Acoustical Physics最新文献

Methods for studying the characteristics of sound waves radiated and scattered by shells in an aqueous medium in the low-frequency range are presented. The results of comparing these characteristics obtained by numerical modeling and measurements in an open water area are shown. The experimental study was carried out using an original methodology, and the calculations were carried out using the authors' software. The results allow a better understanding of the mechanism of sound radiation and scattering in water and optimization of the design of shells for various applications.

A number of novel noninvasive surgical technologies utilizing high-intensity focused ultrasound (HIFU) are based on using nonlinear acoustic effects that lead to distortion of the wave when it propagates from an ultrasound transducer and formation of shock fronts at the focus. Typically, such transducers that generate a high-power ultrasound beam, have near axially symmetric shape with a central circular opening to accommodate a diagnostic probe for visualization purposes. To predict the focal field parameters of such transducer geometries, an equivalent source model of a spherical segment is convenient, as nonlinear effects in its field are well studied. The equivalent source parameters (diameter, focal length, and amplitude) are optimized to closely approximate the focal region of the original transducer along the axial coordinate. This paper investigates the effect of the central opening size on the nonlinear field characteristics and applicability of the equivalent source model for a typical therapeutic ultrasound transducer with a frequency of 1 MHz and F_# = 0.9. It is demonstrated that the size of the central opening significantly affects the degree of nonlinear waveform distortion in the focal region, and the equivalent source model can be applied only when the diameter of the central opening is less than 20% of the transducer diameter.

One of the undesirable effects of using ultrasound for extracorporeal therapy is skin overheating, caused by both ultrasound absorption and contact with the heated surface of the acoustic transducer. To suppress this effect, a forcibly cooled contact medium can be placed between the skin and the irradiating surface. A novel ultrasound applicator implementing this approach has recently been proposed and developed at Southern Federal University. It uses a rectangular piezoelectric transducer bonded to an aluminum plate for volumetric heating of subcutaneous biological tissue. The plate is cooled by circulating cold water through laterally drilled channels. This article presents a numerical algorithm for calculating the three-dimensional temperature field in tissue during the operation of this applicator. The simulation was based on the inhomogeneous heat equation. Experimental acoustic holography data obtained for the developed transducer were used to calculate the heat sources in tissue. An example of heating bovine liver tissue ex vivo is considered, with irradiation times ranging from several seconds to several min. The simulation results were compared with experimental data on tissue thermal ablation at an acoustic power of 12 W and an ultrasound frequency of 6.96 MHz. It is shown that the combination of thermal tissue exposure and contact boundary cooling allows for volumetric tissue heating with a temperature maximum at a depth of 8 to 15 mm, while maintaining a negligible temperature change at depths up to 2–3 mm.

The fine structure of the seismic background in the territory of an oil field was studied using polyspectral analysis and surrogate models. The statistical test for Gaussianity and linearity showed that the natural seismic background in the 1–50 Hz frequency range is a nonlinear process. Bicoherence graphs revealed statistically significant peaks of phase-coupled triplets with a characteristic geometry of peak clustering in the triangular principal domain. To analyze the quasi-noise component of the seismic background, surrogate time series with a randomized phase spectrum were generated, the bispectra of which are free of triplet peaks. Bispectral analysis of surrogate series showed the presence of a non-Gaussian quasi-noise component in the seismic background in the frequency range of 1–6 Hz. Previously, the results of the analysis of the used set of records by two completely different methods, each of which extracts information from different components of the seismic background, quasi-noise and regular, were published. These studies showed that each of the two components contains information sufficient to estimate the total thickness of the productive intervals under the recording point. Based on the results of the bispectral analysis and the features of the algorithms of the two methods, a conclusion was made that the quadratically phase-coupled triplets and the non-Gaussian quasi-noise component in the field territory are manifestations of endogenic seismic emission and are generated by the same nonlinear process developing in the vicinity of an oil-saturated reservoir. The mechanism of seismic emission should be sought in the class of phenomena with quadratic nonlinearity. The statistical characteristics of the internal structure of the seismic background near oil wells have similar features that differ greatly from the statistical characteristics of the seismic record near a “dry” well located outside the reservoir and not producing oil. Previously unknown prognostic features of the oil/water saturation type of rocks have been identified.

The problem of finding the spatiotemporal correlation function of pressure thermal fluctuations of gas near thermodynamic equilibrium is considered. The correlation function of pressure field equilibrium thermal fluctuations is obtained for viscous heat-conducting perfect gas filling an unbounded Euclidean space. The influence of the boundary on the correlation characteristics of thermal fluctuations of pressure is investigated using the example of a viscous isentropic perfect gas filling a small-section pipe with boundary conditions of a solid wall at the ends and periodic conditions for the case of a pipe closed in a circle.