Table of contents

We review the propagation of optical and hydrodynamic perturbations in a gas having nonequilibrium internal degrees of freedom. We discuss the nonequilibrium processes that lead to variation of the refractive index and self-focusing via the effect of kinetic cooling and nonequilibrium vibrational excitation of molecules. We analyze the propagation of small and nonlinear perturbations, as well as shock waves, in nonequilibrium gases.

The linear long-wave approximation is used to analyze the classical interaction between elastic waves and local disorder in dielectric, semiconducting, and metallic crystals. It is shown that carriers of thermal and nonthermal disorder provide significantly different contributions to the acoustic parameters of crystals such as the velocity of sound and the sound attenuation coefficient. The effect of direct and indirect interactions between elastic waves and the ensemble of carriers of disorder on the attenuation of sound is examined. In the former case, deformation by the wave modifies the motion of the carriers of disorder themselves, whereas in the latter case the elastic wave interacts with quasiparticles in the crystal, and the presence of disorder is seen as a change in the character of motion of these quasiparticles. The effects of high concentrations of carriers of disorder produced by melting one of the sublattices of superionic crystals are also described.

Manifestations of order in topologically disordered media are analyzed. Amorphous materials and monatomic melts are considered as examples of disordered media. It is shown that in the case of close-packed liquids geometric and chemical short-range order are most clearly manifested in the acoustic properties. In these materials ordering is of a dynamical character. The temperature dependence of the acoustic properties correlates well with the results of x-ray and neutron diffraction methods of investigation and substantially supplements the latter. The contributions from loss of long-range order and intensification of thermal translational motion on melting are distinguished from one another by comparing the bulk moduli of elasticity of melts, crystals, and amorphous substances.