K. V. Dmitriev

Acoustic double-negative media

We consider the possibility of the existence of media in acoustics that are similar in several effects to the widely discussed electrodynamic left-handed media. The density and compressibility of a medium are shown to be the mechanical analogues of negative permittivity and permeability. We discuss the physical meaning of their negativity and mechanical models with such properties. To identify the effects related to the sign of the density and compressibility, we have performed our analysis based on linearized hydrodynamic equations instead of the wave equation or the Helmholtz equation. We have obtained an analogue of the Lippmann—Schwinger equation and constructed a theory of wave scattering by inhomogeneities in a medium with arbitrary values and signs of the density and compressibility. Our numerical simulations have revealed all of the expected effects. We consider the questions concerning the fulfillment of the causality principle and its consequences generalized to the case of negative media in the form of a connection between the damping and dispersion of waves.

Wave effects in acoustic media with a negative refractive index

A method for analyzing media with a negative refractive index in acoustics has been proposed. An analog of the Lippmann-Schwinger equation is derived from the initial hydrodynamical equation and a theory of wave scattering by inhomogeneities of a medium with an arbitrarily specified value and sign of density and compressibility is constructed. Numerical simulation of the effects related to negative refraction is performed.

Active-passive thermotomographic systems with focusing of acoustic fields

The possibilities of correlation thermotomographic systems, processing focused fields of intrinsic thermoacoustic radiation from an object and background radiation, are discussed. An additional anisotropic “illumination” makes it possible to reconstruct the distribution of all main acoustic and temperature characteristics. The estimated sensitivity and resolution suggest practical applicability of the systems proposed.

Acoustic properties of organic powders as ultrasonic contrast agents

The results of experiments on measuring attenuation and the effective acoustic nonlinear parameter of the second order are given for a suspension of cocoa-powder in water at different concentrations of the suspension. In the process of evaluating the value of the nonlinear parameter the attenuation in the suspension and generation of the second harmonic not only in the suspension but also in water are taken into account. The obtained results are evidence of the possibility of using a suspension of cocoa-powder in water as a technical substitute for ultrasonic contrast agents. The values of attenuation (up to 60 m−1 at the concentration of 1 g of the powder per 1 l of water) and the nonlinear parameter (up to 120 m−1 at the same concentration) mean that the suspension of cocoa-powder in water has smaller attenuation and the nonlinear parameter than ultrasonic contrast agents at the same concentration. However, these values for the suspension differ considerably from corresponding values for water or blood and, therefore, a suspension of cocoa-powder in water is a promising “substitute” for ultrasonic contrast agents in the case of technical testing of systems for nonlinear tomography of a blood flow, but cannot replace them in medical studies.

Principles of obtaining and processing of acoustic signals in linear and nonlinear tomographs

Physical, mathematical, and engineering problems related to creating two fundamentally different types of tomographs (so-called linear and nonlinear tomographs) are considered. The tomographs under development are designed for diagnosing pathological tumors of soft biological tissues at early stages of their growth.

Experimental determination of thermal–acoustic sources by means of focusing correlation tomography

The scheme of an experimental setup for the focused acquisition of thermal—acoustic fields with their subsequent correlation treatment is proposed. The signal-to-interference ratio is evaluated for this scheme and the results from experiments on correlation signal detection from quasi-thermal—acoustic and thermal—acoustic sources are presented.

Acoustic Wave Attenuation and Dispersion in Shallow Water

The propagation of sound in shallow water is studied. Signals are recorded at several distances from the source. The amplitudes of the signals’ spectral components are calculated at these points. The frequency dependence of the attenuation coefficient is found using the maximum likelihood approach.

Maximum power scattered by a point inhomogeneity in problems with different spatial dimensions

Monopole and dipole scattering coefficients of a small wave size acoustic inhomogeneity are introduced. It is shown that the amplitude of these coefficients is limited. The maximum possible power scattered by the small wave size inhomogeneity is found in one-, two-, and three-dimensional cases. The number of metamaterial elements per one wavelength is estimated on this basis.

Matrix Green’s functions and their application in analyzing scattering by density and sound velocity inhomogeneities

Matrix Green’s functions are introduced for a linearized system of hydrodynamic equations. The relations between the retarded and advanced Green’s functions and Green’s functions of the direct and conjugate operators of the system of hydrodynamic equations are determined. An expression for the reciprocity principle and a relation like the Marchenko equation are derived. The proposed mathematical apparatus is used to analyze scattering by a quasi-point refraction-density inhomogeneity of a medium. The phase and amplitude limitations are obtained for the scattering coefficients of such an inhomogeneity. The existence of the largest possible amplitude of the scattered field should be taken into account in designing metamaterials consisting of individual elements whose sizes are small compared to the wavelength, including those with resonance properties.

Thermoacoustical Tomography with Arrays Focusing by Reflection

Spatial distribution of temperature of a biological object is an indicator of a primary tumor process. Perspectives and difficulties of a practical realization of acoustical thermotomography are discussed. The acoustical thermotomography allows, in principle, to reconstruct separately all acoustical and temperature characteristics of an investigated object, if an additional acoustical “illuminating” field is generated, and this field is anisotropic. The “illumination” is additional radiation, similar to the thermoacoustical field of the object. However, the input signal-to-interference ratio at a fixed receiver is very small, and it is necessary to use a large number of receivers or to focus a signal. The possibilities of correlation thermotomographic systems, processing focused fields of intrinsic thermoacoustical radiation from the object and of background radiation, are discussed. System of equations is derived for determining the acoustical and temperature characteristics, when received thermoacoustic signals are pairwise correlated in the mode of difference time delays only. Way of obtaining the necessary set of experimental data is discussed. Realization of the additional anisotropic “illumination” has a number of difficulties in the ring tomographic scheme, but it is sufficiently easy in proposed scheme with using the additional focusing-reflecting devices.