Long-range reverberation in a randomly inhomogeneous shallow water with the use of focused radiation

The low-frequency bottom reverberation in a randomly inhomogeneous shallow water is investigated within the framework of a numerical experiment using vertical transmitting arrays focusing the acoustic field at various distances from the sea bottom. It is assumed that the main source of sound velocity fluctuations in the medium is represented by background internal waves. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that the reverberation level is mainly determined by the presence of internal waves and may vary by 5–20 dB as the distance from the focusing point to the sea bottom increases up to H/2, where H is the channel depth.     

Elastic properties of La<Subscript>0.82</Subscript>Ca<Subscript>0.18</Subscript>MnO<Subscript>3</Subscript> single crystal

The temperature dependences of the velocity of longitudinal sound waves and the internal friction in a La_0.82Ca_0.18MnO₃ single crystal with the Curie temperature T _C = 181 K have been studied. As temperature decreases, the single crystal is shown to undergo the transition from the pseudocubic O* to the Jahn–Teller O’ phase at T ~ 254 K and the reverse transition from O’ to O* phase at T ~ 84 K. The velocity of sound and the internal friction in the O’ phase are found to be significantly smaller than those in the O* phase.     

Sound attenuation in a random waveguide when using a radiating system with different patterns of directivity

Using numerical simulations of sound propagation on Russia’s shallow Arctic shelf, low frequency sound attenuation is analyzed for acoustic sources with different patterns of directivity, i.e., vertical discrete radiating arrays of different length. It is shown that sound attenuation depends largely on the parameters of patterns of directivity and the intensity of surface waves even at short ranges r from a source (r < 250H, where H is the waveguide depth).     

Resonance attenuation of ultrasonic waves in a superconductor with a moving vortex structure

Equations describing the interaction of ultrasonic waves with a moving vortex structure are derived. The addition to attenuation and the relative change in the velocity of longitudinal ultrasonic waves due to this interaction are calculated. It is found that when a longitudinal ultrasonic wave propagates along the direction of motion of the vortex structure and the velocity V of the structure is equal to half the velocity of the wave, then anomalous acoustic attenuation occurs and the contribution from the ultrasound-vortex interaction to the velocity of the ultrasonic wave vanishes. It is shown that if the vortex structure moves at a sufficiently high velocity, then (in contrast to the case of the structure at rest) a weakly damping collective mode propagating with velocity 2V arises in the structure. It is this mode that is responsible for anomalous attenuation of longitudinal ultrasonic waves.     

Amplification of longitudinal ultrasonic waves by a moving vortex structure in type II superconductors

A moving vortex structure can amplify (generate) longitudinal acoustic waves when the velocity of its motion exceeds a certain critical velocity. The critical velocity is determined by the logarithmic derivative of the viscosity coefficient of the vortex structure with respect to the magnetic field and may be much smaller than the speed of sound. In particular, this effect suggests an alternative explanation for the plateau observed in the current-voltage characteristic of superconducting bridges in a perpendicular magnetic field [S.G. Doettinger et al., Phys. Rev. Lett. 73, 1691 (1994)].     

Elastic and kinetic properties of single-crystal La<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript>

An experimental study of the temperature behavior of longitudinal sound velocity, internal friction, electrical resistivity, and thermopower of single-crystal La_0.75Ba_0.25MnO₃ is reported. A structural transition accompanied by a large jump (18%) in the sound velocity was found to occur at T _S ≈170 K. Within the interval 156–350 K, the temperature dependences of the sound velocity and internal friction reveal a temperature hysteresis. An internal-friction peak due to relaxation processes was detected. The metallic and semiconducting regions are separated by a transition domain about 80 K wide lying below the Curie temperature T _C =300 K.     

Ferroelastic phase transition in crystalline K<Subscript>3</Subscript>Na(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>: Acoustic studies

Ultrasonic studies of the temperature behavior of the velocity and damping of sound for the xx and zz longitudinal and yx and zx transverse waves in K₃Na(CrO₄)₂ have been carried out in the temperature interval 185–295 K, which includes the region of the ferroelastic phase transition. The acoustic parameters for both shear and longitudinal waves were found to have anomalies in the region of the phase transition with a Curie temperature of 235.5 K. A theoretical analysis of the softening of the elastic moduli c₄₄ and c₆₆ was performed on the basis of the Landau expansion in terms of the strain tensor components ?₄ and (?₂-?₁)/2 considered as the linearly coupled primary and secondary order parameter, respectively. The absolute values of the elastic moduli c₁₁, c₃₃, c₄₄, c₆₆, c₁₂, and c₁₄ at 295 K were calculated.     

