Volume 70, Nº 4 (2024)

The propagation of flexural elastic waves in notched metal bars with a rectangular cross section with the depth of notches increasing by a power law has been studied by numerical modeling and experimental laser scanning vibrometry. Three types of notch arrangement have been considered: uniform and more frequent and sparse towards the end of a bar. Such structures exhibit the characteristics of an acoustic black hole. For all the studied samples, in the 10–100 kHz frequency range, an increase in amplitude and decrease in length of the flexural wave have been experimentally found as a wave approaches the end of a bar. It has been shown that there is a critical frequency, above which the modes exhibit a section with highly reduced amplitude of oscillations.

The practical task of samples control from various metals using principles of nonlinear acoustics is solved. A surface acoustic wave (surfactant) was used for control, the propagation process of which, due to nonlinear effects, is accompanied by generation of a doubled frequency. An experimental device is used to monitor the structural state of the sample metal by recording a change in a nonlinear acoustic parameter. To excite surfactants, a wedge converter with a resonance frequency of 1 MHz was used. The past wave was recorded by a wedge transducer with a resonance frequency of 2 MHz. It has been shown that NAP for tested materials in the initial state has different significance not only for materials belonging to different classes, but also for materials belonging to the same structural class with different chemical composition (12X17G9AN4 and 12Х18Н10Т). Plastic deformation by 2% does not lead to a change in NAP for alloy AMg6 and steels 12X17G9AN4, 20X13N4G9 and 10XSND. The change as a result of plastic deformation by 2% NAP for stainless steels 12X18N10T and 08X17N4M3 is due to a change in their phase composition associated with martensitic transformation. The presented data on the change in NAP from early stages of elastoplastic deformation to pre-destruction for AMg6 and 10XSND demonstrate the possibility of its use as a prognostic criterion for the limit state of the material.

An acoustic delay line consisting of two Y–X cut lithium niobate plates with a thickness of 0.2 mm, located on top of each other, was experimentally studied. An interdigital converter is located at the edge of each plate. An RF voltage (pulse or continuous) is applied to one converter, which excites a piezoactive acoustic wave with transverse-horizontal polarization traveling in the first plate. The electric field of this wave, penetrating into the second plate, excites an acoustic wave in it, which is converted into an electrical signal using a second interdigital transducer. By changing the distance between the converters by shifting one plate relative to the other, you can change the phase of the output signal and the delay time.

An equation was obtained and the problem of the dynamics of the formation and radiation of a quasi-empty pulsating spherical cavity in a cavitating liquid under the influence of the changing speed of sound in the cavitation zone and the concentration of cavitation nuclei was solved for the first time. Data on the dynamics of the cavity, on radiation and the speed of collapse for the spectrum of internal initial pressure values showed that at the maximum concentration of the gas phase, the pulsations differ in the degree of compression. They have almost the same character — after the first collapse, only one half-cycle occurs, reaching different constant equilibrium radii. The condition of equality of pressure in the cavitation zone and inside the spherical cavity at its boundary made it possible for the first time to establish a dynamic relationship between the volumetric concentration (speed of sound) in the cavitation zone and the radius of the spherical cavity. When calculating and constructing a solution, the condition changes, according to which the initial size of the cavity takes on a value corresponding to the value of the initial pressure. The dependences of the radiation amplitude over the entire range of applied pressures were plotted. It turned out that the radiation amplitude increases by 5 orders of magnitude when the initial pressure in the cavity changes by 3 orders of magnitude from 10^–2 atm to 10^–5 atm.

