Reflection Of Acoustic Waves Research Articles

Transport theory for acoustic waves with reflection and transmission at interfaces

Transport theoretic boundary conditions are derived for acoustic wave reflection and transmission at a rough interface with small random fluctuations. The Wigner distribution is used to go from waves to energy transport in the high frequency limit, and the Born expansion is used to calculate the effect of the random rough surface. The smoothing method is also used to remove the grazing angle singularity due to the Born approximation. The results are presented in a form that is convenient both for theoretical analysis and for numerical computations.

Reflection and refraction of a planar acoustic wave in an anisotropic inhomogeneous layer

The problem of reflection and refraction of a planar acoustic wave by an inhomogeneous elastic layer whose material possesses general-type anisotropy is considered. The equations of motion of the elastic layer are reduced to a system of ordinary differential equations. The boundary-value problem for this system is solved by two methods: by reduction to problems with initial conditions and by the method of power series. Analytical expressions that describe acoustic fields outside the layer are obtained. Calculation results of the transmission factor for transversely isotropic layers inhomogeneous in thickness are presented.

Characterization of lead zirconate titanate piezoceramic using high frequency ultrasonic spectroscopy

Doped piezoceramic lead zircanate titanate has been characterized in the frequency range of 20–60 MHz using ultrasonic spectroscopy. Theoretical analyses were performed for the reflection and refraction of acoustic waves at the interface of water-piezoelectric ceramic. The incident directions of the wave were chosen to be appropriate for ultrasonic spectroscopy measurements. Shear wave spectrum was obtained through mode conversion using a pair of longitudinal transducers submerged in water. The phase velocity shows linear dependence on frequency while the attenuation may be described by a second order polynomial of frequency in the frequency range investigated. The Kramers–Kronig relation between ultrasonic phase velocity and attenuation was compared to measured results.

Polynomial Approximation of SAW Reflection Characteristics for Fast Device Simulation Tools

The paper deals with the derivation of simple polynomials characterising surface acoustic wave (SAW) reflection by metallic gratings with various widths and thicknesses. Such polynomial approximation makes it possible to estimate coupling of mode (COM) parameters in markedly reduced computational time. For very sophisticated SAW devices, this is of great use to develop high-speed simulation and design tools based on the COM method. In the present study, it is shown how simply and accurately the SAW reflection by aluminium gratings on 128°YX-LiNbO3 substrates or 36°YX-LiTaO3 substrates is characterised by polynomial approximation.

Surface acoustic wave resonances in the spreading of viscous fluids

Reflection and attenuation of a Rayleigh surface acoustic wave from a small stripe of viscous fluid spreading across the acoustic path have been observed. As the stripe spreads across the acoustic path, with an accompanying decrease in stripe height to conserve volume, the reflected and attenuated acoustic signals show a distinct pattern of resonances. This paper reports experimental results for the spreading of highly viscous poly(dimethyl)siloxane oils (100 000 cS) in the propagation path of a 169 MHz Rayleigh wave operating in pulse mode and develops a model to explain the observed resonances of the transmission coefficient. Simultaneous optical observations enable the time evolution of the shape of the stripe to be converted into changes in geometric parameters (contact width, spherical radius, height, and contact angle). To model the acoustic attenuation, an approach treating the fluid as a viscoelastic fluid with a single relaxation time (Maxwell model) is developed. This approach is able to explain the structure of the observed transmitted signal. Resonances in the transmission coefficient occur cyclically and correspond to the stripe height matching $n{\ensuremath{\lambda}}_{s}/4,$ where n is odd and ${\ensuremath{\lambda}}_{s}$ is the shear wavelength in the fluid.

Scanning phononic lattices with surface acoustic waves

We examine the transmission and reflection of surface acoustic waves from a two-dimensional hexagonal lattice. Highly anisotropic signals are observed by continuously scanning the wavevector angle. Preliminary models of wave propagation through this phononic lattice predict the acoustic dispersion and a complete band gap; however, the experimental results indicate that more realistic modeling of surface acoustic waves in elastically modulated media is required.

