A method is proposed for reconstructing the acoustic parameters of a medium by iterative processing of the coherence matrices of the acoustic field of random sources, for some of which their power density is known. The possibilities of increasing the stability and accelerating the convergence of the method are discussed. The reconstruction results are compared with the functional-analytical approach based on the processing of the scattering amplitude.
The study analyzes the basic properties of the scalar wavenumber-frequency spectrum of turbulent pressures, representing the total energy of the wave components of the turbulent pressure field with a given wave vector modulus. Consideration of the scalar spectrum, which has independent applied significance, makes it possible to visualize the energy distribution of turbulent pressures in a wide range of frequencies and wave numbers. Based on well-known vector wave field models, relations are proposed for estimating the relative scalar spectrum. The degree and nature of the parametric influence of the Mach and Reynolds numbers are determined.
The results of integrated studies of the slow relaxation of the sound speed in structurally inhomogeneous materials after vibratory action with a finite strain amplitude are presented. The results were obtained using an experimental setup created for these studies, which ensured highly accurate measurements of the linear and nonlinear acoustic characteristics of various materials. The results of an experimental study of relaxation in carbonate rock, which is a structurally inhomogeneous material with a complex system of internal relationships, are presented. The measurements made it possible to determine the dependence of the relaxation parameters on the amplitude and time of excitation, as well as the effects caused by the finite amplitude of the probing wave. The results are interpreted, and effects that have not received an exhaustive explanation are noted.
The influence of developed wind waves on the gain of a vertical array in shallow ocean waveguides is studied analytically and numerically. An algorithm is proposed for calculating the model correlation matrix of the signal at the aperture of the vertical array, taking into account the interference structure of the acoustic field in the sound channel. The antenna gain is analyzed for three spatial processing methods: standard phased array, optimal linear processing method, and optimal quadratic processing. The numerical simulation results for winter hydrological conditions in the Barents Sea are presented. The main focus is the dependence of the antenna gain values, “smoothed” on the scale of the interference structure of the acoustic field in the waveguide, the wind speed, and seabed sediment characteristics. The influence of intermode correlations on the results of gain simulation for various signal processing methods is analyzed in detail. It is shown that ignoring intermode correlations in the case of a vertical array leads to fundamentally erroneous results with optimal processing methods.
Almost no diffraction broadening of self-collimation beams and their easy crossing with low crosstalk offer new opportunities for acoustic wave manipulation. Here, we flexibly steer the self-collimation beam based on the total internal reflection of an airborne square lattice sonic crystal. The properly designed crystal/air interface may serve well as a total internal reflection mirror of the self-collimation beam, with which we are able to achieve the highly controllable beam reversal, snaking and crossing. Our scheme can be extended to more interesting beam steering, and have potential applications in constructing high-density integrated acoustic circuits.
Variations of physical properties of seawater (sound speed, temperature, salinity, and density) over the southern shelf of the Caspian Sea were assessed. Furthermore, the effect of monthly changes in physical structure and layering procedure was evaluated on water characteristics. The influence of the thermal structure and stability of the water column in warm months on the sound speed was greater than that of the salinity parameter. Due to the high correlation between sound speed and temperature in the Caspian Sea water, their vertical structures are mostly correlated. The range of surface layer water changes was within 9–30°C during different months of the year. The water salinity in the areas away from the coast was measured around 12 to 12.5 PSU. The range of measured sound speed data in February was about 1450–1463 m/s, while the values recorded in June‑July were between 1501–1451 m/s. The results indicated formation of a deep acoustic channel in the middle and deep areas of the seawater column. The analysis revealed that the axis of the sound channel occurs at a depth of 140–150 m (intermediate layer) in warm months. The axis of the sound channel was located in upper layers in the warm months.
The temperature dependences of the heat capacity were measured and the general patterns of the formation of the phonon spectrum of single crystals of solid solutions of yttrium-lutetium aluminum garnets Y3 – xLuxAl5O12 at 0 ≤ x ≤ 3 were investigated in the temperature range from 1.9 to 220 K. According to the data obtained at temperatures below 10 K, the Debye temperatures were calculated. The features of the phonon spectrum in the intermediate temperature range are interpreted as a superposition of optical modes for yttrium and lutetium garnets. It is shown that the low heat capacity values due to the contribution of acoustic phonons for Y2.25Lu0.75Al5O12 correlate with anomalies in the concentration dependences of the phonon transport, the absorption of acoustic waves, and the shape of the aluminum NMR line.
Symmetric Lamb waves with a phase velocity exceeding the expansion wave velocity in an infinite medium are considered. It has been proven that internal waves (i.e., solutions of the wave equation, which have zero values of the strain and stress components on the surface and nonzero values in the plate bulk) may exist in this region of phase velocities. Parameters of the internal waves (phase velocity, frequency, and wavelength) are calculated; it has been proven that frequencies of the internal waves with the same phase velocity make an arithmetic progression. Several internal waves are considered, and cross sections of the corresponding strained plates are presented (as well as the distributions of the maximum tension and shear values).
The article focuses on studying the effect of variable thermal conductivity on the reflection of waves propagating through a medium. Considered solid is half space with semiconductor properties. Additionally, we have also introduced the concept of non-local thermoelasticity to develop a stress-strain relation. Using the Helmholtz decomposition principal, we have determined that three longitudinal waves and one transverse wave propagate through the medium after reflection. To account for the thermal signals generated by the elastic vibration, we have used a three-phase lag (3PL) heat conduction model. The numerical computation of the theoretical results is presented graphically for a specific medium. The study of elastic waves passing through the human body is used for diagnosis and treatment. Nature and characteristics of the materials can also be detected by evaluating the behavior of waves reflected and transmitted through them.
An experimental verification of the possibility of estimating the coordinates of a mobile marine robot using spatially offset small-sized vector–scalar arrays, a broadband source placed on board the robot, and a remote polyharmonic permanently installed source—a beacon, which is used to eliminate bearing shifts caused by rotating receiving arrays under the action of underwater currents. It is shown that the use of technical means installed in the waveguide makes it possible to solve the triangulation problem and determine the horizontal coordinates of the robot, while taking into account the ray structure provides a depth estimate.
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