Lens multielement acoustic microscope in the mode for measuring the parameters of layered objects

An acoustic microscope with a cylindrical lens and ultrasound transducer have been considered, as well as the method based on it for the measuring of longitudinal and transverse wave velocities, the thickness and density of the investigated layer. A theoretical model of the microscope has been constructed, and the relation between the spatiotemporal output signal of the transducer and the angular dependence of the sample reflection coefficient has been found. It has been shown that the velocities of body waves and the thickness can be determined by the delays of ultrasound responses reflected from the layer boundaries measured by the transducer elements, and the density, by the amplitudes of these responses. The method was tested experimentally using a 20-element transducer with a central frequency of 15 MHz and a period of 0.8 mm. The example of a duralumin plate has shown that the error in measuring the thickness and velocity of longitudinal waves error does not exceed 1%; the velocity of transverse waves, 2%; and the density can be estimated with an accuracy of about 5%.     

Acoustic identification of the anisotropic nanocrystalline medium with non-dense packing of particles

A two-dimensional model of the anisotropic nanocrystalline (granular) medium being a rectangular lattice of elastically interacting elliptical particles with translational and rotational degrees of freedom was considered. In the long-wave approximation a system of linear equations in partial derivatives describing the propagation of the longitudinal, transverse, and rotational waves in such a system was obtained. The dependences of the wave velocities on the grain size and form were analyzed. It was shown how to determine the moduli of elasticity of the granular material from the change of the velocities of the acoustic waves propagating along different crystallographic directions.     

Experimental investigation of the propagation velocity of ultrasonic waves and calculation of the elastic constants of nanocrystalline pyrocarbon

The dispersion of the propagation velocity of ultrasonic waves in pyrocarbon samples with a monotonically varying structure is studied in a frequency range of 0.5–25 MHz. The elastic constants of pyrocarbon samples that were obtained with the processing temperatures of 2100–3200°C are calculated on the basis of the measured values of propagation velocities of longitudinal and transverse ultrasonic waves.     

Maxima and minima of the displacement components for the Lamb modes

This paper revisits the vanishing of the transverse component of the particle displacement vector in free surfaces of an isotropic homogeneous plate, for both symmetric and antisymmetric Lamb waves. Drawing on well-known analytical expressions from Viktorov's book [(1967) Rayleigh and Lamb Waves: Physical Theory Applications, Chap. II, pp. 67-121], two distinct frequency-thickness product expressions, in cases where this vanishing occurs, are derived: one for the symmetric modes and another for the antisymmetric modes. At these frequency-thickness products, phase and group velocities have appreciable values which are discussed herein. It appears that these velocities depend on the transverse bulk wave velocity only. This is the specific condition of the Lame? modes. Moreover, theoretical and experimental investigations of displacements in the surface of a plate in air have been carried out. The theoretical part shows that the normal and transverse displacements have, respectively, a local maximum and a local minimum in the vicinity of these frequency-thickness products. The experimental part corroborates the presence of the local maximum of the S(0) Lamb mode for various materials.     

Soliton generation via continuous stokes acoustic self-scattering of hypersonic waves in a paramagnetic crystal

A new mechanism is proposed for continuous frequency down-conversion of acoustic waves propagating in a paramagnetic crystal at a low temperature in an applied magnetic field. A transverse hypersonic pulse generating a carrier-free longitudinal strain pulse via nonlinear effects is scattered by the generated pulse. This leads to a Stokes shift in the transverse hypersonic wave proportional to its intensity, and both pulses continue to propagate in the form of a mode-locked soliton. As the transverse-pulse frequency is Stokes shifted, its spectrum becomes narrower. This process can be effectively implemented only if the linear group velocity of the transverse hypersonic pulse equals the phase velocity of the longitudinal strain wave. These velocities are renormalized by spin-phonon coupling and can be made equal by adjusting the magnitude of the applied magnetic field. The transverse structure of the soliton depends on the sign of the group velocity dispersion of the transverse component. When the dispersion is positive, planar solitons can develop whose transverse component has a topological defect of dark vortex type and longitudinal component has a hole. In the opposite case, the formation of two-component acoustic “bullets” or vortices localized in all directions is possible.     

Ultrasonic, hardness and x-ray densitometric analysis of wood

Ultrasonic, hardness and x-ray densitometric methods were used to predict the wood quality of Douglas fir sapwood from pruned and control trees. Measurements were performed on logs and on increment cores.

