Vol 64, No 2 (2018)

The paper considers acoustic wave scattering by inhomogeneities with a small wave size using the Green’s function apparatus, which makes it possible universally to take into account both the refraction and density components of an inhomogeneity. Estimates for the multipole components of a field scattered by a nonresonance inhomogeneity are presented. For an inhomogeneity with small dimensions, it suffices to consider only monopole and dipole scattering. These conclusions are confirmed by an analysis of the field scattered by a circular cylinder with a small wave radius. The results are used to numerically simulate a Lippmann–Schwinger equation. The form of the discretized matrix Green’s function for identical values of the spatial arguments is presented. This makes it possible to take into account multiple scattering processes within a discretization element with a small wave size. Its use automatically fulfills the relations between the phase and amplitude of secondary acoustic field sources.

An axially symmetric compact range reflector with a blended rolled edge was analyzed and optimized in a rigorous formulation of the diffraction problem. The corresponding boundary-value diffraction problem is solved with the analytic regularization method, which reduces the problem to an operator equation of the second kind, thus guaranteeing a numerically stable and effective solution. The distribution of the surface density and the fields at the aperture and in the near-field zone were obtained and analyzed for different types of the reflector-edge curvature. In addition, a “blending function” was used that esures an infinitely smooth contour across the junction between the paraboloid part of the reflector and its rolled edge. The procedure for determining the optimal edge is carried out in the rigorous formulation of the diffraction problem by minimizing the deviation from a plane wave.

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.

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.

The aim of the study whose results are discussed in this paper was to conduct experimental and numerical research on improving a high-accuracy method, developed by the authors, of positioning underwater objects. For this, experimental testing of an improved range-finding technology was carried out, based on the inclusion into the measuring scheme of a block that can measure and monitor the sound velocity on the shelf area of a track connecting a source of navigation signals and an a receiver system imitator consisting of autonomous underwater apparatus. In addition, under natural conditions, we implemented a scenario in which range-finding data was provided to an autonomous underwater apparatus carrying out a mission in the water area at a distance of 300 km from the source of navigation signals using technical tools for controlling variability of the sound velocity on the shelf. A specific example was used to test the acoustic range-finding technology on a track with complex hydrological and bathymetric conditions, and an estimate was obtained for the accuracy of measuring distances during a 4 h drift of the autonomous underwater apparatus imitator.

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.

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.

We have carried out an experimental study on the generation of pressure oscillations in a model of a borehole ring acoustic radiator that moved within a cylindrical chamber. The possibility of preserving a short jet with increasing resonator chamber length has been studied. The influence of the jet length, which is determined by the interval between the nozzle and the ring, on the frequency and intensity of generation has been considered. The constancy of the chamber resonance at the natural frequency, regardless of the length and velocity of the jet, has been noted. Recommendations for choosing the length of the chamber and the generation frequency associated with it are given.

Using a computer-based simulation method, we consider the noise immunity of an individual combined receiver consisting of sound pressure and vibration velocity receivers, as well as the noise immunity of a plane acoustically transparent array consisting of such receivers, are considered. The computer simulation results verify the earlier developed analytic method of calculating the noise immunity coefficient of hydroacoustic arrays for the multiplicative processing algorithm for processes in their channels. It is shown that if the anisotropy of the noise field acting on the combined receiver and array varies in a very wide range, the maximum noise immunity is ensured not by multiplicative but by additive processing.

Results of verifying the levels and directional characteristics of sound radiation are presented for a finite-element numerical model of a submerged inhomogeneous shell. The error in numerical modeling of directional characteristics is determined for sound radiation from shell parts with one-layer (shell–liquid) and multilayer (shell–liquid–shell–liquid) structures.