Acoustic boundary layer phenomena near a corrugated surface

Acoustic resonator techniques for measuring thermophysical properties require a quantitative understanding of the viscous and thermal boundary layers. Near smooth solid boundaries, well-known analytic models of boundary layers have been confirmed by careful measurements. Little is known either experimentally or theoretically about the effects of surface roughness on the boundary layers. Roughness effects have been investigated through numerical calculations of the coupled acoustic, thermal, and shear fields near a corrugated interface, i.e., a surface with a profile y=A cos qx. The calculations were carried out in the limit where the acoustic wavelength is much larger than the scale of surface roughness (A,2π/q) or the penetration lengths δt and δv. Boundary layer phenomena cause phase shifts and energy absorption in the reflection of a plane acoustic wave by a smooth surface. These effects are further modified at a corrugated reflecting surface. The calculated reflection coefficients have been applied to the model relating the speed of sound to the frequencies of spherical and cylindrical acoustic cavity resonances, and to the theoretical model of the Greenspan acoustic viscometer. [Research supported in part by the Office of Naval Research.]

High-frequency acoustic waves reflection on seafloor

The purpose of this research is to study the acoustic waves reflection on seafloor in very shallow water in the frequency range 200–700 kHz. Effectively previous operations of detection of proud objects using sonar imagery showed problems in this environmental context caused by a superficially modified sediment structure and an insonification closed to ‘‘critical angle.’’ A study has been done with experimental data collected in a tank (at scale ‘‘one’’) on calibrated sand. This approach eases control of a lot of acoustical and geophysical parameters. Mono- and bistatic scattering measurements are recorded. Temporal broadband signals are analyzed by Fourier transforms. Results are compared with various theories. It especially emphasizes the Biot propagation model and a composite roughness model. The input sediment structure parameters are debated with discussion about their influence on seafloor reflection. Concerning the nontotal internal reflection below critical angle, the debate is not yet closed and expected additional data experiments on acoustic wave field inside the sediment itself would allow us to understand the mechanism of acoustic waves propagation in very shallow water and extract better models fitting experimental results.

Ultrasound contrast agents: Basic principles

Introduction. Ultrasonography lacked substances to be administered to patients to improve or increase the diagnositc yield, which is peculiar considering that contrast agents have long been used with all the other imaging techniques. Fortunately some contrast agents, most of them consisting in gas microbubbles, have been recently introduced for ultrasound imaging too: this review will focus on their history, behavior, current applications and future developments. Echocontrast agent research is in progress and many new agents are expected to be marketed this and next year, to be added to Leovist™ by Schering AG (Berlin, Germany), to enhance the ultrasound signal safely and effectively. No definitive conclusions can be drawn yet on the actual merits of each contrast agent, but all of them seem to be both effective and safe, meaning that their future success will depend on the relative cost-effectiveness and peculiarities. The basic principles of echocontrast agents. The microbubbles act as echo-enhancers by basically the same mechanism as that determining echo-scattering in all the other cases of diagnostic ultrasound, namely that the backscattering echo intensity is proportional to the change in acoustic impedance between the blood and the gas making the bubbles. The different acoustic impedance at this interface is very high and in fact all of the incident sound is reflected, even though not all of it will of course go back to the transducer. But the acoustic wave reflection, though nearly complete, would not be sufficient to determine a strong US enhancement because the microbubbles are very small and are sparse in the circulation. Moreover, reflectivity is proportional to the fourth power of a particle diameter but also directly proportional to the concentration of the particles themselves. Second harmonic imaging. As we said above, the microbubbles reached by an ultrasound signal resonate with a specific frequency depeding on microbubble diameter. However, the main resonance frequency is not the only resonance frequency of the bubble itself and multiple frequencies of the fundamental one are emitted, just like in a musical instrument. These harmonic frequencies have decreasing intensity, but the second frequency, known as the second harmonic, is still strong enough to be used for diagnostic purposes. The theoretical advantage of the harmonic over the fundamental frequency is that only contrast agent microbubbles resonate with harmonic frequencies, while adjacent tissues do not resonate, or else their harmonic resonation is very little. Thus, using a unit especially set to produce ultrasounds at a given frequency (3.5 MHz) and receive an ultrasound signal twice as powerful (7 MHz) it will be possible to show the contrast agent only, without any artifact from the surrounding anatomical structures, with a markedly improved signal-to-noise ratio. A similar effect to digital subtraction in angiography can thus be obtained, even though through a totally different process. Moreover, second harmonic imaging permits to show extremely small vessels (down to 40 μm) with very slow flow, which would be missed with a conventional method. B-mode imaging can also depict the microbubbles in the myocardium suppressing nearly all the artifacts from cardiac muscle motion. Recently a peculiar behavior of microbubbles has been observed which may permit contrast agent detection even in capillaries. This method is variously known as sonoscintigraphy, loss of correlation, stimulated acoustic emission and transient scattering. The contrast agent microbubbles reached by an ultrasound beam powerful enough explode producing a strong and very short backscatter echo which is read by the unit as a Doppler signal and results in a color pixel where the individual microbubble exploded. Conclusions. The microbubble contrast agents developed and introduced as safe and effective echo-enhancers in present-day clinical practice will open up new oppurtunities and result in an amazing revolution of ultrasonography as we know it now.