On logs, hardness was tested by a pilodyn instrument. Longitudinal waves (80 kHz) were used to measure the velocity in the radial anisotropic direction of the wood. Surface waves of the same frequency were employed to measure the velocity on the circumference of the log in the longitudinal anisotropic direction of the wood.

On increment cores, 1 MHz waves, longitudinal and transverse, were used to measure the velocity of ultrasonic pulses along three anisotropic axes of the wood, using the through-transmission technique.

The predictive power of the pilodyn test and of the surface wave velocity method for logs was judged in relation to several independent variables on cores given by the densitometric method and by ultrasonic velocities and elastic constants of the wood.     

Determination of isotropic layer parameters from spatiotemporal signals of an ultrasonic array

The paper discusses a method for measuring the velocities and attenuation of longitudinal and transverse ultrasonic waves and the density and thickness of the isotropic layer with an array placed in an immersion liquid parallel to the sample. The method is based on the recording of the total spatiotemporal signal of the array and its expansion into a spatial spectrum of pulse plane wave response. The ultrasonic velocity and sample thickness depend on the response delay of the plane wave in the layer from the transverse projection of the slowness vector. The density and attenuation are determined from the behavior of the amplitudes of spectral responses. To confirm this method in experiment, the parameters of a polystyrene plate have been measured using a linear 32-element array with a central frequency of 17 MHz.     

Transverse wave band gaps and longitudinal wave band gaps in solid phononic crystals

Based on the properties of transverse (divergenceless) waves and longitudinal (irrotational) waves, we divided the transverse wave modes and longitudinal wave modes from the mixed eigen modes in solid phononic crystals. By investigating the transverse wave and longitudinal wave band structures at low frequency, we found that transverse bands and longitudinal bands exhibit different behaviors in solid systems including spherical scatterers. Phononic crystal with a large density ratio of solid spheres to the background can guarantee both the large longitudinal and large transverse band gap, but solid spheres with a small ratio of longitudinal wave velocity to transverse wave velocity can only help to enlarge the longitudinal band gap, and do not help to enlarge the transverse band gap.     

Characterization of subwavelength elastic cylinders with the decomposition of the time-reversal operator: theory and experiment

The decomposition of the time-reversal operator provides information on the scattering medium. It has been shown [Chambers and Gautesen, J. Acoust. Soc. Am. 109, 2616-2624 (2001)] that a small spherical scatterer is in general associated with four eigenvalues and eigenvectors of the time-reversal operator. In this paper, the 2D problem of scattering by an elastic cylinder, imbedded in water, measured by a linear array of transducers is considered. In this case, the array response matrix has three nonzero singular values. Experimental results are obtained with linear arrays of transducers and for wires of different diameters smaller that the wavelength. It is shown how the singular value distribution and the singular vectors depend on the elastic velocities cL, cT, the density rho of each wire, and on the density rho0 and velocity c0 of the surrounding fluid. These results offer a new perspective towards solution of the inverse problem by determining more than scattering contrast using conventional array processing like that used in medical ultrasonic imaging.     

Two-dimensional wave propagation in a rotating elastic solid with voids

Two-dimensional wave propagation is studied in an isothermal linear isotropic elastic material with voids rotating with constant angular velocity based on a theory of elastic material with voids developed by Ie?an (1986) in the thermoelastic context. It is found that there exist three coupled plane waves propagating with distinct phase speeds. The presence of voids and the rotation of the medium are responsible for this coupling. In the absence of voids, the classical longitudinal and transverse waves are found to be coupled through the rotation of the medium. At very large frequency or when the angular rotation is very small relative to the wave frequency the waves are decoupled and propagate with distinct phase speeds. These are (i) a longitudinal wave, (ii) a transverse wave and (iii) a longitudinal wave corresponding to the change in void volume fraction. The first two correspond to the waves of classical elasticity, while the third is new and arises from the presence of the voids. The results are illustrated graphically.     

On the Instabilities of the Collisionless and Non-Magnetized Active Plasmas

Spontaneously excited instabilities in a uniform, collisionless and non-magnetized active plasma are studied, under the assumption that the polarisability of its active constituent is due to transitions of only one optical electron between two inversely populated energy levels. The attention is focused on non-resonant situations in which the modal frequencies of the excited waves differ significantly from the frequency of coherent electromagnetic radiation arising due to the presence of the active constituent. The longitudinal (‘acoustic’) waves in both low and high phase velocity domains, as well as the transverse (‘optical’) waves, are investigated. It is shown that the instabilities excited are most frequently aperiodical, and appear within, a comparatively large wavelength range. Their amplitude increments are evaluated. Under certain particular conditions, specified in the paper, the instabilities turn out to be periodical and to appear in very narrow wavelength bands only. Their modal frequencies and amplitude increments are also evaluated.     

