Vol 65, No 6 (2019)

The distribution of the wave energy transmitted by a contactless ultrasonic piezoelectric transducer into the environment among the reflected, transmitted, and guided waves excited when sounding an immersed elastic plate is studied. The dependence of the time-averaged amount of wave energy transferred by each of the excited wave of different types (acoustic bulk waves, Lamb waves, and Scholte-Stoneley waves) on the relative source size, its distance to the plate, and frequency as well as a spatial structure of energy fluxes are analyzed. The numerical study is carried out in the framework of a semi-analytical model based on the integral and asymptotic representations for the Green function of the coupled problem under consideration. The plots of the wave energy dependence on the input parameters indicate the existence of local maxima (sweet spots) of the excited traveling waves that do not coincide with the maxima of the total source power.

This paper describes the phenomenology of the reflection of acoustic body waves in an acoustooptical paratellurite crystal in the case of arbitrary elastic wave incidence on a free interface between the crystal and vacuum. The analysis is performed at arbitrary angles of incidence of acoustic waves in the XOY plane of the material. Anomalous variants of acoustic wave reflection are considered, which fundamentally differ from the known ones and for which the transformation of incident elastic wave energy into reflected wave energy is studied. In the case of oblique incidence, cases of strict reflection of a wave towards the incident wave with an energy conversion efficiency close to 100% are noted.

A new layered surface acoustic wave (SAW) structure was proposed by Murata Co., Ltd recently. It was reported that such structure could achieve an incredible high performance including a higher quality factor Q and electromechanical coupling coefficient K². For deeply understanding propagating characteristics and optimizing the performance of the SAWs in such structure, a layered structure of IDT/θ° YX-LiTaO₃/SiO₂/AlN/Diamond with different structural parameters were theoretically investigated by FEM method. The calculated admittance shows that four eigenmodes simultaneously exist in such layered structure including the main mode SH SAW. And compared with the Traditional SAW structure, the main mode could achieve a higher value of K² with sacrificing its velocity a little. Different metal layers (Au, Al, and Cu) were examined as the electrode material. With Au employed, the K² of the main mode is a little larger resulting from better suppression of the spurious modes. The optimum thickness of electrode and piezoelectric layer are 0.2 and 0.02λ, respectively. In this case, the K² for the SH SAW achieves its maximum value of 12.20% with a large phase velocity of 3608 m/s. Furthermore, the pure SH SAW can be obtained with the Y-cut Euler angle θ from 0° to 60°, where its K² has a wide range of 8.70 to 13.69%. Consequently, the work provides a theoretical guide for designing SAW devices of different bandwidth and operation frequency with such structure.

The article presents the results of studies of the elastic properties of the YBa₂Cu₃O_{(7 –x)} high-temperature superconducting ceramic in the vicinity of the phase transition to the superconducting state at a temperature of 91.3 K and room temperature of 293 K. Near the phase transition temperature with a width of ~10 K, a local increase in longitudinal nonlinear acoustic parameter N was revealed for the first time, which characterizes anharmonic interatomic interaction in the material. Experimental studies of the linear and nonlinear elastic properties of the YBa₂Cu₃O_{(7 –x)} HTSC ceramic near the superconducting transition zone show that the electronic phase transition significantly influences its elastic properties.

A brief description of physically meaningful exact solutions of nonlinear partial differential equations that describe concrete physical objects, processes, etc., is presented. New exact solutions of the Khokhlov–Zabolotskaya equation that have a physical meaning (including a variant of the stationary equation with respect to the direction of propagation of a wave structure) are presented that correspond to the description of localized wave structures either only with spatial localization (beams in the self-trapped propagation mode), or with spatiotemporal localization (pulse-type structures, which are sometimes referred to as wave (acoustic) bullets). The solutions and the localized structures that are described by them are not obvious for nonlinear acoustics or predictable from “physical considerations” (as, for example, for the self-trapped propagation of beams in nonlinear optics), since there are no explicit conditions for the balance of the “necessary” effects in the dispersion-free state.

The paper considers the interference structure during underwater pulsed sounding in the ocean associated with acoustic field diffraction by wind waves. When calculating the matrix of waveguide modes scattered by an agitated surface in oceanic waveguides, a two-scale surface wave model was used, in which the acoustic field is scattered by a resonance harmonic of waves, the phase velocity of which is modulated by larger waves. It is shown that when wind waves are undeveloped, i.e., the wave spectrum is nonisotropic and described by the JONSWAP model, the frequency matched filter response during observations of surface reverberations becomes asymmetric along the frequency axis and depends on the wind direction.

