Authors:A. A. Belous; A. I. Korol’kov; A. V. Shanin Pages: 158 - 163 Abstract: A method for experimentally determining the frequency-dependent reflection coefficient of a sound-absorbing material at oblique incidence is presented. The M-sequence method and a monopole source are used to measure pulse responses of melamine foam for different angles of incidence of an acoustic wave. Frequency dependencies of the reflection coefficient obtained at different angles of incidence are compared with that calculated theoretically using the Biot model and approximate inversion of the Fourier–Bessel integral. PubDate: 2018-03-01 DOI: 10.1134/s1063771018020021 Issue No:Vol. 64, No. 2 (2018)

Authors:P. V. Artel’nyi; A. L. Virovlyansky; A. Yu. Kazarova; P. I. Korotin; L. Ya. Lyubavin; A. V. Stulenkov Pages: 175 - 185 Abstract: The paper presents the results of processing measurement data on the spatiotemporal structures of sound fields in Lake Ladoga. Measurements were taken with an extended vertical receiver array. The aim of processing was to isolate the field components that were stable with respect to small variations in the waveguide parameters. Since a model of the medium is inevitably inaccurate, such components can be predicted with greater accuracy than the total field. In terms of the ray approach, a stable component is generated by a beam of rays propagating over close trajectories. The discussed experiment analyzed sound fields excited by a point source that emitted wideband pulses, as well as the fields of wave beams excited by the emitting vertical array at fixed frequencies. In both cases, the processing results showed that the isolated stable components, as expected, coincide substantially better with the prediction of theoretical calculation (by the wide angle parabolic equation method) than with the total wave field. PubDate: 2018-03-01 DOI: 10.1134/s1063771018010025 Issue No:Vol. 64, No. 2 (2018)

Authors:O. E. Gulin; I. O. Yaroshchuk Pages: 186 - 189 Abstract: The study is devoted to statistical modeling of low-frequency acoustic signal propagation in a twodimensionally inhomogeneous random shallow sea with a thermocline and differing penetrability of the bottom. Calculations are performed using the local-mode representation of the solution in the one-way propagation approximation. Plots are presented for the behavior of the mean acoustic field intensity for different sound velocity and density values in the bottom. It is shown that the earlier described effect of a decrease in propagation losses in a model randomly inhomogeneous shallow sea with an absorbing bottom significantly depends on the parameters of bottom sediments and is more strongly manifested for bottom boundaries with greater penetrability. PubDate: 2018-03-01 DOI: 10.1134/s1063771018020069 Issue No:Vol. 64, No. 2 (2018)

Authors:I. P. Smirnov; V. I. Kalinina; A. I. Khil’ko Pages: 205 - 214 Abstract: We solve the problem of layer-by-layer reconstruction of the parameters of bottom layers using parametric models of the formation of signals reflected from a layered half-space for coherent probing of the sea shelf. We use a decision rule based on the sequential application of the MUSIC projection algorithm. We study the operability and stability of the search and decision-making algorithms with limited a priori data. PubDate: 2018-03-01 DOI: 10.1134/s106377101802015x Issue No:Vol. 64, No. 2 (2018)

Authors:Jun Tang; P. S. Petrov; Shengchun Piao; S. B. Kozitskiy Pages: 225 - 236 Abstract: In this study the method of source images for the problem of sound propagation in a penetrable wedge [G. Deane and M. Buckingham, J. Acoust. Soc. Am. 93 (1993) 1319–1328] is revisited. This solution is very important three-dimensional (3D) benchmark in computational underwater acoustics, since a wedge bounded from above by the sea surface and overlying a sloping penetrable bottom is the simplest model of a shallow-sea waveguide near the coastline. The corrected formulae for the positions of the source images and bottom images are presented together with the explanation of their derivation. The problem of branch choice in the reflection coefficient is thoroughly discussed, and the corresponding explicit formulae are given. In addition, numerical validation of the proposed branch choice schemes and the resulting wedge problem solutions are presented. Finally, source images solution is computed for a series of examples with different ratios of shear and bulk moduli in the bottom. The interplay between the acoustic-elastic waves coupling and the horizontal refraction in the wedge is demonstrated. PubDate: 2018-03-01 DOI: 10.1134/s1063771018020070 Issue No:Vol. 64, No. 2 (2018)

