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.     

Rayleigh wave dispersion and attenuation on a statistically rough free surface of a hexagonal crystal

Dispersion and attenuation of Rayleigh surface acoustic waves on a statistically rough free surface of a Z-cut hexagonal crystal were analytically studied using a modified mean-field method within the perturbation theory. Numerical calculations were carried out in the frequency range accessible for the perturbation theory using expressions for the real and imaginary parts of the complex frequency shift of Rayleigh waves caused by a slight surface roughness. The Rayleigh wave dispersion and attenuation in the Z-cut hexagonal crystal were shown to coincide qualitatively with those in an isotropic medium, differing only quantitatively. In the long-wavelength limit λ?a, where a is the lateral roughness correlation length, explicit analytical expressions for the relative change in the phase velocity and the inverse damping depth of Rayleigh waves were derived and used in numerical calculations.     

Dispersion and attenuation of Rayleigh waves at a one-dimensional random roughness of the free surface of a hexagonal crystal

Analytical expressions are derived for dispersion and attenuation of Rayleigh waves propagating along the statistically rough free surface of a hexagonal crystal (Z cut). The roughness under consideration is one-dimensional (the profile function of the roughness depends on one coordinate) and has the form of hollows of a random lattice. The results obtained earlier in the solution of an analogous problem for a two-dimensional roughness are used in the one-dimensional case. The relationships derived for the dispersion and attenuation of Rayleigh waves are treated analytically and numerically over the entire range of frequencies acceptable in the framework of the perturbation theory. It is shown that the dispersion and attenuation of Rayleigh waves are qualitatively similar to those observed in an isotropic medium.     

Roughness-trapped shear horizontal surface acoustic waves

Shear horizontal surface acoustic waves do not exist on the flat surface of a semi-infinite elastic medium. It has been shown by several authors recently that such waves can exist on a periodically corrugated, planar surface. We show here on the basis of the Rayleigh method that shear horizontal surface acoustic waves exist on a randomly rough planar surface of an isotropic elastic medium. These waves are only weakly localized to the surface and they have a lifetime that is long due to their roughness-induced scattering into other surface acoustic waves and into bulk waves.     

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.     

Scattering of bulk acoustic waves with different polarizations on a statistically rough free surface of a solid at oblique incidence

In the first Born approximation of the perturbation theory by a Green's function method developed by Maradudin, Mills [7] and Kosachev, Lokhov, Chukov [8,9] the problem of scattering bulk acoustic waves with different polarizations at oblique incidence on a statistically rough free boundary of an isotropic solid was solved. When the correlation function of the surface roughness is of a Gaussian form, the expressions for the transformation energy factor of the incident wave in the scattered volume and surface Rayleigh waves with respect to polarization, frequency and grazing angle of the incident wave as well as the roughness parameters and the Poisson coefficient of the medium were obtained. These results are helpful in accounting for the experiments on residual losses [15–17].     

Reflection of surface acoustic waves in the bulk of a hexagonal single crystal

It has been shown that surface acoustic waves in some hexagonal crystals can include not only the waves outgoing from the surface into the bulk of the crystal but also the reflected waves, i.e., those coming from the bulk of an infinite single crystal. The outgoing and reflected waves decay exponentially with the distance from the surface. It has been found that the reflected wave can exist if the velocity of its propagation is below the critical value, which does not exist for some crystals. The numerical calculations have shown in what real hexagonal crystals the reflected wave can exist. The values of the critical velocity have been found for a number of hexagonal crystals.     

The viscous effects on shear horizontal surface acoustic waves in semi-infinite superlattices

In this paper, by a semi-analytical method, the propagation characteristics of shear horizontal surface acoustic waves in semi-infinite superlattices containing viscous materials are investigated. The factors that influence the attenuation and phase velocity of the surface waves are analyzed in detail. The results may be useful for the design of acoustic wave devices.     

Fourth moments of acoustic waves forward scattered by a rough ocean surface

Three types of statistical fourth moments of acoustic waves forward scattered by a randomly rough ocean surface are derived and numerically evaluated. The first one is related to the scintillation index which characterizes intensity fluctuations. The second one is the two-position intensity correlation function which describes the spatial correlation of wave intensity. The third is the fourth-moment two-position coherence function which carries information on the phase fluctuations of the scattered wave. In the range of weak scattering, the ratio of the absolute value of the fourth-moment two-position coherence function over the two-position intensity correlation exactly describes the mean-square fluctuation of the relative phase between the two positions. The acoustic frequency is high so that the Kirchhoff approximation can be used. Two types of spectral functions for surface-height fluctuations are considered: a Gaussian spectrum and the Donelan-Pierson spectrum. The latter is obtained from a model for the fluctuations of the ocean surface height which are controlled by the wind speed at the ocean surface.     