Nonlinear reflection of acoustic waves at the dissipative isotropic solid–solid interfaces

The nonlinear reflection at the solid–solid interfaces is investigated under the condition that a SV wave is incident from a nondissipative solid medium to a dissipative solid medium. The amplitude of the freely propagating reflected second harmonic SV wave is calculated. Comparing with previous calculation without consideration of dissipation, it is observed that the peaks of the reflected second harmonic SV wave in the vicinity of critical angles for refractive P and SV waves become smooth and the influence of nonlinear properties of the refractive medium on the amplitude of the reflected second harmonic wave is more notable. A experiment is carried out for glass–copper or rock interface. The SV wave of 8 MHz is incident from glass in which the attenuation of the acoustic wave is negligible upon glass–copper or rock interface. The reflected second harmonic SV wave is detected by a transducer with the working frequency of 16 MHz. The amplitudes of the second harmonic wave at the different incident angles are measured. The resultant angular dependence is in basic agreement with the calculation. [Work supported by NSF of China.]

The effect of a layer of varying density on high-frequency reflection, forward loss, and backscatter [seafloor acoustics

This paper examines the impact that a thin layer of varying density would have on high-frequency reflection, forward loss, and backscattering of acoustic plane waves from the seafloor. A functional form for density stratification was found by examination of several high-resolution density profiles obtained from X-ray computed tomography (CT) scans of seafloor cores. A solution based on these general profiles was used to estimate the reflection coefficient. The influence of the density profile on reflection loss and backscatter was then calculated using the estimated reflection coefficient. Parameter values used in simulations were obtained from the literature or from the CT scans of cores. It was found that inclusion of a density profile adds a strong frequency dependence to estimates of the reflection coefficient and forward loss. The largest effect on total scattering strength is near normal incidence where returns are dominated by interface scattering. The effect of the density profile on the strength of acoustic returns suggests that care should be taken when using high-frequency systems for measuring sediment properties, especially near normal incidence.

Reflection of surface acoustic waves by localized wetting liquids

Rayleigh wave propagation on unloaded solid surfaces becomes severely damped when the surface is covered by a liquid. Our experimental results show that, when the liquid is localized on the surface, reflections of the wave may occur. Moreover, if the liquid shape dynamically evolves, these reflections display a distinct series of maxima and minima.

Reflection of Transient Acoustic Waves by a Continuously Layered Halfspace with Depth-Dependent Attenuation

An integral transformation-type method is presented for the analysis of the space-time domain acoustic wavefield that is reflected by a halfspace consisting of a continuously layered, lossy, isotropic (equivalent) fluid. The application of a vertically varying compliance memory function makes it possible to model a large class of depth-dependent loss properties. The method combines higher-order WKBJ-like asymptotics with the Cagniard-De Hoop method of inverse transformation. The coefficients that occur in the WKBJ asymptotics follow a recurrence scheme that is easy to evaluate by means of a symbolic manipulation program. The form of the transform domain expressions leads to a very fast inversion process. Numerical results are presented that show the reflections from continuously layered lossless and lossy halfspaces.

Pressure methods of space charge measurement in dielectrics

The objective of this paper is to compare two pressure methods of space charge measurement in dielectrics: pressure wave propagation (PWP) and electrically stimulated pressure wave (ESPW). There are some problems connected with the acoustic wave reflections, noise, wave matching and resolution, and they are illustrated using polarized samples of the lead zirconate titanate-polystyrene composites (PZT-PS) and PTFE double sheet layers. The paper also presents a trial of explanation of the observed electric signal responses to acoustic waves.