Acoustic properties of a human chest

A cross-spectral method for determining the longitudinal velocity of sound in the tissues of a human chest in vivo is proposed and substantiated. The method is based on the detection of a percussion stroke by two acoustic sensors positioned over opposite parts of a lung. Statistical estimates are obtained for the longitudinal velocity of sound in chest tissues (the middle part of the right lung) from a group of three men (40–47 years old) without any evident lung disorders in the frequency ranges of 80–130, 170–290, and 350–500 Hz. The adequacy of the double-resonance acoustic model of the human respiratory tract, which combines the resonance of the air volume in the human chest and the wave resonances of the bronchial tree as a narrow pipe, is experimentally verified.     

On the error of the time–pulse method of measuring air consumption in mines

The derivation of a formula for the time during which a sound signal propagates between two given points A and B in a stationary gas flow is considered. It is shown that the gas flow changes the signal reception time by a quantity proportional to the consumption, regardless of the detailed velocity profile. The difference between the reception time of signals from point B to the point A and vice versa is proportional to air consumption with high accuracy. It is shown that the relative error of the obtained formula does not exceed the squared maximum Mach number in the gas flow. This allows measurement of the consumption of gas moving in a mine with an arbitrary stationary subsonic velocity field.     

Effect of correlation between the shallow and deep metastable level subsystems on the excitonic photoluminescence spectra in <Emphasis Type="Italic">n</Emphasis>-GaAs

The excitonic photoluminescence spectra of GaAs epitaxial layers are studied. Changes in the relative arrangement of shallow and deep centers in the tetrahedral lattice are shown to bring about changes in the decay kinetics and the shape of the (D⁰, x) emission line (corresponding to an exciton bound to a shallow neutral donor). This change in the excitonic photoluminescence spectra is caused by dispersion in the exciton binding energy of shallow donors E_D, the dispersion being a result of the influence of the subsystem of deep metastable defects in n-GaAs crystals.     

Effect of the sediment layer on the excitation of acoustic modes and lateral waves in shallow sea

The simplest model of a shallow sea in the form of an isovelocity water layer and a fluid sediment layer overlying a homogeneous elastic halfspace is used to investigate the effect of the thickness of the sediment layer and the sound velocity in it on the behavior of the frequency dependences of the amplitudes of trapped and leaky modes and shear and longitudinal lateral waves that are excited by an acoustic point source in a shallow-water oceanic waveguide.     

Hydraulic jump as a white hole

In the geometry of the circular hydraulic jump, the velocity of the liquid in the interior region exceeds the speed of the capillary-gravity waves (ripplons), whose spectrum is “relativistic” in the shallow water limit. The velocity flow is radial and outward, and thus the relativistic ripplons cannot propagate into the interior region. In terms of the effective 2 + 1 dimensional Painlevé-Gullstrand metric appropriate for the propagating ripplons, the interior region imitates a white hole. The hydraulic jump represents the physical singularity at the white-hole horizon. The instability of the vacuum in the ergoregion inside the circular hydraulic jump and its observation in recent experiments on superfluid ⁴He by Rolley et al. [3] are 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.     

A statistical model of a metallic inclusion in semiconducting media

The atomic dynamics of the binary Al_100–xCu_x system is simulated at a temperature T = 973 K, a pressure p = 1.0 bar, and various copper concentrations x. These conditions (temperature, pressure) make it possible to cover the equilibrium liquid Al_100–xCu_x phase at copper concentrations 0 ≤ x ≤ 40% and the supercooled melt in the concentration range 40% ≤ x ≤ 100%. The calculated spectral densities of the time correlation functions of the longitudinal \({\tilde C_L}\)(k, ω) and transverse \({\tilde C_T}\)(k, ω) currents in the Al_100–xCu_x melt at a temperature T = 973 K reveal propagating collective excitations of longitudinal and transverse polarizations in a wide wavenumber range. It is shown that the maximum sound velocity in the v_L(x) concentration dependence takes place for the equilibrium melt at an atomic copper concentration x = 10 ± 5%, whereas the supercooled Al_100–xCu_x melt saturated with copper atoms (x ≥ 40%) is characterized by the minimum sound velocity. In the case of the supercooled melt, the concentration dependence of the kinematic viscosity ν(x) is found to be interpolated by a linear dependence, and a deviation from the linear dependence is observed in the case of equilibrium melt at x < 40%. An insignificant shoulder in the ν(x) dependence is observed at low copper concentrations (x < 20%), and it is supported by the experimental data. This shoulder is caused by the specific features in the concentration dependence of the density ρ(x).     

