Nondispersive delay line on magnetostatic waves

A new approach to the design of a nondispersive delay line on magnetostatic waves is proposed, whereby the cascades operating on the surface waves and the backward volume waves have a common passband in the same magnetic system due to a difference in the crystallogrphic orientations of ferrite waveguides cut from a {100} yttrium iron garnet film. The possibility to obtain a pulse delay time of up to 113 ± 3 ns in a 300-MHz-wide frequency band is experimentally demonstrated. The central frequency of this band can be tuned within 4–6 GHz by changing the applied magnetic field.     

New method of change in temperature coefficient delay of acoustic waves in thin piezoelectric plates

As is well-known, the development of high-effective and thermostable acoustic devices assumes using the acoustic waves with high coefficient of electromechanical coupling (K2) and low temperature coefficient of delay (TCD). At present, it also is well-known that fundamental shear horizontal (SH0) acoustic waves in thin piezoelectric plates possess by significantly more electromechanical coupling compared to surface acoustic waves (SAW) in the same material. However, although the value of TCD of SH0 waves is insignificantly less than for SAW, this is not enough for development of thermostable devices. This paper suggests a new way of decreasing TCD of SH0 waves in piezoelectric plates at a high level of electromechanical coupling. This way assumes to use the structure containing the piezoelectric plate and liquid with the special dependence of permittivity on temperature. Theoretical and experimental investigation showed that, for SH0 wave in YX LiNbO3 plate at hf = 700 m/s (h = plate thickness, f = wave frequency) the presence of butyl acetate can decrease the value of TCD by six times at K2 = 30%. In a whole the obtained results open the wide prospect of using SH0 wave in thin piezoelectric plate for development of high effective and thermo-stable acoustic devices.     

Nonlinear surface acoustic waves on an isotropic solid

A systematic derivation of the approximate coupled amplitude equations governing the propagation of a quasi-monochromatic Rayleigh surface wave on an isotropic solid is presented, starting from the non-linear governing differential equations and the non-linear free-surface boundary conditions, using the method of mulitple scales. An explicit solution of these equations for a signalling problem is obtained in terms of hyperbolic functions. In the case of monochromatic excitation, it is shown that the second harmonic amplitude grows initially at the expense of the fundamental and that the amplitudes of the fundamental and second harmonic remain bounded for all time.     

Nonlinear mode coupling of surface acoustic waves on an isotropic solid

The nonlinear mode coupling between two co-directional quasi-harmonic Rayleigh surface waves on an isotropic solid is analysed using the method of multiple scales. This procedure yields a system of six semi-linear hyperbolic partial differential equations with the same principal part governing the slow variations in the (complex) amplitudes of the two fundamental, the two second harmonic and the two combination frequency waves at the second stage of the perturbation expansion. A numerical solution of these equations for excitation by monochromatic signals at two arbitrary frequencies, indicates that there is a continuous transfer of energy back and forth among the fundamental, second harmonic and combination frequency waves due to mode coupling. The mode coupling tends to be more pronounced as the frequencies of the interacting waves approach each other.     

Propagation of nonlinear acoustic waves in gallium

We study nonlinear phenomena which occur during strong sound signal propagation in superpure Ga and are due to the perturbation of the electron subsystem in the metal. Although the observations are in qualitative agreement with theoretical concepts in a number of aspects, there are effects which still lack adequate theoretical interpretation, in particular, an oscillatory dependence of the harmonic amplitudes on magnetic field which is linear with respect toH ^1/2, and appreciable nonlinear distortions of the sound pulse envelope accompanied by the acoustic noise generation.     

Hybridization of acoustic waves in piezoelectric plates

The conditions for hybridization of the zero-order and high-order acoustic waves propagating in a piezoelectric crystal plate have been studied. The dependence of the phase velocity of the hybrid waves on the parameter hf (h is the plate thickness and f is the wave frequency) is established for the potassium niobate and lithium niobate plates possessing various crystallographic orientations and conductivities. It is found that hybridization takes place when the conductivity of a thin surface layer exceeds a certain critical value, which can vary within broad limits depending on the plate material and orientation. The degree of dispersive repulsion of the coupled modes grows with increasing electromechanical coupling coefficient.     