Self–orthogonal sextic formalism for simple reflection and mode conversion at the free surface of an anisotropic elastic half–space

Simple reflection and mode conversion of acoustic waves at a free surface of a semi–infinite anisotropic elastic medium are investigated using the Stroh sextic formalism. By introducing new sets of self–orthogonal wavefunctions in the self–orthogonal sextic formalism, we establish a series of linear dependence relations which enable us to analyse various kinds of simple reflection and mode conversion under a unified theoretical framework. It is shown that the properties of simple reflection and mode conversion can be described in terms of three independent constants.

Acoustic wave propagation and reflection on nonlinear boundaries

Unlike optics reflection phenomenon in acoustics has long been considered a linear process. In the present paper a general approach to acoustic reflection problems at nonlinear solid–solid and liquid–solid interfaces will be discussed in terms of perturbation theory. Fresnel formulas for the second-order reflection waves were derived for ideally bonded contacts of isotropic solids. Nonlinear bulk wave reflection was shown to result in efficient second harmonics generation, particularly, for overcritical angles of incidence. Unlike the linear case, nonlinear reflection generally leads to the rotation of the polarization plane for the harmonics of shear waves. Reflected harmonics are to be observed even when acoustic impedance is continuous across the boundary forbidding linear reflection to exist. Based on the nonlinear sound reflection problem, the method for analysis of boundary wave nonlinear propagation has been developed. It uses the idea of partial mode decomposition in wave reflection and takes into account self- and cross-interactions of acoustic waves reflected and transmitted at complex angles. The examples of analytical solutions for the second harmonics, as well as numerical calculations of nonlinear wave profiles, will be demonstrated for the Rayleigh, Stoneley, and Stoneley-Sholte wave propagation along nonlinear boundaries. The results obtained permit general characteristics of boundary acoustic nonlinearity that were shown to be different from those in the bulk of nonlinear medium to be defined.

Surface acoustic waves interaction with twin structure in the LiNbO3 crystal

This paper reports a scanning electron microscopy study of the surface acoustic waves interaction with twin structure in the LiNbO3 crystal. The twin was formed spontaneously in the process of the LiNbO3 crystal growth. We investigated the surface acoustic waves reflection and refraction on the twin structure. Also, we demonstrate the shift of the surface acoustic wave potential contrast on the twin walls.

Reflection and transformation of acoustic waves at the interface in superfluid He3-A

The reflection and transformation of acoustic waves in He3-A and He3-A1 is considered in two cases: (1) at the boundary of a solid impermeable wall for an arbitrary angle of incidence, and (2) for a normal incidence of waves at the interface between a free liquid and a system of periodic plane-parallel capillaries filling the half-space. In the first case, the reflection coefficient of first and second sounds, spin and spin–temperature waves, and the coefficients of mutual transformation of these waves are calculated. It is shown that a longitudinal spin wave is not transformed into other waves and experiences instead the total reflection at the solid wall. The angle of incidence for which the energy attenuation coefficient of first sound attains its maximum value is calculated as well as the angular interval corresponding to attenuation and total internal reflection of second sound. In the second case, the coefficients of excitation of fourth sound and of a magnetoacoustic wave by the first and second sounds, the reflection coefficients for the first and second sounds and for a longitudinal spin wave, the coefficient of transformation of first sound into second sound and vice versa, and the coefficient of reflection of fourth sound at the interface between the capillary system and the free liquid, and the coefficient of excitation of a longitudinal spin wave in free helium by the same wave in a capillary are calculated.

Compliant layer requirements for eliminating the plane wave reflection from submerged inertial panels

It is shown that the plane acoustic wave reflection from a submerged panel may be nulled with a compliant coating of constant storage modulus and modest loss factor at all frequencies for which the panel and coating are ‘‘thin.’’

Ultrasonic wave attenuation during water sorption in poly(2‐hydroxyethyl methacrylate) hydrogels

AbstractA scanning laser acoustic microscope (SLAM) was used to measure the acoustic attenuation during water sorption in a film of crosslinked poly(2‐hydroxyethyl methacrylate) [poly(HEMA)] hydrogel. The contribution of reflection, scattering, and absorption of acoustic waves to the measured attenuation is evaluated. The analysis of a model accounting only for the possible reflection of the acoustic waves at the swollen/unswollen boundaries indicates that the time dependence of the attenuation during water sorption cannot be explained simply by accounting for the presence of two additional interfaces. A predominant contribution of acoustic wave absorption during the glass transition, occurring at the two swollen/unswollen interfaces, is assumed. Also, microvoids cannot be excluded as contributors to craze growth in the glassy matrix at the penetrant/polymer interface.