Optimization of the Electromagnetic (EM) Perturbative Effects Produced by High-Frequency Gravitational Waves

For the relic gravitational waves in high frequency band, we survey the electromagnetic resonance effect generated from the high frequency gravitational waves, which can be described in the transverse perturbative photon fluxes. Under the fixed tensor-scalar ratio r = 0.2, spectral index n _t = 0 and running index α _t = 0.01, we discuss several properties and quantity changes of the transverse perturbative photon fluxes, which can be improved significantly through setting the longitudinal magnetic component of background EM field in the standard gaussian form, and wave impedance analysis on the transverse direction. Through the theoretical calculation, the transverse perturbative photon fluxes can reach up to 10³ s ^?1 with some optimal parameters such as waist of EM field W ₀ = 0.05m, initial stochastic phase of gravitational waves δ = (0.21 + n)π(n = 0,1,2...). Furthermore the interference of the background transverse photon fluxes can be removed completely through establishing a suitable wave impedance function.     

Acoustic Parameters of the Bottom In Lake Baikal

A technique for estimating the effective acoustic parameters of the bottom in shallow water areas under ice cover has been developed. The methodology compares the experimental and simulated dependences of the sound field amplitude on depth at a distance from the source about an order of magnitude greater than the depth of the water area. The effective parameters are the values of the sound speed in the bottom, density, and attenuation coefficient of acoustic waves, which provide maximum agreement with experimental data in the calculations. The methodology was tested in a field experiment on Lake Baikal and can be recommended for developing autonomous acoustic monitoring systems.

     

Rayleigh wave attenuation due to the scattering by two-dimensional irregularities of the walls of an empty borehole

Attenuation of the Rayleigh waves propagating along an irregular surface of an empty borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular boundary of a half-space. The technique used to evaluate the attenuation coefficient is based on the perturbation method and the mean field method. As a result, an expression is obtained that relates the partial attenuation coefficients of the surface Rayleigh wave to the scattering by the irregular surface of an empty borehole into the bulk longitudinal and transverse waves (the R → P and R → S processes) and into the surface Rayleigh waves (the R → R processes). The frequency-dependent behavior of the partial attenuation coefficients is analyzed for different correlation functions of irregularities.     

Spin waves on chains of YIG particles: dispersion relations,Faraday rotation,and power transmission

We calculate the dispersion relations for spin waves on a periodic chain of spherical or cylindrical Yttrium Iron Garnet (YIG) particles. We use the quasistatic approximation, appropriate when kd ? 1, where k is the wave number and d the interparticle spacing. In this regime, because of the magnetic dipole-dipole interaction between the localized magnetic excitations on neighboring particles, dispersive spin waves can propagate along the chain. The waves are analogous to plasmonic waves generated by electric dipole-dipole interactions between plasmons on neighboring metallic particles. The spin waves can be longitudinal (L), transverse (T), or elliptically polarized. We find that a linearly polarized spin wave undergoes a Faraday rotation as it propagates along the chain. The amount of Faraday rotation can be tuned by varying the off-diagonal component of the permeability tensor. We also discuss the possibility of wireless power transmission along the chain using these coupled spin waves.     

Transmission of low-frequency sound through the water-to-air interface

L.M. Brekhovskikh revealed and studied the important role played by inhomogeneous waves emitted by a point source when they pass through an interface with a medium in which the velocity of sound is lower, for example, from water to air. This paper studies the energy characteristics of sound emitted into air by an underwater point source. The energy transfer due to inhomogeneous waves is shown to cause the phenomenon of anomalous transparency of the interface for low-frequency sound. The anomalous transparency manifests itself in that the energy flux through the interface increases with decreasing frequency of sound and, at sufficiently low frequencies, almost all of the acoustic energy produced by the underwater source is emitted into air. Conversely, at high frequencies, when the contribution of the inhomogeneous waves becomes negligible, the water-to-air interface is similar to a perfectly reflecting surface and almost all of the acoustic energy produced by the source is emitted into water. The anomalous transparency phenomenon changes the conventional opinion on the possibility of acoustic coupling between points in water and air and on the role played by physical processes evolving in the water column in generating atmospheric acoustic noise.     

Acoustic testing of the vortex structure produced by an air flow about an array of cylinders

Results of experiments on the scattering of a plane ultrasonic wave from a vortex wake formed in an air flow behind a lattice of vertical cylinders are presented. The lattice is periodic in the direction perpendicular to the oncoming flow. The experiments are performed in a wind tunnel for two values of the Reynolds number, namely, Re = 75 and 500, and for lattices with different numbers of cylinders and with different lattice periods g = (2.5–15)d (where d is the diameter of the cylinders). The measured parameters of the scattered waves are used to estimate the degree of transverse correlation between the vortex wakes formed behind the cylinders for flows with different Reynolds numbers. The results obtained from an analysis of the characteristics of the scattered sound are compared with the results of direct hot-wire anemometer measurements and with the data obtained by other researchers.