The features of using the acoustic method for accurately determining the concentration of components in two-component composite materials by measuring the speed of sound of long waves are described in this paper. Furthermore, explicit expressions for the volume concentrations of the matrix material and reinforcing particles or fibers of composite materials obtained by acoustic measurements are found. In addition, the advantages, features, and limits of the application of acoustic quality control of composite materials of various compositions and purposes are described. It is established that the methods for determining the concentration of components are valid for all types of composite materials, which are conveniently considered as phonon crystals. These results make it possible to more accurately determine or select a measuring cell for the experimental determination of the speed of sound. The mathematical problem to be solved is a purely exact inverse problem.

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The conditions of the onset of aeroacoustic resonance phenomena near a plate in a gas flow in a rectangular channel are studied theoretically and ezperimentally in a two-dimensional formulation. The eigenfrequency as a function of the plate's chord and its position in the channel, the shape of the eigenfunctions, and the effect of the Mach number of the basic gas flow versus the eigenfrequencies and ei#enfunctions and the mechanism of self-ezcited oscillations are determined. A mathematical model by means of which the dependence of the resonance phenomena on the geometrical parameters of the structure were performed is proposed and substantiated.Introduction. The investigation of self-excited acoustic oscillations in a rectangular channel with a plate --the simplest model of the periodic lattice of plates --is of importance, because this shape of an unbounded region is typical and occurs frequently in engineering. Self-excited oscillations that are generated in real structures are, as a rule, caused by the interaction of the eigenoscillations of an infinite region with nonlinear sources: the formation of coherent structures in a flow, unsteady combustion, etc. The important conditions of the onset of intense auto-oscillations are the coincidence of the frequency of an oscillation source with the acoustic eigenfrequency of an open infinite region and the nonorthogonality of the function that describes the source-generated acoustic waves and the eigenfunction.The first studies of self-excited oscillations near a symmetric lattice of plates in a rectangular channel were performed by Parker [1], who showed that self-excited oscillations are purely acoustic, are not connected with plate vibrations, and are caused by coherent structures in the wake behind the plates. Usually, these oscillations are called a vortex sound or aeolian tones (Aeolus is the Greek god of the winds). Aeolian tones near a plate in a channel or near a regular grid of plates were studied theoretically and experimentally in [2][3][4][5][6][7][8][9]. In a two-dimensional formulation, self-excited vibrations near a plate in a rectangular channel in a two-dimensional formulation were treated by Sukhinin in [10]. He proposed and substantiated a mathematical model of eigenoscillations and investigated numerically the dependence of the eigenfrequendes of oscillations on the geometrical parameters of the plate in the channel.Theoretical and experimental investigations of acoustic oscillations near a plate in a channel can be grouped as follows:9 Studying Resonance Properties of Unbounded Media. The existence of the acoustic resonance, the shape of self-excited oscillations (auto-oscillations), the dependence of resonant (eigen) frequencies on the geometrical parameters and the main gas flow. Mathematical simulation, numerical and experimental studies of self-excited acoustic vibrations in unbounded and semi-bounded regions.9 Studying the Nature of a Source of Auto-Oscillations. Formation and development of coherent structures in the wake behind a plate i...