Fourth moments of acoustic waves forward scattered by a rough ocean surface

Abstract

Three types of statistical fourth moments of acoustic waves forward scattered by a randomly rough ocean surface are derived and numerically evaluated. The first one is related to the scintillation index which characterizes intensity fluctuations. The second one is the two-position intensity correlation function which describes the spatial correlation of wave intensity. The third is the fourth-moment two-position coherence function which carries information on the phase fluctuations of the scattered wave. In the range of weak scattering, the ratio of the absolute value of the fourth-moment two-position coherence function over the two-position intensity correlation exactly describes the mean-square fluctuation of the relative phase between the two positions. The acoustic frequency is high so that the Kirchhoff approximation can be used. Two types of spectral functions for surface-height fluctuations are considered: a Gaussian spectrum and the Donelan-Pierson spectrum. The latter is obtained from a model for the fluctuations of the ocean surface height which are controlled by the wind speed at the ocean surface.     

Time delay temperature behaviour of surface acoustic waves propagation on crystal substrate

A method is described for the computation of the time delay temperature behaviour for surface acoustic waves propagated in a surface layer of piezoelectric crystals. The linear field equations for small vibration superposed on thermally induced deformation were employed in the determination of the time delay temperature behaviour. The computed time delay temperature dependence of the surface acoustic waves propagated in the digonal axis direction on the ZX-cut quartz plate is given as an example.     

Low-frequency scattering of acoustic waves by a bounded rough surface in a half-plane

The problem of the scattering of harmonic plane waves by a rough half-plane is studied here. The surface roughness is finite. The slope of the irregularity is taken as arbitrary. Two boundary conditions are considered, those of Dirichlet and Neumann. An asymptotic solution is obtained, when the wavelength lambda of the incident wave is much larger than the characteristic length of the roughness iota, by means of the method of matched asymptotic expansions in terms of the small parameter epsilon= 2piiota/lambda. For the Dirichlet problem, the solution of the near and far fields is obtained up to O(epsilon2). The far field solution is given in terms of a coefficient that have a simple explicit expression, which also appears in the corresponding solution to the Neumann problem, already solved. Also the scattering cross section is given by simple formulas to O(epsilon3). It is noted that, for the Dirichlet problem, the leading term is of order epsilon3 which, by contrast, is different from that of the circular cylinder in full space, that is, of order epsilon(-1) (log epsilon)(-2). Some examples display the simplicity of the general results based on conformal mapping, which involve arcs of circle, polygonal lines, surface cracks and the like.     

具有单相单向换能器的水平剪切声表面波压力传感器插入损耗特性

为实现水平剪切声表面波压力传感器低损耗设计,系统研究了均匀叉指换能器(IDT)和单相单向换能器(SPUDT)器件的声波能量损耗性能。在确定最优反射系数的SPUDT结构的基础上,建立3D周期性有限元仿真模型,计算器件表面振动位移和双向输出电压瞬态响应。通过对电压信号进行傅里叶变换获得器件模型的插入损耗,并制备两种不同换能器的声学传感器,测试其频率特性和灵敏度。与均匀IDT相比,基于SPUDT器件插入损耗的仿真和实验结果分别降低了5.2 dB和5.6 dB,SPUDT器件灵敏度约为均匀IDT器件的两倍。结果表明,SPUDT能有效降低SH-SAW压力传感器声波能量的单向损耗,提高检测灵敏度,且构建的3D周期性有限元仿真模型有助于声学传感器的声波损耗分析,实现高精度和小型化的声学测量系统。     

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.     

Focussed surface acoustic waves on an anisotropic crystal, studied by optical probing

Efficient focussing of surface acoustic waves has been achieved using a properly shaped gold film deposit on the −22.3° rotated Y-cut surface of quartz. The acoustic wave field was studied with laser probing techniques. A more than threefold increase in intensity and a tenfold decrease in beam width was observed at the focal point. The focussing action was obtained with the elastic wave equivalent of the Fresnel phase-reversal zone plate of optics. The multiple foci of this device allowed a simultaneous generation of acoustic waves in nearly all directions on the surface. Consequently, the surface wave velocity anisotropy could be determined completely. The experimental results are in very good agreement with the calculated velocity anisotropy. Electromagnetic diffraction theory is adopted to the two dimensional anisotropic system to analyse the performance of the focussing device.     

Dispersion characteristics of surface magnetostatic waves with dissipation

It is shown that unlike undamped waves, the dispersion characteristics of spin surface waves with dissipation have a maximum wave number at which there is a downward reversal in the dispersion curve of a wave number. This forms the upper branch of a dispersion curve with inverse dispersion and high attenuation, leading to an unclear frequency dependence of the wave vector. The lower primary dispersion branch corresponds to waves with forward dispersion, and attenuation is proportional to the small parameter of dissipation. However, the coefficient of wave attenuation grows sharply near the maximum wave number. Some angular and frequency limits of surface wave propagation change as well.     

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     

The analogy between acoustic waves and water surface waves with refraction

The similarity between acoustic waves in an inhomogeneous medium and water surface waves with variable water depth is established. The two systems are analogous if they both are characterized by the same index of refraction field. Based on this analogy, acoustic refraction problems are treated by utilizing a surface wave simulation achieved with a water wave tank. Experimental results obtained from the wave tank for various situations are used to deduce the behavior of the corresponding acoustic systems. It is concluded that there are two major advantages in employing a water wave tank. The first is that the entire wavefront configuration can visually be observed; the second is that water surface waves can be investigated in the laboratory much more readily and more economically than acoustic waves.