Visual characteristics of inhomogeneous acoustic waves

Experimentally obtained visualizations of propagating inhomogeneous acoustic waves driven by zero-order antisymmetric Lamb waves (flexural waves) in water are presented. The inhomogeneous waves are visualized by optical holographic interferometry. A series of photographs show the evolution in time of instantaneous acoustic pressure distributions associated with propagating inhomogeneous waves. The photographs reveal characteristic features of flexurally driven inhomogeneous waves such as transversely attenuated wavefronts oriented perpendicularly to the plate boundary and a phase propagation velocity along the boundary approximately equal to the plate wave velocity (250 meters/second). Effects due to the dispersive nature of the flexural plate waves are also noted in the photographic series. Features distinguishing these subsonic, inhomogeneous surface waves (also called trapped or evanescent waves) from the leaky, lateral or head wave and also from incompressible fluid motions associated with low frequency vibrations of fluid loaded plates are identified. The relevance of inhomogeneous acoustic waves driven by subsonic flexural waves to practical sound-structure interaction problems is discussed.     

Acoustic attenuation in the layers of a microwave transducer at the excitation of longitudinal and shear elastic waves by a piezoelectric of the 6<Emphasis Type="Italic">mm</Emphasis> symmetry class

An electroacoustic transducer in the form of a piezoelectric of the 6mm symmetry class with an arbitrary orientation of the sixfold axis and with two finite-thickness metal electrodes is considered taking into account the acoustic attenuation in the transducer layers. A system of equations is obtained to determine the impedance of the transducer, the radiation resistances for shear and longitudinal waves, the power ratio of these waves in the acoustic line, and the transformation factors for transverse and longitudinal waves. The effect of attenuation on the characteristics of a specific transducer operating in the 15-GHz frequency range is numerically analyzed.     

脉冲等离子体辐射微波机理的初步研究

 在分析了等离子体辐射微波的电流波形和喇叭天线接收到的微波信号波形的基础上,对脉冲等离子体辐射微波的机理进行了理论分析：阴阳极间存在TEM和TM两种模式的波;流经等离子体的电流产生的磁场使等离子体中的波成为慢波;高频场的激励因素有两个,一是作加速运动的电子,二是阳极斑点溅射时阳极电位突降;同一时刻具有不同速度的电子将能量转换给相应相速的波是其产生超宽带辐射的原因。     

Natural beam focusing of non-axisymmetric guided waves in large-diameter pipes

The propagation of non-axisymmetric guided waves in larger diameter pipes is studied in this paper by treating the guided waves as corresponding Lamb waves in an unwrapped plate. This approximation leads to a simpler method for calculating the phase velocities of hollow cylinder guided waves, which reveals a beam focusing nature of non-axisymmetric guided waves generated by a partial source loading. The acoustic fields in a pipe generated by a partial-loading source includes axisymmetric longitudinal modes as well as non-axisymmetric flexural modes. The circumferential distribution of the total acoustic field, also referred as an angular profile, diverges circumferentially while guided waves propagate with dependence on such factors as mode, frequency, cylinder size, propagation distance, etc. Exact prediction of the angular profile of the total field can only be realized by numerical calculations. In particular cases, however, when the wall thickness is far less than the cylinder diameter and the wavelength is smaller than or comparable to the pipe wall thickness, the acoustic field can be analyzed based on the characteristics of Lamb waves that travel along a periodic unwrapped plate. Based on this assumption, a simplified model is derived to calculate the phase velocities of non-axisymmetric flexural mode guided waves. The model is then applied to discussions on some particular characteristics of guided-wave angular profiles generated by a source loading. Some features of flexural modes, such as cutoff frequency values are predicted with the simpler model. The relationship between the angular profiles and other factors such as frequency, propagation distance, and cylinder size is obtained and presented in simple equations. The angular profile rate of change with respect to propagation distance is investigated. In particular, our simplified model for non-axisymmetric guided waves predicts that the wave beam will converge to its original circumferential shape after the wave propagates for a certain distance. A concept of "natural focal point" is introduced and a simple equation is derived to compute the 1st natural focal distance of non-axisymmetric guided waves. The applicable range of the simplified equation is provided. Industrial pipes meet the requirement of wall thickness being far less than the pipe diameter. The approximate analytical algorithms presented in this paper provides a convenient method enabling quick acoustic field analysis on large-diameter industrial pipes for NDE applications.     

