Место издания: Barcelona , Год издания: 2010

The characteristic size of the structure governs the fact that difference relations do not automatically transform into differential ones. It is impossible to distinguish an infinitesimal volume of a body, to which we could apply the major conservation laws, because the minimum representative volume of the body should contain at least a few elementary microstructures. This leads to motion equations of infinite order. Solutions of such equations include, along with sound waves, perturbations propagating with abnormally low velocities not bounded below. It is shown that in such media weak perturbations can increase or decrease without limit. The dispersion of structure sizes plays a double role. First of all, the intensity of instabilities decreases by dispersion and it therefore stabilizes the medium, whereas frequency diapason of unstable solutions is extending, and catastrophes may be beginning by very small frequencies. The equation of equilibrium is not valid in any point of the medium, this one valid in average only. Hence there is a possibility to have a lot of micro dynamic acts, in spite of static macro state in average. This paper describes some conditions, where some dynamic acts create wave motion of the media in usual sense.