Reflection and refraction of acoustic waves from the insulator-magnetoacoustic material interface

The reflection and refraction of longitudinal and transverse acoustic waves by the plane interface between an insulator and an easy-plane antiferromagnet undergoing a magnetic-field-induced orientational phase transition are analyzed. The angles of refraction, as well as all four coefficients of wave conversion, can be effectively controlled by varying the field. Conditions for the appearance of the critical angles of internal reflection and the effect of the magnetic field on these angles are considered. A glancing wave radiated into the material is shown to be a possibility near the phase transition.     

Reflection of structural waves at a solid/liquid interface

This paper investigates the reflection characteristics of structural or guided waves in rods at a solid/liquid interface. Structural waves, whose wavelengths are much larger than the diameter of the rod, are described in a first approximation by classical one-dimensional wave theory. The reflection characteristics of such waves at a solid/liquid (melting) interface has been reported by two different ultrasonic measurement techniques: first, measuring the fast regression rate of a melting interface during the burning of metal rod samples in an oxygen-enriched environment, and second, monitoring the propagation of the solid/liquid interface during the slow melting and solidification of a rod sample in a furnace. The second work clearly shows that the major reflection occurs from the solid/liquid interface and not the liquid/gas interface as predicted by plane longitudinal wave reflectivity theory. The present work confirms this observation by reporting on the results of some specially designed experiments to identify the main interface of reflection for structural waves in rods. Hence, it helps in explaining the fundamental discrepancy between the reflection characteristics at a solid/liquid interface between low frequency structural waves and high frequency bulk waves, and confirms that the detected echo within a burning metallic rod clearly represents a reflection from the solid/liquid interface.     

Ultrasonic fields radiated through matching layers with nonparallel boundaries

In the case of ultrasonic measurements in aggressive media piezoelectric elements of ultrasonic transducers are separated from a medium by thick protective layers, which may posses nonparallel front and back surfaces. This enables to reduce significantly the amplitude of multiple reflections, but the structure of the ultrasonic field radiated through a layer with nonparallel boundaries becomes complicated. The main objective of this paper is to present a method suitable for simulation of ultrasonic fields radiated through a layer with nonparallel boundaries in a transient mode. The proposed simulation method is based on the transformation of a multilayered medium into a virtual one without internal boundaries, equivalent to the actual medium from the point of a view of the relative times of arrival of direct and edge waves. The simulated ultrasonic fields in water are compared with the measurement results and a good correspondence between calculated and measured fields is obtained.     

Ultrasound fields in attenuating media

For medical ultrasonic imaging and for nondestructive testing, the attenuation of pressure waves and the resulting shift in wave velocity are important features in commonly used transmission media such as biological tissue. An algorithm for the numerical evaluation of pressure field distributions generated by ultrasonic transducers is presented. The attenuation and dispersion of the sound transmission medium are taken into consideration. The sound fields are computed numerically for continuous wave as well as pulse excitation. The transducer has plane or gently curved geometry and is embedded in a plane rigid baffle. The numerically determined pressure fields are presented as 3D plots, as gray-scale images for a fixed time stamp (like a snapshot), or as isobars regarding the maximum values over time for each local point in the area under investigation. The algorithm described here can be utilized as a tool for design of ultrasound transducers, especially array antennas.     

Using streamlines to visualize acoustic energy flow across boundaries

For spherical waves that radiate from a point source in a homogeneous fluid and propagate across a plane boundary into a dissimilar homogeneous fluid, the acoustic field may differ significantly from the geometric acoustic approximation if either the source or receiver is near the interface (in acoustic wavelengths) or if the stationary phase path is near the critical angle. In such cases, the entire acoustic field must be considered, including inhomogeneous waves associated with diffraction (i.e., those components that vanish with increasing frequency). The energy flow from a continuous-wave monopole point source across the boundary is visualized by tracing acoustic streamlines: those curves whose tangent at every point is parallel to the local acoustic intensity vector, averaged over a wave cycle. It is seen that the acoustic energy flow is not always in line with the "Snell's law" or stationary phase path. Also, plots of acoustic energy streamlines do not display unusual behavior in the vicinity of the critical angle. Finally, it is shown that there exists a law of refraction of acoustic energy streamlines at boundaries with density discontinuities analogous to Snell's law of refraction of ray paths across sound speed discontinuities. Examples include water-to-seabed transmission and water-to-air transmission.     