Heating of gadolinium plasma ions by the ion cyclotron resonance method

Resonance rf heating of gadolinium plasma ions is calculated in the configuration when an electric field travels along a permanent magnetic field and simultaneously rotates in the direction normal to the latter. Two model functions are taken as initial ion distribution functions over longitudinal velocities: one is a linear function of the velocity in the low velocity range and the other is a shifted semi-Maxwellian function. The ion transverse velocity distribution function is calculated under the assumption that the initial “transverse” distribution function is Maxwellian with a temperature of 5 eV. Ion fluxes toward collector plates are calculated by integrating the total distribution function over the allowed ranges of longitudinal and transverse velocities and transverse coordinates of the guiding center of the ions before the collector. The calculation is performed as applied to the ¹⁵⁷Gd target isotope and its two nearest neighbors. The effect of the longitudinal temperature on the width of the heating efficiency resonance line and of the longitudinal magnetic field on the ion heating selectivity is studied. Also, the influence of the longitudinal wavenumber of the warming traveling electric field on the selectivity of an ion cyclotron resonance reactor is investigated. The heating efficiency is estimated from the frequency dependence of the fraction of ions heated to an energy above a given value.     

Measurements of the longitudinal wave speed in thin materials using a wideband PVDF transducer

A flat transducer was constructed, using a 9-microm-thick PVDF (polyvinylidene fluoride) film for generation and detection of high-frequency ultrasonic waves, and used for measurements of the phase velocity of longitudinal waves traveling along the thickness direction in a very thin material. The transducer has a useful wideband frequency characteristic extending from 10 MHz to over 150 MHz. Measurements of the phase velocity of the longitudinal waves are carried out using a 0.212-mm-thick glass slide and a 0.102-mm-thick stainless-steel shim, using water as a coupling medium. The thickness limit for this measurement appears to be approximately 20 microm. The phase velocity of the longitudinal mode is obtained as a function of frequency in the frequency domain by using a modified sampled continuous wave (cw) technique. It can also be measured in the time domain by using a broadband pulse of short duration.     

用于储罐底板缺陷检测的超声兰姆波模式研究

从超声兰姆波的声场方程出发,得到板材表面离面位移(法向位移)为零的条件,即当超声兰姆波的相速度等于板材介质的纵波声速时,在板材表面的离面位移为零。在给定适当频厚积的条件下,分别数值模拟了仅有切向位移而无离面位移的A1、S1、A2和S2兰姆波模式在有液体负载的单层钢板中的传播情形。结果表明:离面位移为零的S2模式频散较小且对板中缺陷更为敏感。     

Diffuse wavefields in cylindrical coordinates

A diffuse wavefield is normally defined in terms of plane waves--to quote one textbook definition "plane waves are incident from all directions with equal probability and random phase." In some vibro-acoustic problems the response of a two-dimensional component such as a plate is more conveniently expressed in terms of cylindrical waves, and it is not immediately obvious what properties should be assigned to the cylindrical waves to constitute a diffuse field. It is shown here that a diffuse wavefield can be modeled as a summation of statistically independent cylindrical waves, apart from the fact that each outgoing wave of a particular order is fully correlated to an incoming wave of the same order. A simple relationship is derived between the energy flow P in each wave component and the energy density e of the wavefield: P = ec(g)/k, where c(g) is the group velocity and k is the wavenumber. This result is shown to hold true for both bending waves and in-plane waves (longitudinal and shear) in a plate. The work has application to the calculation of coupling loss factors in statistical energy analysis.     

Detection of moving objects and flows in liquids by ultrasonic phase conjugation

The possibility for the application of the method of parametric phase conjugation of ultrasonic waves in measuring the velocity of moving objects and flows is investigated. Results of experimental measurements of the Doppler frequency shift are presented for a low-frequency wave (1 MHz) generated by phase-conjugate waves (10 MHz and 11 MHz) propagating in opposite directions in the presence of a moving scatterer. The super high sensitivity of the phase of the low-frequency wave to variations in the spatial position of the scatterer is used to measure the velocity of the object. The presence of flows in the region of propagation of phase-conjugate waves returned leads to an uncompensated Doppler shift of the phase of the phase-conjugate wave at the primary radiation source. The implementation of this feature of ultrasonic phase conjugation for the detection and measurement of the flow velocities in a liquid is demonstrated experimentally.