Pleiades Publishing Ltd

Measurements of response, gain, and noise immunity are carried out for an underwater compensated additive receiving array with randomly spaced hydrophones that is moored at the bottom of a man-made lake with multimode sound propagation. The in-sea locating ability of a similar array is demonstrated with the sources of noiselike signals at frequencies of 5–100 Hz. A dedicated numerical processor is developed and tested for processing the signals received by a random underwater array.

The applicability conditions for the concept of a directivity pattern (a scattering amplitude) in the problems of waveguide propagation are formulated. The consideration is based on the solution of the Sturm-Liouville problem. The results of the comparison between these conditions and the analogous conditions obtained earlier in the ray approximation are discussed. The expression for the scattering matrix of waveguide modes is modified on the basis of the suggested conditions in such a way that it involves only the quantities determined from the solution of the Schrödinger equation. This makes it possible to perform numerical calculations by using the results of numerous studies of the propagation in inhomogeneous waveguides and the diffraction by complex-structured bodies in free space.

Several structural variants of a new acoustic metamaterial are proposed with unusual wave properties, and normal wave dispersion is studied. One of the variants is a bandpass filter in whose passband the phase velocity of a normal wave is negative. In another variant, there are two passbands: in the first, the phase velocity is positive; in the second, negative. As a result, unusual physical phenomena can be observed in it, in particular, a special type of birefringence and the combined Doppler effect. In the long-wavelength range, where the metamaterial behaves like a continuous medium, the corresponding equations have been derived in partial derivatives. Essential differences from the common wave equation are noted.

Methods based on wavelet analysis are used to study longitudinal elastic waves propagating in aluminum single crystals in the [111] and [001] directions at frequencies of 5 and 10 MHz. Results of the wavelet filtering of responses of the laser Doppler interferometer to these acoustic signals are reported. The efficiency of filtering the time dependencies of the interferometer response in the presence of white and colored noise is studied. Recommendations concerning the choice of the filtering parameters are given.

Relations between the amplitudes of acoustic waves excited by a thin elastic plate under the effect of external forces and the amplitudes of waves scattered by this plate are obtained. Two cases are considered: when the plate separates acoustic media filling two half-spaces and when it separates acoustic media filling an acoustic waveguide. The energy conservation law is used to derive the identities that determine the relations between the amplitudes of acoustic waves radiated by a thin elastic plate under the action of forces.

Abstract—The paper studies the features of the cross-correlation function of a chirp signal reflected from a rotating propeller with an emitted replica signal. In the obtained correlation function, in addition to the main peak corresponding to the propagation time of the signal to the scatterer and back, additional informative maxima are observed in the form of multiplets, the presence of which is determined by sound scattered by the rotating propeller. The parameters of the additional maxima depend on the diffuser characteristics (distance to the propeller, its rotation rate, number of blades, orientation relative to the direction of irradiation). The results of numerical simulation and experimental data processing are presented. The results point to the possibility of using additional correlation function peaks as an informative parameter to identify a rotating propeller.

The evolution of acoustic waves that propagate in cylindrical channels containing permeable regions and surrounded with a porous medium is studied. The quantitative and qualitative features of the wave dynamics are determined in relation to the condition of the inhomogeneous porous medium. In particular, cases when the channel is surrounded with radial fractures or with a low-permeability mudcake are considered.

Features of underwater sound scattering by small crustaceans are considered. The scattering data are obtained with the use of unique instrumentation that allows one to measure quantitative scattering characteristics (backscattering cross sections and angular scattering patterns) for crustaceans of different sizes, at different frequencies (20–200 kHz) and different insonification aspects. A computational model of crustaceans is considered with allowance for both the soft tissues of the main massive part of the animal's body and the stiff armour. The model proves to be advantageous for explaining some scattering features observed in the experiments. The scattering cross sections of crustaceans measured by other researchers are presented in a unified form appropriate for comparison. Based on such a quantitative comparison, relatively simple approximate empirical formulas are proposed for estimating the backscattering cross sections of small (within several centimeters) marine crustaceans in a broad frequency range.