Anomalous transparency of water-air interface for low-frequency sound

Sound transmission through a water-air interface is normally weak because of a strong mass density contrast. We show that the transparency of the interface increases dramatically at low frequencies. Almost all acoustic energy emitted by a sufficiently shallow monopole source under water is predicted to be radiated into air. Increased transparency at lower frequencies is due to the increasing role of inhomogeneous waves. For sources symmetric with respect to a horizontal plane, transparency is further increased by a destructive interference of direct and surface-reflected waves under water. The phenomenon of anomalous transparency has significant geophysical and biological implications.     

The layer multiple-scattering method for calculating transmission coefficients of 2D phononic crystals

We extend the layer multiple-scattering theory (LMST) to elastic waves propagating in two-dimensional (2D) periodical composites. The formalism to calculate the reflection and transmission coefficients for elastic waves through finite slabs is presented. In this spirit, the crystal is viewed as a sequence of identical monolayer which has one-dimensional (1D) periodicity along a given direction. The reflection and transmission coefficients for a multilayer slab can be obtained by a double-layer scheme through the calculation of the scattering matrix of a monolayer. To demonstrate the application of this formalism, we calculate transmission coefficients for systems consisting of pure solid components or mixing (solid and fluid) components. The validity of this method is checked by both band structure calculations and transmission measurement of ultrasonic experiment.     

The thermoelastic excitation of air-solid interface waves using the pulsed laser

In the 1920s, the solid-solid interface wave, Stoneley wave, was first studied by Stoneley. From the 1930s to 1940s, the fluid-solid interface waves, usually called Scholte wave or Scholte-Stoneley wave, were studied by Cagniard and Scholte respec-tively[1]. The Scholte wave corresponds to the real root of the fluid-solid interface secu-lar equation, which is usually called the Scholte equation, and the velocity of Scholte wave is only slightly lower than the longitudinal velocity of the f…     

Asymptotic Analysis of Vertical Branch-Cut Integral of Shear Waves in a Fluid-Filled Borehole Utilizing the Steepest-Descent Method

We obtain an asymptotic solution to the vertical branch-cut integral of shear waves excited by an impulsive pressure point source in a fluid-filled borehole, by taking the effect of the infinite singularity of the Hankel functions related to shear waves in the integrand at the shear branch point into account and using the method of steepest-descent to expand the vertical branch-cut integral of shear waves. It is theoretically proven that the saddle point of the integrand is located at ks-i/z, where ks and z are the shear branch point and the offset. The continuous and smooth amplitude spectra and the resonant peaks of shear waves are numerically calculated from the asymptotic solution. These asymptotic results are generally in agreement with the numerical integral results. It is also found by the comparison and analysis of two results that the resonant factor and the effect of the normal and leaking mode poles around the shear branch point lead to the two-peak characteristics of the amplitude spectra of shear waves in the resonant peak zones from the numerical integral calculations.     

带斜楔相控声束偏转聚焦延时特性研究

带斜楔的相控阵超声检测广泛应用于低碳钢薄壁工件焊缝的检测,研究带斜楔的相控阵超声偏转聚焦检测延时法则和对应的声场将使检测可靠性进一步提高。本文利用费马原理探究了相控阵超声探头辐射声波至楔块中,在楔块-工件平面界面发生模式转换入射到工件中的横波聚焦时各阵元延时的计算方法,数值求解指定焦点时各阵元的延时,并利用计算得到的延时进行声场仿真和实验测量,发现声波能很好的聚焦在目标点,仿真和实验结果吻合。     

The size effects of a rigid reflector in the scattering of transient ultrasonic field

A model is proposed to assess the transient ultrasonic field radiated by a planar transducer and scattered by a target with a known dimension and geometry. The approach is based on the hypothesis of linear acoustics for a perfectly rigid reflector immersed in an isotropic, homogeneous and lossless fluid. The detected acoustic pressure is explained in terms of the plane and edge waves. In order to determine the boundary effects on the detected pressure, targets of different sizes were used in our simulations. An experimental verification of the proposed model is presented for the case of circular planar targets with different radii made of duralumin and immersed in distilled water. In general, the theoretically predicted results are in good agreement with the experimentally measured results.     