Authors:J. Poblet-Puig; A. V. Shanin Pages: 252 - 259 Abstract: A numerical method of solving the problem of acoustic wave radiation in the presence of a rigid scatterer is described. It combines the finite element method and the boundary algebraic equation one. In the proposed method, the exterior domain around the scatterer is discretized, so that there appear an infinite domain with regular discretization and a relatively small layer with irregular mesh. For the infinite regular mesh, the boundary algebraic equation method is used with spurious resonance suppression according to Burton and Miller. In the thin layer with irregular mesh, the finite element method is used. The proposed method is characterized by simple implementation, fair accuracy, and absence of spurious resonances. PubDate: 2018-03-01 DOI: 10.1134/s1063771018020148 Issue No:Vol. 64, No. 2 (2018)

Abstract: The paper studies the dependence of the potential noise immunity of a detection system and noise immunity of a detection system with an interference compensator on the array parameters, angular signal position and local noise, the degree of correlation of distributed noise and spectral densities of the signal power, noise, and interference. The gain in noise immunity of the detection system when the optimal spatial filter is used with respect to the use of an interference compensator is estimated as a function of the degree of correlation of distributed noise and the power of random amplitude–phase errors of the weight coefficients of the array. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030089

Abstract: Sound propagation in monodisperse emulsions with arbitrary volume concentrations is studied theoretically using a cell model. It is assumed that emulsion cells are bounded by thin and imponderable rigid shells allowing realization of the minimum energy dissipation principle with viscous acoustic losses. Solutions covering many particular cases and wide parameter and variable ranges have been obtained. These solutions are suitable for studying acoustic properties of emulsions and suspensions, marine sediments, fogs, and smoke, as well as elastoviscous materials with solid or liquid inclusions, etc. Sound propagation and absorption in emulsions and suspensions are considered in more detail. The experimental data in the literature is compared. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030090

Abstract: We present the methods and results of numerical experiments studying the low-frequency sound propagation in one of the areas of the Arctic shelf with a randomly inhomogeneous gas-saturated bottom. The characteristics of the upper layer of bottom sedimentary rocks (sediments) used in calculations were obtained during a 3D seismic survey and trial drilling of the seafloor. We demonstrate the possibilities of substituting in numerical simulation a real bottom with a fluid homogeneous half-space where the effective value of the sound speed is equal to the average sound speed in the bottom, with averaging along the sound propagation path to a sediment depth of 0.6 wavelength in the bottom. An original technique is proposed for estimating the sound speed propagation in an upper inhomogeneous sediment layer. The technique is based on measurements of acoustic wave attenuation in water during waveguide propagation. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030053

Abstract: A mathematical model is proposed for a Kirchhoff–Love-type nonlinear elastic cylindrical shell surrounded by an elastic medium and containing a viscous incompressible liquid. The model is used to analyze wave processes both analytically and numerically. On the basis of the proposed computational algorithm, a software package is developed, which makes it possible to plot diagrams and to obtain numerical solutions to Cauchy problems with initial conditions taken in the form of exact solutions to dynamic equations of shells in the absence of the liquid. PubDate: 2018-05-01 DOI: 10.1134/S106377101803003X

Abstract: Nonlinear evolution of a standing acoustic wave in a spherical resonator with a perfectly soft surface is analyzed. Quadratic approximation of nonlinear acoustics is used to analyze oscillations in the resonator by the slowly varying amplitude method for the standing wave harmonics and slowly varying profile method for the standing wave profile. It is demonstrated that nonlinear effects may lead to considerable increase in peak pressure at the center of the resonator. The proposed theoretical model is used to analyze the acoustic field in liquid drops of an acoustic fountain. It is shown that, as a result of nonlinear evolution, the peak negative pressure may exceed the mechanical strength of the liquid, which may account for the explosive instability of drops observed in experiments. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030144

Abstract: The paper considers active wave processes in changes in the lumina of cylindrical hollow organs. A mathematical model is proposed for autowave transport of the internal contents of an organ based on mechanochemical interactions. The self-organization of changes in the shapes of organs are discussed in application to lymphatic vessels of living organisms. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030107

Abstract: A numerical study of acoustic streaming in a cylindrical cavity subjected to vibrational action with a small vibration amplitude is performed. The dependence of the streaming character on the intensity of heat exchange with the surrounding medium is studied. A change in the forms of the acoustic streaming vortices is shown for a smooth transition from adiabatic to isothermal boundary conditions, which occurs via variation in the heat transfer coefficient. The values of the dimensionless heat transfer coefficient are determined for which the acoustic streaming pattern is close to the limiting cases corresponding to adiabatic and isothermal boundary conditions. For limiting cases of thermal boundary conditions, comparison with an analytical solution is performed. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030077

