Vol 65, No 4 (2019)

A high-frequency diffraction problem is considered for a Gaussian beam incident parallel to the axis of a strongly elongated spheroid. The parabolic equation method in spheroidal coordinates is used to construct the leading order term of the field asymptotics in the boundary layer near the surface in the form of an integral containing Whittaker functions. The field amplitudes on the surface of a perfectly hard spheroid are computed. High-frequency diffraction effects are discussed.

Modified discrete source method and diagram equation method are used to develop two algorithms for solving a three-dimensional scattering problem in the case of scattering from an acoustically soft or acoustically hard doubly periodic rough surface. Calculations use the efficient algorithm developed earlier for determining a periodic Green’s function in the problem of diffraction by a grating consisting of bodies of revolution. For the algorithm based on the diagram equation method, applicability conditions are determined. The two methods are compared in application to a sine-shaped doubly periodic surface. Fulfillment of both the energy conservation law and the boundary condition at the central surface element is verified. Dependences of reflected harmonic amplitudes on wave parameter and angle of incidence of plane waves are plotted.

The article investigates the response of composite skin-stringer joint to broadband acoustic loading. To describe the frequency and spatial structure of the acting sound field, the study used a model of a field completely correlated over the surface of a sample with a uniform frequency spectral density. The finite element method (FEM) was used to simulate the response of the joint in the 50–1550 Hz frequency band at different total sound pressure levels. To validate the FEM, a series of experiments on a vibration table were carried out, in which vibration simulated acoustic loading. The data obtained were used as the input data for integrating the equations of motion. The results of calculating of the RMS strains in the zone of maximum stresses showed good convergence with the experimental results. For samples containing simulated defects, the resonance frequencies and dynamic response parameters were calculated. Comparison of these results with experimental data characterizing the change in the resonance frequency as a function of defect size made it possible to specify the applied failure criterion and adapt the FEM for analyzing the durability of the composite joint.

Nowadays, biodiesel is considered as an important alternative source of energy. In this work, we introduce theoretically a phononic crystal (PnC) model acts as a biodiesel sensor. Such a sensor can detect and measure biodiesel physical properties efficiently. The sensor configuration is a 1D PnC structure and a defect layer filled with biodiesel in-between. Using the transfer matrix method, the transmission spectrum of the biodiesel PnC structure is calculated. We pay more attention to the resonant peak (the transmitted peak inside the band gap), which is related directly to the biodiesel properties. The obtained simulated results have revealed that the resonant peak is very sensitive to the physical properties of biodiesel. Also, the resonant peak position is highly affected by the biodiesel layer thickness, temperature, and pressure applied to the PnC structure. Moreover, we obtained a high \(Q\) value for the biodiesel sensor at room temperature up to the value of 1980.

The frequency dispersion of the sound propagation speed in a marine waveguide makes it possible to compress an acoustic signal with an increase in its intensity. The paper presents the results of experimental research of this effect during the propagation of a broadband acoustic signal from a parametric array in shallow water. The parametric array provided single-mode excitation for the marine waveguide. The possibility of compressing a broadband signal during its propagation in shallow water with special selection of the frequency modulation mode is discussed. Waveguide dispersion leads to an increase in the efficiency of parametric radiation in shallow water.

The study analyzes experimental data on amplitude–time parameters p₊, p_–, t₊, t_–, t_R+, t_R– of the first positive and negative phases of acoustic waves in the atmosphere from different pulsed sources. The experimental data are compared with numerical simulation results for spherically and cylindrically symmetrical propagation of a blast wave in homogeneous air. Comparison reveals the features of how the stability of the atmospheric boundary layer influences the parameters and shape of acoustic signals propagating in it. Approximations are given for the experimental values p_–, t_– and t_R– of the recorded acoustic signals for a wide range of variations in the source energy values of 10^–8 < Q < 10¹⁰ kg TNT and distances of 1 < R/Q^1/3 < 4 × 10⁴ m/kg^1/3 to them.

The article investigates the possibility of reconstructing a reflector image using the compressive sensing (CS) method based on an incomplete set of echo signals measured by an antenna array in double or triple scanning mode. For comparison, we also considered reflector reconstruction methods used in ultrasonic testing (UT): the correlation, combined SAFT (C-SAFT), and maximum entropy (ME) methods. The last method allows reconstruction of superresolution images from an incomplete set of measured echo signals. Numerical and model experiments have demonstrated the possibility of reconstructing a reflector image with superresolution for a significant decrease in the amount of data used. Reconstructed CS images were compared with images reconstructed by other methods.

An adaptive modal MUSIC algorithm is constructed to localize an acoustic source by a vertical array operating under conditions of incomplete information on a waveguide propagation channel . The results of statistical modeling are presented, which demonstrate the probabilities of correct source localization versus the input signal-to-noise ratio and the sample size. The method is validated by its application to the experimental data observed in the Ladoga Lake. It is shown that this method ensures greater stability of the estimation procedure to mismatch between the true and expected signal replica compared to the conventional element-space MUSIC.

In this study, we used passive acoustic thermometry to measure the core (acoustic brightness) temperature in a subject’s forearm while applying warming ointments to the skin. A decrease of 3–6°C in the acoustic brightness temperature was recorded in the first 5–10 min. After this, the acoustic brightness temperature returned to the level prior to application of the ointment. We attribute the decrease in core temperature to the use of a gel at room temperature in the measurements. On the one hand, the application of a cold gel to the skin should reduce the superficial blood flow. However, the use of warming ointments temporarily blocked this process. As a result, cold blood from near-surface capillaries entered the internal tissues of the forearm, cooling the tissue. The effect was taken into account in the heat conduction equation with blood flow by changing a parameter used in the equation: the temperature of flowing blood. The time dependence of the acoustic brightness temperature calculated with the heat conduction equation is consistent with the experimental data.