration: A review on the micromanipulation system based on sub-MHz acoustic waves

Manipulation of micro-objects have been playing an essential role in biochemical analysis or clinical diagnostics. Among the diverse technologies for micromanipulation, acoustic methods show the advantages of good biocompatibility, wide tunability, a label-free and contactless manner. Thus, acoustic micromanipulations have been widely exploited in micro-analysis systems. In this article, we reviewed the acoustic micromanipulation systems that were actuated by sub-MHz acoustic waves. In contrast to the high-frequency range, the acoustic microsystems operating at sub-MHz acoustic frequency are more accessible, whose acoustic sources are at low cost and even available from daily acoustic devices (e.g. buzzers, speakers, piezoelectric plates). The broad availability, with the addition of the advantages of acoustic micromanipulation, make sub-MHz microsystems promising for a variety of biomedical applications. Here, we review recent progresses in sub-MHz acoustic micromanipulation technologies, focusing on their applications in biomedical fields. These technologies are based on the basic acoustic phenomenon, such as cavitation, acoustic radiation force, and acoustic streaming. And categorized by their applications, we introduce these systems for mixing, pumping and droplet generation, separation and enrichment, patterning, rotation, propulsion and actuation. The diverse applications of these systems hold great promise for a wide range of enhancements in biomedicines and attract increasing interest for further investigation.     

The acoustic response in a liquid layer overlying a multi-layered viscoelastic half-space

A Green function is constructed that represents the steady-state acoustic response in a liquid layer overlying a Voigt viscoelastic multi-layered half-space. The effect of the solid half-space is characterized by a ratio related to the impedance of the sub-bottom. This ratio is evaluated by first developing a matrix recurrence relation for the potentials in the solid which, when repeatedly applied, directly relates the first viscoelastic layer's response to the last layer's. After boundary conditions are applied at the liquid-solid interface, the impedance ratio is obtained from another matrix relation. The result is in a form convenient for digital computer calculations. The response for the homogeneous single layer or half-space is shown to be consistent with existing results.     

Research on the propagation of guided waves in multi-layered plates

The dispersion characteristics and excitation mechanisms of the guided waves in multilayered plates are studied in this paper. Firstly, the dispersion equation is obtained by the propagator matrix method. Then, the bisection technique is employed to find all the roots of the dispersion equation. The dispersion characteristics of the guided waves are investigated and analyzed. For the multilayered plates in which the S-wave velocity increases or decreases from top to bottom, it is found that the velocity limits in high frequency of the first and high modes are equal to the Rayleigh wave and S-wave velocities of the low-velocity layer, respectively. It is also found that the velocity limits in the high frequency of all modes are equal to the S-wave velocity of the low-velocity layer for the plate with a low-velocity middle layer. The normal displacement spectrum of all the modes excited by the normal force source with a definite width on the surface of the plate is also investigated. It is proved that the dominant mode is the first mode when the S-wave velocity increases from top to bottom layer and the dominant mode is different in different frequency range for the plate with a low-velocity middle layer.     

Dispersion and attenuation of surface acoustic waves of shear horizontal polarization on a free surface of a hexagonal crystal with a structurally damaged isotropic surface layer

The phase velocity dispersion and the inverse attenuation length of surface acoustic waves of shear horizontal polarization propagating along a free flat (smooth) surface of a hexagonal crystal (Z cut) in the presence of a thin (compared to the wavelength) structurally damaged surface layer are found in the analytical form. It is shown that, in the long-wavelength limit (the wavelength is large compared to the characteristic size of layer inhomogeneities), which is of the greatest interest to experimenters, the change in the phase velocity dispersion and the change in the inverse attenuation length are proportional to the third and sixth powers of the wave frequency, respectively. The inverse attenuation length is numerically calculated.     