A ray technique to calculate reflected and transmitted waves in layered media

This paper considers the problem of calculating the propagation of acoustic waves within an ideal isotropic multilayer plate structure. In such a situation the process of mode conversion as the wave interacts with each interface of the plate creates an ever increasing number of waves to track, and to perform calculations on, as the wave propagates within the layered media. Exploring this problem by examining the ray paths of the multiple reflections within the plate structure, it is possible to show that upon careful consideration many of these paths will travel equivalent distances in time and space becoming coincident. The principle of superposition can then be used to combine these coincident paths, this superposition reduces the number of waves to track, and simplifies the problem so that the necessary calculations can be performed in a time efficient manner.     

Ultrasonic wave propagation in heterogeneous solid media: theoretical analysis and experimental validation

This research deals with the ultrasonic characterization of thermal damage in concrete. This damage leads to the appearance of microcracks which then evolve in terms of volume rate and size in the material. The scattering of ultrasonic waves from the inclusions is present in this type of medium. The propagation of the longitudinal wave in the heterogeneous media is studied via a homogenization model that integrates the multiple scattering of waves. The model allows us to determine the phase velocity and the attenuation according to the elements which make the medium. Simulations adapted to the concrete are developed in order to test the responses of the model. These behaviors are validated by an experimental study: the measurements of phase velocity and attenuation are performed in immersion, with a comparison method, on a frequency domain which ranges from 160 kHz to 1.3 MHz. The analysis of different theoretical and experimental results obtained on cement-based media leads to the model validation, on the phase velocity behavior, in the case of a damage simulated by expanded polystyrene spheres in granular media. The application to the case of a thermally damaged concrete shows a good qualitative agreement for the changes in velocity and attenuation.     

Plane-wave analysis of the near field of light diffracted by ultrasound.

Although the phenomenon of light diffraction by ultrasound has been studied very extensively during the last 40 years, almost all investigations were concentrated on the individual far field (Fraunhofer) diffraction orders. In the present paper, the basic theory is developed for studying the near field (Fresnel region) of light diffracted by an arbitrary plane ultrasonic wave and the fundamental periodicity properties are stated. The general plane-wave theory of Raman-Nath has been taken as a starting point. From the analysis, the near field of the diffracted light is seen to be highly sensitive to variations of the ultrasonic amplitude and this feature provides a useful technique for observing weak ultrasonic waves. In particular, for the specific case of Raman-Nath-type diffraction, a procedure is presented allowing the reconstruction of the time waveform of the ultrasonic wave from the diffracted light intensity signal.     

The use of an orthogonality relation for reducing the size of finite element models for 3D guided waves scattering problems

The scattering of guided waves by complex shaped defects in three-dimensional (3D) waveguides is considered. For such problems, analytical solutions do not exist, and modal decomposition techniques based on the establishment of the displacement and stress fields in the vicinity of the scatterer are quite heavy and complicated to perform. On the other hand, finite elements (FE)-based methods constitute a powerful way to obtain solutions, but they are known to be very memory consuming. This paper proposes a post-processing technique, based on a 3D orthogonality relation, to decompose a complex acoustic field produced by a scatterer and predicted by a 3D FE model, into plane waves, the amplitudes of which are quantified in the far field. This technique allows important reductions in the size of the FE models to be made. Two applications are presented to demonstrate the potential of this method. The first one concerns the scattering of the S₀ Lamb wave incident on a flat bottom circular hole. In this example, the amplitude of each mode is calculated via the orthogonality relation-based method, and compared to that obtained by simply monitoring the displacements at appropriate through-thickness positions. In the second application, the incident S₀ Lamb mode is converted into five modes scattered by a defect of complex geometry.     

