Vol 71, No 5 (2025)

Matrix Klein–Gordon equation (MKGE) describes waveguides with discrete structure of their cross-section. It can be also considered as a model of a continuous waveguide obtained in the result of application of the waveguide Finite Element Method. In the current work we study applicability of MKGE to flexural vibrations of a thin plate. The question is not trivial since MKGE contains time and coordinate derivatives up to the second order, while the plate’s vibrations are obeyed to an equation with the fourth order coordinate derivative. Here we derive MKGE of different dimensions for the plate. We show that linear approximation of the displacements in the plate across its width leads to an overestimation of the plate’s rigidity, while more complicated models lead to the right values.

In the experiment, the source of thermal acoustic radiation was localized using the correlation technique. A receiving array of four sensors was used for this purpose. A 5.5 mm diameter Teflon cylinder cooled by 34°C relative to the environment was used as the source; it was located at a distance of 770 mm from the 20 mm sensors. Localization was performed at two positions of the source, the distance between which was 10 mm. The average reception frequency was 1.5 MHz. During localization, spatial correlation functions calculated for three pairs of adjacent sensors and for two pairs of sensors located “every other” were summed up. The source images obtained in the experiment were spaced by 8.5 mm, the image size was 6 mm for the source in the center and 6.5 mm for the shifted source. Thus, the spatial resolution obtained with the given geometry corresponds to the calculated data for the sensor array and is significantly higher than the spatial resolution with non-correlation reception.

The article is devoted to the study of the possibilities of using granules obtained from nanosized silicon dioxide powders as materials that absorb low-frequency acoustic vibrations. Granules with sizes from hundreds of microns to several millimeters were obtained from silicon dioxide by different methods and were classified by the sizes of the resulting granules. Some of the granules were solid, and some had open porosity with pores in the nanometer range, the volume of which was more than 50% of the granule volume. The granules were used as an independent layer of noise-absorbing material, as well as a modifier of traditional materials used in mechanical engineering. It was found that granulated nanoporous materials based on nanoparticles can significantly suppress acoustic vibrations at low frequencies.

The paper presents the results of a comparative study of elastic losses and dispersion in dense and porous ferroelectrically “hard” piezoceramics of the same chemical composition based on the lead zirconate titanate (PZT) system. To measure and analyze the real and imaginary parts of the complex elastic parameters, as well as their frequency dependences, we used a previously developed method for analyzing piezoresonance spectra for the fundamental and higher-order resonances of thickness oscillations mode of piezoceramic disks. Experimental samples of dense and porous piezoceramics were prepared using conventional sintering techniques and a modified pore-former burnout method. The study revealed regions of anomalous elastic dispersion in porous piezoceramics, caused by a change in the ratio of the wavelength of resonant oscillations of the piezoceramic element to the spatial heterogeneity scale of its porous microstructure as the frequency increases.

The article discusses theoretical and experimental results of studies of propagation of pulse pseudorandom signals in the Sea of Japan on an acoustic path with a length of 144.4 km under the influence of typhoon consequences on hydrological conditions. The case of propagation of signals with a central frequency of 400 Hz from an extended shelf to the deep sea with reception at depths of 69, 126, 680 and 914 m is studied. To receive signals, a drifting system with hydrophones distributed over a depth of up to 1000 m with the possibility of long-term recording at fixed depths was used. Analysis of experimentally obtained pulse characteristics showed that at all horizons with the same time range, a group of ray arrivals with a duration of about 0.5 s with a maximum in the center is recorded. Based on the results of the experiment, the effective propagation speeds of signals received at various depths were calculated, and conclusions were formulated on the possibility of solving problems of high-precision positioning of underwater objects for various purposes at depths of up to 1000 m and at a distance of hundreds of kilometers from control centers.