Abstract: Plane wave imaging is a relatively new technique in ultrasound imaging. However, in traditional methods, the coherent information of different emissions and different elements are not considered. In fact, the sign coherent factor (SCF) can improve the lateral resolution of the imaging greatly. In addition, the delay multiply and sum (DMAS) beamformer is mainly based on the spatial correlation of background scattering signals, it has higher contrast and resolution, but suffers from energy loss at great depths. In this paper, combining the advantages of SCF and DMAS, the sign coherent factor delay multiply and sum (SCF-DMAS) beamformer for plane wave imaging is proposed. Unlike the traditional plane wave imaging, the proposed SCF-DMAS beamformer is based on the 2-D echo data set, which improves the imaging speed greatly. Finally, we simulated the point targets and the cyst phantom to evaluate the performance of proposed method. Compared with the traditional plane wave imaging, the lateral resolution of SCF-DMAS beamformer improves greatly for the point targets, and for the cyst phantom the contrast ratio (CR) and contrast-to-noise ratio (CNR) increased by 96.97 and by 79.98% respectively without reducing the frame rate. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030156

Abstract: A novel numerical algorithm based on the wide-angle parabolic approximation is developed for modeling linear and nonlinear fields generated by axially symmetric ultrasound transducers. An example of a strongly focused single-element transducer is used to compare the results of ultrasound field simulations based on the Westervelt equation, Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation with differently modified boundary condition, and nonlinear wide-angle parabolic equation. It is demonstrated that having a computational speed comparable to modeling the KZK equation, the use of wide-angle parabolic approximation makes it possible to obtain solutions for highly focused ultrasound beams that are closer in accuracy to solutions based on the Westervelt equation. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030168

Abstract: Effect of sound pressure level on acoustic impedance of an orifice in a baffle is investigated based on the measurements performed in an impedance tube by the two-microphone method. Dependences of imaginary and real parts of impedance on the orifice diameter are obtained in nonlinear conditions. Special attention is paid to the attached length (the end correction) of the orifice. Dependence of the attached length of an orifice on the oscillating velocity in it is approximated by analytic functions. PubDate: 2018-05-01 DOI: 10.1134/S1063771018020100

Abstract: We have studied the dynamics of an artificial nonlinear element representing a flexible membrane with oscillation limiters and a static pressing force. Such an element has the property of “bimodularity” and demonstrates “modular” nonlinearity. We have constructed a mathematical model that describes these oscillations. Their shapes have been calculated. We follow the analogy with a classical object—Galileo’s pendulum. We demonstrate that for a low-frequency excitation of the membrane, the level of the harmonics in the spectrum is higher than in the vicinity of the resonance frequency. We have established a strong dependence of the level of the harmonics on the magnitude of the pressing force for a weak perturbation. We propose a design scheme for a device in the quasi-static approximation possessing the property of bimodularity. We perform an experiment that confirms its operability. We show a qualitative coincidence of the experimental results and calculations when detecting an amplitude-modulated signal. PubDate: 2018-05-01 DOI: 10.1134/S1063771018020112

Abstract: Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030120

Abstract: Consideration of the vertical sound velocity profile is highly important for solving problems of sound propagation in waveguides and scattering problems. A pulsed echo signal reflected from a spherical scatterer in a waveguide is modeled for the case of a waveguide characterized by sound velocity increasing with depth. The simplest model of the medium is considered in which the scatterer, the source, and the receiver are positioned in a layer with constant sound velocity. Below this layer, the sound velocity increases with depth so that the square of refractive index varies according to linear law. The scattering coefficients for the sphere are calculated using the normal wave method. The number of normal waves forming the echo signal is determined by the preset directionality of the source. Modeling is performed in a frequency band of 70−90 kHz for distances between the scatterer and the transmitter-receiver within 500−1000 m. The transmitted signal has the form of a pulse with cosine envelope and central frequency of 80 kHz. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030065

Abstract: The paper presents the results of theoretical and experimental studies of the occurrence and locations of aerodynamic noise sources in air blowers related to air flow around stationary and moving elements inside a machine body. These studies were based on basic research by Lighthill and Curle and used a developed method for measuring pressure pulsations on rotating blades and stationary elements of a machine body. The most significant sources of discrete and broadband components of aerodynamic noise were revealed. The role of blades in an impeller in the emission of discrete noise components was studied. It was established that broadband peaks in the emitted noise are associated with acoustic resonances of the internal volume of the air blower. It was shown that the turbulence and velocity of the incoming flow influence the intensity of aerodynamic sources inside the body. Our studies spurred both deeper research into the nature of aerodynamic noise sources that form in air blowers and recommendations for reducing the noise produced by these sources. PubDate: 2018-05-01 DOI: 10.1134/S1063771018030028