Acoustical Physics

The problem of scattering a plane sound wave on a diffraction grating composed of semi-infinite thin screens located at the same distance from each other is considered. It is shown that at certain values of the impedance of the screen surface, the grating completely reflects the wave incident at any angle. The result obtained is useful for the development of soundproof blown structures.

This paper presents acoustic properties of potassium titanyl arsenate crystal, advanced acousto-optic material. Stiffness ratios for this crystal were determined, as well as phase velocity of acoustic waves and angles between vectors of phase and group material velocities. Acoustic parameters of the crystal were calculated based on experimental data from Schaefer–Bergmann diagram and elastic wave velocity measured with phase method.

A comparison has been made of the noise immunity of local source direction finding algorithms with linear scalar and vector-scalar antennas using different methods of processing signals from pressure receivers and oscillatory velocity receivers. Calculations were carried out for equidistant antennas with equal apertures and equal inter-element distances. It has been calculated and experimentally shown that vector-scalar antennas have a significantly smaller lateral field than scalar antennas, and the additional maximum, which is formed only at “oblique” reception angles, is 3–5 times smaller than the “mirror” lobe of scalar antennas. Good agreement between the experimental and calculated spatial spectra was established for all directions towards the source. A computational and experimental justification for the use of vector-scalar antennas for unambiguous direction finding of a noise source, including at oblique angles of incidence of the wave front, both “on the foot” and in the antenna towing mode, is presented. It is shown that receiving signals using vector-scalar antennas ensures separation of signals “on the left and right sides” and separation of sources located “in the front and rear hemisphere”.

The paper presents a description of the results of a full-scale experiment conducted in August 2023 in the Sea of Japan on the transmission and reception of pulsed acoustic signals for the “shallow-to deep” scenario. A feature of the experimental waveguide is its division into shallow and deep-water parts, approximately equal in length. The results of mathematical modeling of the propagation of pulsed acoustic signals from the shelf zone to the deep sea for this track are discussed. The modal structure of the field in the waveguide is described, theoretical estimates of the arrival times of the modal components of the acoustic signal are obtained. The formation of a dense group of modal components of small numbers, which is not typical for this class of problems, is discovered and theoretically explained. This phenomenon is associated with both the configuration of the waveguide, divided in approximately equal parts into deep-water and shallow-water parts, and with the orientation of the acoustic path at an acute angle relative to the depth gradient, which creates conditions for the occurrence of the phenomenon of horizontal refraction. A comparison of the experimental impulse responses of the waveguide and the estimates of the arrival times gives reason to believe that a distinctive feature of acoustic paths of this type is the formation of two maxima of the impulse response function.

Based on the wave equation exact within the framework of linear acoustics of moving media, a scalar equation on acoustic pressure in the atmospheric boundary layer is obtained. The main approximations for diffraction and turbulent pulsations transport effects are derived. Based on these, HOWARD and KZK-type models are developed. The two-dimentional KZK-type equation is applied to the problem of calculating the sonic boom characteristics from civil supersonic aircraft demostrator “Strizh” under development in TsAGI in conditions of strong velocity pulsations in atmospheric boundary layer. Grid and statistical convergence of simulation results is achieved. The three-step structure of the demonstrators pressure signature front shock leads to a small change in amplitude and a significant loudness reduction in PL metric compared to the results obtained for N-waves under similar conditions.

A stationary medium containing inhomogeneities of sound speed, density, and frequency-dependent absorption coefficient is considered. These inhomogeneous acoustic characteristics, including a power index of frequency dependence of the absorption coefficient, are unknown and must be reconstructed based on scattering data at many frequencies. First, complex scatterer function, which contains contributions from various types of the inhomogeneities, is reconstructed by solving the inverse problem. Then, method for extracting the individual spatial distributions of all the sought-for acoustic characteristics from the scatterer function is proposed. Numerical simulation results are presented, which illustrate capabilities and restrictions of the proposed method for various noise levels in initial data. It is shown that result of reconstructing the power index of the frequency dependence of the absorption coefficient has the lowest noise resistance. At the same time, the sound speed, density, and absorption coefficient are reconstructed with acceptable accuracy and high resolution.

Toothed whales and dolphins produce complex acoustic signals. However, how they develop and function is unclear. In this study, the sounds of a newborn (NB) bottlenose dolphin (Tursiops truncatus) were recorded by a dual-channel recording system in the frequency band 0.4–220 kHz with a dynamic range of 81 dB and a sampling frequency of 1 MHz, simultaneously with video recording of its position relative to hydrophones, 22, 46, 46.5 and 47 h after birth. The results of the analysis of the recordings indicate that the NB produced 20 series containing 1640 echolocation clicks. In doing so, it varied the duration of series in the range of 0.8–7 s, the number of clicks in a series from 20 to 280 and the interpulse intervals from 6 to 220 ms. When moving along the pool, it shifted the position of the maximum of the click radiation direction in space, and each subsequent click was produced after receiving an echo from the previous one and a time delay (tens of ms) for echo processing. The spectral and temporal characteristics of its clicks differ slightly from those of adult dolphins. The apparent sours level of clicks (ASL) at a distance of 1 m from the NB was 188–164 dB relative to 1 μPa. Consequently, the abilities of the NB of the bottlenose dolphin (Tursiops truncatus) to echolocate, considered for the first time for toothed whales in this paper, are innate, while physiological, social and cognitive aspects of echolocation, not considered in this paper apparently still have to develop.

The effectiveness of using acoustic emission diagnostics in conjunction with vibration diagnostics and video recording to assess the load-bearing capacity of a composite sample, as well as to identify the mechanisms of the evolution of its destruction under compression, has been studied. Before the compression test, a sample made of multilayer high-strength carbon fiber plastic was subjected to impact action with an energy of 90 J in the central part. Before the occurrence of developing macrodamages in the structure of the material, the amplitude spectra recorded during vibration diagnostics remained practically constant. The picture changed sharply with the emergence and development of macrodamages, which was reflected by the scalogram of the recorded vibration signal emissions at the characteristic stages of the evolution of the destruction of laminated carbon fiber reinforced plastic. The dynamics of changes in the peak frequencies of local maxima of the amplitude spectra of vibration signals generated by the processes of collapse of the ends of the sample, its delamination, local buckling during deflection, and breakage of layers with increasing compressive load until the load-bearing capacity of the carbon fiber plastic is lost has been studied. The use of vibration diagnostics in conjunction with video recording made it possible not only to verify the results of acoustic emission diagnostics to assess the level of load-bearing capacity of layered carbon fiber reinforced plastic, but also to monitor the kinetics of macro-damage in its structure.