In this study a technique for the modeling of propagation of acoustic pulses in shallow-water waveguides with three-dimensional bottom inhomogeneities is described. The described approach is based on the ray theory of sound propagation and the method of modified Maslov canonical operator. Representation of acoustical field in terms of the canonical operator gives several important advantages in practical computations. In particular, it is possible to compute the time series of a pulse at a reception point located on the caustics of a family of rays. Besides, a significant part of calculations within the proposed approach can be performed analytically; therefore, overall computational costs are substantially reduced. As an example, sound propagation in a wedge-shaped waveguide representing a shelf area near the coast line is considered. The ray geometry in such a waveguide is discussed both in the isovelocity case and in the presence of the thermocline in the water column. For both cases, the time series of an acoustical pulse propagating along the track aligned along the isobaths (parallel to the apex edge of the wedge) is calculated.

The article analyzes difficult to explain experimental data on acoustic signals recorded from explosions and injections of matter at heights of 120–150 km. It is noted that a significant part of the acoustic energy of high-altitude explosions reaches the Earth’s surface without significant absorption. Based on an analysis of a nonlocal three-flux gas-dynamic model, transformation of a strong blast wave in air into a continuous disturbance, a hypothesis is proffered about the sequential decay of a wave from a high-altitude explosion into wave packets (wave trains). A significant fraction of the blast energy should be transferred by such wave packets during their motion from top to bottom throughout the atmosphere, which should be virtually “transparent” to such acoustic disturbances.

The main acoustic requirements in organ concert halls, their evolution in recent decades, and the problems arising in the creation and rebuilding of these halls are considered. The problems associated with the specific features of organs in the acoustic design of halls are briefly discussed.

The article presents the basic relations for correcting sampling estimates of the correlation matrices of adaptive algorithms for detecting the weakest signals in complex interference conditions. The proposed algorithms were used to process field data obtained from a multielement planar array deployed in a maritime zone at a depth of 200 m in coastal wedge conditions. The paper describes the method for processing field data and presents the results of constructing direction-finding reliefs for the case of more than 25 maritime navigation sources in the field of view. It is demonstrated that adaptive algorithms aimed at detecting the weakest signals can yield better results than those obtained by classical adaptive and nonadaptive algorithms.

A modified model for the operation of the semilunar valve is proposed, which makes it possible to estimate the spectrum of acoustic oscillations that occur as it closes. The authors have analyzed closing of the pulmonary valve cusps under a drop in pressure in the pulmonary artery and the right ventricle. Filling of the semilunar valve cusps with blood was taken into account, which leads to an increase in its closing time. This time increases in proportion to the square root of the increase in mass. The second part of the article considers elastic vibrations of a valve that is already closed. The vibrations involve both the cusps and walls of the pulmonary artery, as well as the surrounding fluid. The simplest model of this complex oscillatory system can be an elastic stretched membrane immersed in a fluid and excited by a pulsed force action from the side of the closing valve cusps. It is demonstrated that the second heart sound, heard during auscultation, is a train of damped oscillations, the ultimate duration of which is controlled by the hydraulic resistance of the vibrating cusps. Both the closing process and excited vibrations are nonlinear and multimode. The spectrum and duration of vibrations depend on their amplitude and tension of the valves. The average vibration frequency decreases with time. These results can be useful both for understanding the physical features of the process and for developing new diagnostic methods. In particular, increased pressure in the pulmonary artery can be diagnosed by increasing the frequency and decay time of vibrations in the pulmonary component of the second heart sound.

The problems associated with the absence in the Russian Federation of a single center for the collection and provision of information on Russian-language scientific publications are discussed. A technological scheme is proposed that is useful in creating such a center. The cluster of this center is described via example of acoustic topics, already implemented at the Department of Acoustics, Faculty of Physics, Moscow State University. The corresponding Acoustics portal (http://akdata.ru) is publicly available on the Internet. Entrance points of the portal are the following: the full-text archive of the Acoustics Journal, “Signal Information” on acoustics, and “Information Retrieval System ‘Acoustics’. Russian-language sources”. Work with the system is described. Scientometric data from analyzing the information content of the system are presented for journals, headings, and authors. Bradford’s Law of Scattering of scientific publications is verified using acoustics topics. A list of authors demonstrating publication activity in the field of acoustics is given.