海底表层声场垂直衰减分析

声波在海底中的传播衰减规律是海洋声学中值得研究的课题。根据简正波理论推导了高声速海底表层声场垂直衰减系数的解析表达式,并给出了其理论上限。数值仿真与实验研究表明:对液态海底模型,海底表层声场垂直衰减系数与声波频率基本呈正比;对于特定频率,海水-海底声速比是影响垂直衰减系数的最主要因素,比值越小垂直衰减系数越大;对于斜率较大的楔形海底地形,且声源在深水区、接收点在浅水区的情形,垂直衰减系数会大大小于其理论上限。     

Surface acoustic waves at a free-metallized interface

Light scattering was used to study surface acoustic waves at a free-metalized interface on piezoelectric lithium niobate. An interaction region of several acoustic wavelengths was observed in which a reflected surface wave and a continuous spectrum of bulk waves are generated.     

On the acoustic wave attenuation in a turbulent medium

Abstract

The equation for the mean acoustic field has been obtained for a random turbulent medium using the Green function approach. The correlation function was described by the Karman distribution with the index n=2 approximately?11/6. Applying Bourret's approximation, the exact expression for the mass operator has been calculated analytically. The frequency dependence of the scattering coefficient of the mean field has been derived. Conditions of Cherenkov radiation are discussed.     

On the acoustic wave attenuation in a turbulent medium

The equation for the mean acoustic field has been obtained for a random turbulent medium using the Green function approach. The correlation function was described by the Karman distribution with the index n=2 approximately≃11/6. Applying Bourret's approximation, the exact expression for the mass operator has been calculated analytically. The frequency dependence of the scattering coefficient of the mean field has been derived. Conditions of Cherenkov radiation are discussed.     

Dispersion and attenuation of surface shear acoustic waves with horizontal polarization on the free statistically rough surface of the hexagonal crystal

Expressions for dispersion of the phase velocity and inverse damping depth of surface acoustic waves with shear horizontal polarization are derived in an analytical form within perturbation theory using the modified mean-field method for the Z-cut hexagonal crystal with a free statically rough surface. Both two-and one-dimensionally rough surfaces are considered. The one-dimensionally rough surface is considered as a special case of the two-dimensionally rough surface. It is shown that shear surface waves with horizontal polarization cannot exist on the flat surface of the Z-cut hexagonal crystal. The derived expressions are studied analytically and numerically in the entire frequency range accessible in perturbation theory. The long-wavelength limit (most interesting from the experimental point of view) is considered, where the wavelength is much longer than the roughness correlation radius. The conditions for the existence of SH-polarized waves are determined for both roughness types. It is shown that dispersion and attenuation of SH polarized waves are qualitatively similar in character to those we considered previously for an isotropic medium.     

Effects of double spectral electron distribution and polarization force on dust acoustic waves in a negative dusty plasma

The nonlinear features of dust acoustic waves (DAWs) propagating in a multicomponent dusty plasma with negative dust grains, Maxwellian ions, and double spectral electron distribution (DSED) are investigated. A Korteweg de Vries Burgers equation (KdVB) is derived in the presence of the polarization force using the reductive perturbation technique (RPT). In the absence of the dissipation effect, the bifurcation analysis is introduced and various types of solutions are obtained. One of these solutions is the rarefactive solitary wave solution. Additionally, in the presence of the dissipation effects, the tanh method is employed to find out the solution of KdVB equation. Both of the monotonic and the oscillatory shock structures are numerically investigated. It is found that the correlation between dissipation and dispersion terms participates strongly in creating the dust acoustic shock wave. The limit of the DSED to the Maxwell distribution is examined. The distortional effects in the profile of the shock wave that result by increasing the values of the flatness parameter, r, and the tail parameter, q, are investigated. In addition, it has been shown that the proportional increase in the value of the polarization parameter R enhances in both of the strength of the monotonic shock wave and the amplitude of the oscillatory shock wave. The effectiveness of non-Maxwellian distributions, like DSED, in several of plasma situations is discussed as well.