It is known that disturbances of a localized vortex can have two specific mechanisms of interaction with the surrounding flow. The first is associated with energy loss by the flow, which leads to instability when the energy of the vortex disturbances is negative. The second is the Miles mechanism of interaction between vortex core oscillations and disturbances in the vicinity of the critical layer (the streamline along which the phase velocity of the disturbances coincides with the velocity of the mean flow). This is accompanied by an energy flux from this vicinity, leading to damping of the oscillations in the case of negative energy (and, conversely, to Miles instability when the energy of the core disturbances is positive). A flow in which both of these mechanisms are realized simultaneously is considered for the first time. For this purpose, disturbances of circular vortices with negative energy are considered, for which both acoustic instability and Miles damping are realized. It is shown that in the case of weak compressibility, the Miles mechanism can completely suppress acoustic instability. However, in the case of stronger energy loss due to acoustic radiation, acoustic instability will dominate. The influence of various parameters on these effects is analytically studied, and a quantitative criterion for the acoustic instability of a vortex with a smoothed vorticity profile is established. The effect of acoustic instability is considered for high velocities in the vortex core, including supersonic flow. Flow velocity enhances the acoustic instability increment due to more efficient sound radiation, which enables instability of vortices with stronger smoothness. This effect demonstrates that the behavior of vortex structures in high-speed jets can differ fundamentally from the case characterized by a low Mach number and, due to acoustic instability, intensity vortex oscillations, which in the subsonic case are characterized by strong damping. It is also shown that in an incompressible flow with a vortex confined by impedance walls, an alternative energy loss mechanism to the acoustic one is realized. In this case, Miles damping can also be overcome. Moreover, unlike the mechanism realized by outgoing acoustic waves, energy loss by the vortex due to absorption by the cylinder walls can be significantly more effective, leading to an expansion of the instability region to flows with smoother vorticity profiles.

The process of propagation and disintegration of long low-frequency nonlinear acoustic-gravity waves into the upper atmosphere is investigated. One spectral wave mode is considered, for which an approximate nonlinear Korteweg–de Vries–Burgers equation is derived using the formulated version of the variational principle for a liquid layer. The coefficients of the derived equation depend on the height. The issue of wave destruction into smaller-scale solitary waves is investigated in the work. The conditions for wave destruction are written out and formulas for estimating the scales of secondary soliton waves formed during the disintegration of the primary wave are written out. It is shown that wave destruction can occur only in certain layers determined by the vertical structure of the wave.

The creation of quiet green areas to reduce acoustic stress contributes to improving the quality of life of citizens in noisy cities, reducing stress levels and enhancing the uniqueness of urban spaces. Quantitative indicators for assessing the quality of the sound environment are in demand when designing such zones, ensuring harmony between urban dynamics and the human need for silence. It is believed that human adaptation to the sound environment leads to the fact that residents of megacities stop noticing annoying sounds, although this does not negate their harmful effects on the nervous system, causing its exhaustion. In the interest of studying the properties of perception of the sound environment depending on its properties, as well as the degree of adaptation of residents to the environment, a cross-comparison of assessments of the quality of the sound environment, as well as the values of acoustic characteristics of spaces in the territories adjacent to the building, was carried out. The survey was conducted by two groups of respondents in academic buildings of two leading universities — MSU and MSTU. The first group consisted of 30 third-year students from Moscow State University, the second group consisted of 25 third-year students from Moscow State Technical University. To assess the quality of the sound environment, the hearing examination method introduced by the International Organization for Standardization for the Analysis of Soundscapes (ISO) was used. We assumed that the students of the two universities are well adapted to the sound and landscape environment of Moscow, and over the years they have developed a sustainable attitude towards the environment of their university. A comparison of estimates of the quality of sound media and their acoustic properties indicates that MSU students were less sensitive to loud noises and quiet sounds of nature in the territories of the two universities. The reason for such differences may be the adaptation of MSU students to the environment of their university, which promotes natural audiovisual relaxation and increases psycho-emotional stability.