For a low-frequency sound signal propagating in a horizontally inhomogeneous waveguide of a shallow sea, the influence of a fluctuating interface between the water layer and liquid bottom sediments was studied based on statistical modeling within the framework of the cross-sectional method. The modeling was carried out for hydrological conditions, in many situations corresponding to the shallow shelf zones of the Russian Arctic seas. A feature of these water areas is the presence of an almost homogeneous water layer lying on weakly consolidated bottom sediments with various characteristics, including a high degree of gas saturation. The dependence of the average intensity of the sound signal and its individual modes on the parameters of the problem has been studied: the characteristic scale of fluctuations of the interface and the impedance of this interface, which determines its transmitting properties. It is shown that the influence of bathymetry fluctuations on the average intensity of acoustic modes has its own characteristics in comparison with the influence of random volumetric inhomogeneities of sound speed in the water layer and sediments, established earlier. Thus, bottom roughness of a relatively small-scale lead, on average, to increased attenuation of a sound signal when propagating in a waveguide, and this can occur at relatively short distances from the source. An increase in the reflectivity of an irregular bottom boundary weakens the effect of increased sound attenuation so that for typical values of sound speed in the bottom, the attenuation at distances of 10–20 km from the source differs little from that for an undisturbed horizontal boundary.

Remote sensing of seeps, the release of gas (mainly methane) from the seabed, is an urgent task. The importance of detecting seeps in Arctic shelf zone region is constantly growing due to the degradation of underwater permafrost and the release of gas hydrates. Gas bubbles scatter underwater sound and their resonant frequencies correspond are in the kilohertz range for seeps observed in nature. A promising method for detecting and studying seeps is probing with underwater sound near the denoted resonant frequency. This corresponds to a decrease in the operating frequency relative to the traditional method of studying high-frequency sonars, so the proposed method will be classified as low-frequency in this study. This method expands the study area due to the low sound attenuation in water and the high scattering level near at bubble resonances. Estimates of the scattering strength were carried out taking into account collective interaction (group effects) of bubles. The possibility of using low-frequency hydroacoustic systems to detect seeps has been demonstrated using the results of a full-scale experiment using a simulated bubble jet as an example. A data processing method for detecting nonstationary scatterers is proposed.

The results of measurements of cavitation thresholds and some hydrological and hydrochemical parameters of seawater in various areas of the World Ocean are presented and discussed. The stable temporal variability of the cavitation thresholds on the time scales of several days is shown. The daily, semi-daily and other periodicities of changes in the magnitude of cavitation thresholds were revealed.

A modification of Dean’s method is proposed for determining the impedance in the case of a nonuniform sound field on the front and bottom surfaces of a resonator. Instead of acoustic pressures in Dean’s formula, the modification uses the coefficients of eigenfunctions, which correspond to a uniform acoustic pressure distribution on the front and bottom surfaces of the resonator. The eigenproblem is solved by the finite element method; the coefficients of the eigenfunctions are found by the least squares method. At the current stage of research, the full-scale experiment has been replaced by numerical simulation in a linear formulation of sound propagation in an impedance tube with normal wave incidence with a honeycomb resonator attached to it. The inhomogeneity of the pressure field over the cross section of the resonator is created from the different positions of holes in the resonator face plate. The study is done for a different number of acoustic pressure measurement points at the bottom of the resonator. Calculations show that the proposed method is efficient and provides good agreement with the straight method for determining impedance. However, the possibilities of using modification of Dean’s method in full-scale measurements are limited, because accurate resonator impedance determination requires a large number of measurement points.

The acoustic and electrical properties of a 128-element ultrasonic transducer designed to generate high-intensity focused ultrasound in air in the low-frequency ultrasonic range are investigated. To reduce parasitic grating maxima of the acoustic field, a spiral arrangement of piezoelectric elements on a spherical base was used. The operating frequency of the transducer was 35.5 kHz, and the diameter of the source and focal length were approximately 50 cm, significantly exceeding the wavelength (approximately 1 cm). This selection of parameters allowed for effective focusing, with localization of wave energy in a small focal region, thereby achieving extremely high levels of ultrasonic intensity. The parameters of the ultrasonic field were studied using a combined approach that included microphone recording of the acoustic pressure and measuring the acoustic radiation force acting on a conical reflector. Acoustic source parameters were determined from the two-dimensional spatial distribution of the acoustic pressure waveform, which was measured by scanning the microphone in a transverse plane in front of the source. Numerical modeling of nonlinear wave propagation was also used based on the Westervelt equation to simulate the behavior of intense waves. The acoustic pressure level reached 173 dB, with a focal spot size comparable to the wavelength.