Vol 64, No 3 (2018)

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.

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.

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.

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.

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.

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.

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.

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.

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.