Ultrafast generation of acoustic waves in copper

The ultrafast generation of acoustic waves in copper films is investigated with a femtosecond optical pump and probe technique. By studying the generation at times before the electrons and the lattice come into equilibrium, the strength of their interaction can be measured and the dynamics of ultrafast electron diffusion can be studied. The acoustic strain pulses observed are bipolar in shape with exponential tails that are much broader than expected from simple thermoelastic stress generation. This can be explained by the supersonic diffusion of electrons over distances larger than the optical skin depth. The nonequilibrium diffusion equations governing stress generation are nonlinear, and are solved numerically. Using a linearized formulation, we also solve them analytically to a good approximation. The acoustic strain profile provides a `snapshot' of the initial spatial temperature distribution of the lattice, thus allowing a sensitive probe of the nonequilibrium dynamics of the diffusion. The electron-phonon coupling constant can be estimated directly from the acoustic pulse duration, provided that the sound velocity and thermal conductivity are known. In general, the relaxation and diffusion of carriers is specific to the sample in question, whether metal or semiconductor, suggesting the use of this method for thin film characterization     

Hybridization of backward acoustic waves in piezoelectric plates

The phenomenon of hybridization of the backward acoustic waves propagating in a piezoelectric crystal plate has been studied. In an electrically free plate (in particular, of potassium niobate) with a crystal orientation for which a sagittal plane is the symmetry plane, the dispersion curves of backward acoustic waves exhibit points of intersection and hybridization is absent. However, for a small change in the direction of wave propagation, the dispersion curves exhibit “repulsion” and the waves become coupled. The degree of hybridization is quantitatively evaluated in terms of the hybridization coefficient, which is defined as the ratio of the total mutual energy density and the total energy density of the interacting waves. It is demonstrated that the extent of repulsion of the dispersion curves for the interacting waves is determined by the dependence of the hybridization coefficient on the product of the plate thickness and the wave frequency.     

Propagation of acoustic waves in unsaturated porous media

A mathematical model of propagation of acoustic waves in a proous medium saturated with a two-phase mixture is proposed. The mechanism of initiation of slow motion as a result of prolonged nondestructive acoustic action is analyzed. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 3, pp. 433–440, May–June, 1998.     

Review of shear surface acoustic waves in solids

General considerations about physical reasons for the existence of surface acoustic waves and, in particular, shear surface acoustic waves in solids are presented. The results of calculations for various types of shear surface acoustic waves are described, and corresponding physical explanations are given.     

Resonant interaction of acoustic waves in nonequilibrium gases

Nonlinear equations are derived, which describe the evolution of two-dimensional acoustic disturbances in a thermodynamically nonequilibrium gas. The characteristic features of parametric interaction of wave packets in acoustically active media are analyzed. The transmission band under conditions of critical and noncritical matching and the threshold of parametric amplification are determined. Conditions are obtained, under which a giant parametric pulse may form in an acoustically active medium, with the amplitude of this pulse exceeding that of a pumping wave.     

Velocity dispersion of acoustic waves in cancellous bone

Measurement of ultrasonic attenuation and velocity in cancellous bone are being applied to aid diagnosis of women with high fracture risk due to osteoporosis. However, velocity dispersion in cancellous bone has received little attention up to now. The overall goal of this research was to characterize the velocity dispersion of human cancellous bone based on a spectral analysis of ultrasound transmitted through the bone specimens. We have followed a systematic approach, beginning with the investigation of a test material, moving on to the investigation of bone specimens. Particular attention is given to diffraction effect, a potential source of artifacts. Parametric images of phase velocity (measured at the center frequency of the pulse spectrum), slope of attenuation coefficient (dB/cm/MHz) and velocity dispersion were obtained by scanning 15 bone specimens. We have demonstrated that the diffraction effect is negligible in the useful frequency bandwidth, and that the ultrasonic parameters reflect intrinsic acoustic properties of bone tissue. The measured attenuation showed approximately linear behavior over the frequency range 200 to 600 kHz. Velocity dispersion of cancellous bone in the frequency range 200 to 600 kHz was unexpectedly found to be either negative or positive and not correlated with the slope of attenuation coefficient. There was a highly significant correlation between the slope of attenuation coefficient and phase velocity at the center frequency of the spectrum. This behavior contrasts with other biological or nonbiological materials where the local form of the Kramers-Kronig relationship provides accurate prediction of velocity dispersion from the experimental frequency dependent-attenuation for unbounded waves.     

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform. From images of a small source scanning across the interferometer beam it is estimated that the spatial resolution of the imaging system is in the range of 100 to about 300 mum, depending on the interferometer beam width and the size of the aperture formed by the scan length divided by the source-detector distance. By taking an image of a phantom it could be shown that the imaging system in its present configuration is capable of producing three-dimensional images of objects with an overall size in the range of several millimeters to centimeters. Strategies are proposed how the technique can be scaled for imaging of smaller objects with higher resolution.     