Low-frequency sound transmission through a gas-liquid interface

Typically, sound speed in gases is smaller and mass density is much smaller than in liquids, resulting in a very strong acoustic impedance contrast at a gas-liquid interface. Sound transmission through a boundary with a strong impedance contrast is normally very weak. This paper studies the power output of localized sound sources and acoustic power fluxes through a plane gas-liquid interface in a layered medium. It is shown that, for low-frequency sound, a phenomenon of anomalous transparency can occur where most of the acoustic power generated by a source in a liquid half-space can be radiated into a gas half-space. The main physical mechanism responsible for anomalous transparency is found to be an acoustic power transfer by inhomogeneous (evanescent) waves in the plane-wave decomposition of the acoustic field in the liquid. The effects of a liquid's stratification and of guided sound propagation in the liquid on the anomalous transparency of the gas-liquid interface are considered. Geophysical and biological implications of anomalous transparency of water-air interface to infrasound are indicated.     

Negative refraction in two-dimensional photonic crystals

We present some of our recent results for negative refraction in photonic crystals. The concept of negative refraction in photonic crystals is firstly introduced. Then, the propagation of electromagnetic waves in photonic crystals is systematically studied. By the layer Korringa–Kohn–Rostoker method, the coupling efficiency between external plane waves and the Bloch waves in photonic crystals is investigated. It is found that the coupling coefficient is highly angular dependent even for an interface between air with n=1 and a photonic crystal with effective index n_eff=-1. It is also shown that, for point imaging by a photonic crystal slab, owing to the negative refraction, the influence of the surface termination on the transmission and the imaging quality is significant. Finally, we present results experimentally demonstrating negative refraction in a two-dimensional photonic crystal at optical communication wavelengths. PACS 42.70.Qs; 41.85.Ct; 42.30.Va     

A finite-element model of the aperture method for determining the effective radiating area of physiotherapy treatment heads

This paper describes a theoretical study of the way in which a circular aperture placed in front of a plane-piston modifies the ultrasonic field it generates. Specifically, the radiated acoustic power transmitted by the aperture and the radiation force experienced by an absorbing target placed in the transmitted beam, are evaluated at a distance from the exit-side of the aperture. The calculations used a finite element (FE) method, in conjunction with a surface Helmholtz integral formulation to solve the fluid/structure interaction problem. The PAFEC (Program for Automatic Finite Element Computation) vibroacoustics software was used for the FE calculations and the implementation of the surface Helmholtz integral formulation method. Acoustic pressures and particle velocities were computed at required points, whilst accounting for the presence of the aperture in the medium, together with its dynamic properties when subjected to an incident sound field. This enabled the calculation of the radiation force on the absorber and its variation with the applied aperture diameter was investigated. As part of the validation process for the new FE aperture model, the ratio of radiation force to radiated acoustic power obtained using the FE method in the unapertured case, through the use of the Rayleigh integral, yielded good agreement with results obtained through an analytical solution. The study has been carried out to provide a better understanding of the factors affecting the measurement uncertainty for the aperture method of determining the effective radiating area (A(ER)) of physiotherapy ultrasound treatment heads.     

Negative refraction imaging of solid acoustic waves by two-dimensional three-component phononic crystal

By using of the multiple scattering methods, we study the negative refraction imaging effect of solid acoustic waves by two-dimensional three-component phononic crystals composed of coated solid inclusions placed in solid matrix. We show that localized resonance mechanism brings on a group of flat single-mode bands in low-frequency region, which provides two equivalent frequency surfaces (EFS) close to circular. The two constant frequency surfaces correspond to two Bloch modes, a right-handed and a left-handed, whose leading mode are respectively transverse (T) and longitudinal (L) modes. The negative refraction behaviors of the two kinds of modes have been demonstrated by simulation of a Gaussian beam through a finite system. High-quality far-field imaging by a planar lens for transverse or longitudinal waves has been realized separately. This three-component phononic crystal may thus serve as a mode selector in negative refraction imaging of solid acoustic waves.     

Special features of oblique wave propagation through the interface of media with dislocations

The mechanisms of propagation of oblique plane harmonic waves through the interface of Voigt and Maxwell viscoelastic media were studied in the context of field theory of defects which describes dislocation continuum dynamics. The defect field waves structure for different displacement wave polarizations was determined. Analytical expressions were derived for the reflection and refraction coefficients of displacement waves and defect field waves propagating in the media. The dependences of Fresnel coefficients on the incidence angle of a primary wave and parameters of the contacting media were analyzed.