A statistical analysis of the noise immunity of a combined receiver is performed for the observation of a fluctuating tone signal against the background of underwater dynamic noise. The analysis is based on the experimental data obtained in deep ocean with the use of two four-component combined hydroacoustic receivers positioned at depths of 150 and 300 m. Theoretical expressions are obtained for the signal-to-noise ratio of a combined receiver, for reciprocal spectral levels of the signal and noise in both narrow and wide frequency bands. The definition of the combined receiver gain is introduced in terms of the functions of a common single-point coherence for the acoustic pressure and the particle velocity in an acoustic wave. According to the experimental data obtained, in the case of multiplicative processing, the maximal gain in the signal-to-noise ratio of a combined receiver, as compared to a hydrophone-based square-law detector, can reach 15–16 dB for the horizontal channel and 30 dB for the vertical channel of the combined receiver in the case of the compensation of opposing flows of the signal and noise energy.

The frequency and time averaging of the fluctuations that occur in the cross-correlation function of a radiated noiselike acoustic signal with the signal received after its reflection from a rough water surface is considered. The variance and temporal correlation function are calculated for the output effect of a correlation receiver for different ratios between the averaging time and the time correlation interval of fluctuations, the band width of the radiated signal, and the frequency correlation interval of the transfer function fluctuations.

A theoretical study of the propagation of unipolar strain pulses in a solid medium with an arbitrary power-law hysteretic nonlinearity is carried out. Exact analytical expressions are derived for describing the propagation and evolution of initially sine-shaped and triangular pulses in such media. The solutions found are analyzed both numerically and graphically.

The characteristic features of the mode structure of low-frequency sound field in the ocean are investigated for the ocean regions containing the benthic front, which represents the boundary between the Antarctic bottom water and the deep water. The hydrographic characteristics of the benthic front are described with the use of the data of the international WOCE experiment. On the basis of mathematical simulation, the changes introduced by the benthic front into the phase and group velocities and the vertical structure of modes are estimated for a frequency range of 5–20 Hz. Possible ways of reconstructing the hydrographic characteristics of the benthic front by the mode tomography methods are considered.

The pattern equations method is extended to solving three-dimensional problems of wave diffraction by an ensemble of bodies. The method is based on the reduction of the initial problem to a system of N (N is the number of scatterers in the ensemble) integro-operator equations of the second kind for the scattering patterns of scatterers. With the use of the series expansions of the scattering patterns in angular spherical harmonics, the problem is reduced to an algebraic system of equations in the expansion coefficients. An explicit (asymptotic) solution to the problems is obtained in the case when the scattering bodies are separated by sufficiently long distances. It is shown that the method can be used to model the characteristics of wave scattering by complex-shaped bodies.

The possibility of determining the actual paths of moving sound sources from the signals received by a linear horizontal array whose size is large compared to the wavelength is investigated with the use of procedures suggested earlier for the imaging of dynamic objects moving below an inhomogeneous layer. Two cases of signal reception are considered: when the signals propagate in the oceanic waveguide and when the signals propagate through an inhomogeneous layer located near the array. It is shown that, unlike the standard spatial processing procedures, the proposed methods allow one to measure the absolute angular displacements to within the diffraction resolution of the array and to eliminate the ambiguity in angular measurements. An important point is that the proposed methods require no prior data on the parameters of the inhomogeneous layer or the multimode waveguide.

The features of the sound propagation at a frequency of 1.1 GHz in the isotropic phase of cholesteryl miristate are studied. The analysis of experimental data in terms of the theory of interacting modes shows that the peculiarities of the temperature behavior of sound velocity and absorption coefficient are determined by the interaction between the hypersonic wave and the fluctuations of the cholesteric helix pitch.

A stratified stationary flow around a circular cylinder is accompanied by the formation of a thin, acoustically contrasting layer extending along the horizontal axis of the flow. The theoretical and experimental studies show that the pressure differential and the backscattering coefficient of the layer can be quantitatively estimated in the framework of the problem on a stratified ideal incompressible liquid flow around a cylinder.

The scattering efficiencies of hard and soft cylindrical scatterers are compared using the Novikov-Grinevich-Manakov functional algorithm designed to reconstruct two-dimensional scatterers. The existence of a rigid relationship between the amplitude and phase of the wave scattered by a quasi- point-like scatterer and by scatterers with small wave sizes in the form of a soft cylinder, a soft sphere, and an air bubble in a liquid is confirmed by numerical simulations.