Surface acoustic waves in a gallium arsenide-conducting layerstructure

The electron attenuation and SAW velocity versus the surface conductivity and magnetic field induction for the structure consisting of the (001) cut anisotropic GaAs substrate and an isotropic conducting layer was calculated. The SAW propagated along the [110] piezoactive direction distinguished by the greatest electromechanical coupling coefficient     

Matrix methods applied to acoustic waves in multilayers

Matrix methods for analyzing the electroacoustic characteristics of anisotropic piezoelectric multilayers are described. The conceptual usefulness of the methods is demonstrated in a tutorial fashion by examples showing how formal statements of propagation, transduction, and boundary-value problems in complicated acoustic layered geometries such as those which occur in surface acoustic wave (SAW) devices, in multicomponent laminates, and in bulk-wave composite transducers are simplified. The formulation given reduces the electroacoustic equations to a set of first-order matrix differential equations, one for each layer, in the variables that must be continuous across interfaces. The solution to these equations is a transfer matrix that maps the variables from one layer face to the other. Interface boundary conditions for a planar multilayer are automatically satisfied by multiplying the individual transfer matrices in the appropriate order, thus reducing the problem to just having to impose boundary conditions appropriate to the remaining two surfaces. The computational advantages of the matrix method result from the fact that the problem rank is independent of the number of layers, and from the availability of personal computer software that makes interactive numerical experimentation with complex layered structures practical.     

Temperature-sensitive cuts for surface acoustic waves in quartz

Temperature is the main parameter that affects the stability of surface acoustic wave (SAW) devices such as the external and internal stress. The temperature sensitivity of the frequency as a function of quartz crystal anisotropy is presented by a perturbation equation because of a temperature change. The analytical result shows which cut is suitable for high temperature sensitivity of the quartz and which cut is associated with low temperature sensitivity. The theoretical sensitivity values are compared with experimental results. Both the experimental and theoretical results are in good agreement.     

延迟利用的多节点协同预约式水声网络MAC协议

水声网络媒体接入控制(Media Access Control,MAC)层为水声节点提供接入和冲突避免服务。由于水声信道面临着传播延迟高、带宽窄等困难,水声网络MAC协议的性能较低。文章在MACA-U协议的基础上,针对水声网络传播延迟高的特点,对信道预约的长等待时间进行利用,提出了一种RS-MACA协议。该协议在负载较低时与MACA-U协议兼容;在负载较高时,邻近节点可采用"并行发送"的方式发送,显著提高了吞吐量性能。发射节点在信道预约等待时间内,同时接受符合条件的邻节点的发送预约,邻节点不需要消耗额外时间进行信道预约。完成预约的节点按照时隙进行数据发射,避免了数据包的碰撞。NS3软件仿真结果表明,与MACA-U协议相比,改进后的协议吞吐量性能有明显的提升。     

Negative birefraction of acoustic waves in a sonic crystal

Optical birefringence and dichroism are classical and important effects originating from two independent polarizations of optical waves in anisotropic crystals. Furthermore, the distinct dispersion relations of transverse electric and transverse magnetic polarized electromagnetic waves in photonic crystals can lead to birefringence more easily. However, it is impossible for acoustic waves in the fluid to show such a birefringence because only the longitudinal mode exists. The emergence of an artificial sonic crystal (SC) has significantly broadened the range of acoustic materials in nature that can give rise to acoustic bandgaps and be used to control the propagation of acoustic waves. Recently, negative refraction has attracted a lot of attention and has been demonstrated in both left-handed materials and photonic crystals. Similar to left-handed materials and photonic crystals, negative refractions have also been found in SCs. Here we report, for the first time, the acoustic negative-birefraction phenomenon in a two-dimensional SC, even with the same frequency and the same 'polarization' state. By means of this feature, double focusing images of a point source have been realized. This birefraction concept may be extended to other periodic systems corresponding to other forms of waves, showing great impacts on both fundamental physics and device applications.     

Ray representation of longitudinal lateral waves in acoustic microscopy

For object materials having a large enough Rayleigh velocity, the V(z) (where V is the output voltage and z is the defocus distance) variation is mainly due to interference between the fields of the geometrically reflected wave and the leaky Rayleigh wave. However, for materials, such as organic compounds, having a low Rayleigh velocity, the leaky Rayleigh wave is not excited. For this case, the lateral wave resulting from propagation along the surface of the longitudinal wave plays a significant role in determining the V(z) dependence. The effect of the lateral wave contribution on V(z) is studied. Ray optics is to derive an expression giving the influence of the longitudinal lateral wave. Good agreement is found between the theory and measurements for z not near zero. Because of the ease with which the longitudinal wave velocity can be obtained from V(z), one can conveniently determine the elastic constant c(11 ) of isotropic materials using the acoustic microscope.