卷 64, 编号 5 (2018)

The paper reviews and analyzes the state of the art of the problem on sound absorption in the linear formulation and methods for solving it. It is shown that the majority of publications are reduced, from the viewpoint of absorption efficiency, to the realization of one of two ideal absorbers: a Kirchhoff blackbody and an optimal absorber. These two absorbers have fundamentally different properties: the relative cross section of the blackbody absorber is equal to unity, whereas for the optimal absorber, this is the largest possible value, which can be much larger than unity. The practical problems they solve also differ. Ways of creating modern efficient absorbers are discussed, the most promising of which are absorbers made of acoustic metamaterials, which realize ideal bodies in the optimal way and require developed additive technologies for their manufacture.

A gently sloping perturbation of the rigid wall of a planar acoustic waveguide can cause a Wood anomaly, reflected in an enormously rapid change in the diffraction pattern when the cutoff frequencies are approached from below inside a continuous spectrum. Asymptotic analysis has been used to establish the restrictions imposed on the wall perturbation profile that ensure the appearance, enhancement, or disappearance of the anomaly. The asymptotics of the eigenfrequencies is studied and the complex and threshold resonances are discussed.

We study the interaction of noise and regular signals with a front whose steepness increases or decreases owing to nonlinear distortion of the profile of an intense pumping wave. Projective transformation is used, which is a result of one of the Burgers equation symmetries. Signal interaction with the pumping wave at its leading edge results in an increase in signal amplitude, a decrease in its time scale, an increase in the signal evolution rate, and earlier merging of discontinuities. At the trailing edge, an increase in signal amplitude, an increase in the time scale, and deceleration of the evolution rate occur. Formulas are obtained that describe the transformation of the spectrum and the correlation function of noise. Laws of the change in noise energy for both small and large Reynolds numbers are found. We study the interaction of weak noise with a nonstationary shock front in a medium with a finite viscosity. It is shown that, owing to competition between amplification at the shock front and high-frequency attenuation, the dependence on the noise intensity on distance has a nonmonotonic character, and at large distances, the intensity tends to zero, while the correlation time tends to a finite value.

The paper presents experimental research data on the frequency and concentration dependences of additional ultrasound attenuation in mixtures of asphaltene in toluene. The results indicate that additional attenuation is determined by several different processes and has a relaxation nature. By juxtaposing the results to the data from several methods for studying similar model disperse systems, we show that changes in the concentration attenuation coefficient and its frequency dependence can provide additional information on structure transformations and phase transitions. The frequency dependence was measured in a range of 10–23 MHz. The concentration dependence was measured in the same frequency range from 0.1 to 10 wt %.

We consider the patterns manifesting themselves in the formation of the spatial distribution of the intensity of an acoustic field excited by a point source emitting a tone signal in a deep-water oceanic waveguide with an underwater sound channel open to the surface and an acoustically transparent bottom. It is shown that at a certain emission frequency, the diffraction and ray caustics can merge, which leads to an appreciable increase in the acoustic field intensity along characteristic caustic lines on the horizontal distance–depth plane.

A possibility to create controlled anisotropic illumination in an acoustic tomographic system with correlation processing of experimental data is considered. Fundamental difficulties encountered in setting up anisotrpic illumination in a ring array system are discussed. It is shown that, in systems with focusing elements, illumination is realizable; the output signal-to-interference ratio is estimated.

The proposed ultrasound imaging mode allows detection of objects, which essentially differ in their scattering properties from the surrounding tissues and liquids. The objects in question are primarily microcalcifications, renal and urinary stones. Our previous study has shown that the Doppler signals from these objects have two components common for echoes from solid mineral inclusions. They can be in superposition with the blood and noise signals. One of these two mineral-related components is characterized by cavitation, the other – by elastic vibrations of the object presumably caused by acoustic radiation force. According to statistical and energy parameters, these components differ from each other, as well as from noise and blood echoes. The article proposes a practical method for identifying signals with mineral-related components. This method is the base for the novel diagnostic visualization mode specifically designed for the mineral inclusions detection with ultrasound.