Nonlinear acoustics
of structurally inhomogeneous media
This trend of physical acoustics was successfully developed at the IAP RAS on the foundation laid by the works made by L. A. Ostrovsky and his disciples in the 1970—1980s. Theoretical and experimental studies on this topic are substantially due to a rapid development of the physics of nonlinear wave phenomena as a whole, and the formation of nonlinear acoustics was parallel with the development of nonlinear optics and nonlinear plasma physics at the IAP RAS. The "classical" subject of the study performed at that time was the nonlinear effects in liquids with gas bubbles, which are a typical example of a structurally inhomogeneous medium containing inhomogeneities (air bubbles) with acoustic properties strongly different from those of the mediummatrix (liquid). As distinct from a weak nonlinearity of homogeneous liquids, caused by the intermolecular interaction, such media have a strong acoustic nonlinearity due to a strong compressibility of bubbles. Observation of the nonlinear effects caused by bubbles can be effectively used for their diagnostics even at very low concentrations of gas, which already indicates the potential of improving the sensitivity of nonlinearly acoustic diagnostics of media with "soft" inhomogeneities of the structure.
Since the end of the 1980s and especially from the beginning of the 2000s, great attention was paid to studying the microstructureinduced nonlinear effects in solids, e.g., in rocks, polycrystalline metals, construction materials, etc. These materials having typical structural peculiarities, such as microcracks, dislocations, and intergranular boundaries and contacts, also acquire distinct nonlinear acoustic properties. These properties are essentially different from the atomic nonlinearity of monocrystals and homogeneous amorphous bodies both in values of the effects and in character of their manifestations in specific media and materials. Besides the general physical interest, an investigation of such nonlinear effects is challenging due to the radically new possibilities of diagnostics for some important applications, e.g., in material science and nondestructive testing.
A series of laboratory and field experiments was carried out to explore numerous nonlinear acoustic effects in various (natural and artificial) microinhomogeneous media, such as the generation of difference frequency and higher harmonics, the nonlinear limitation of amplitude and selfinduced transparency, the selfdemodulation and nonlinear pulse delay, the amplitude and phase selfaction, the amplitude crossmodulation of two interacting waves, the nonlinear shift of the resonance frequency, the slow dynamics of acoustically activated samples, and the manifestations of dilatatancy, polarization anisotropy, and dispersion of nonlinearity.
One of the recent results in this field is associated with nonlinear geophysical acoustics. The use of highpower coherent sources allowed one for the first time to find the nonlinear effects of Raman scattering and selfaction of seismic waves under natural conditions, which indicates the prospects for developing the nonlinear methods for diagnostics and monitoring of the rock state. The nonlinear approach to diagnostics of loose granular media also opens up new possibilities for monitoring of avalanche instabilities in such media. To clarify the nonlinear effect of seismic noise modulation by tidal deformations of the Earth's crust (discovered over 30 years ago), a physical mechanism substantiating the empirical use of some peculiarities of this effect as a sign of approaching strong earthquakes was proposed for the first
time in cooperation with the Kamchatka Branch of the Geophysical Service of RAS.

