  Our goal is to see how density waves of different scale combine to form voids
between galaxy systems of various scale. We perform numerical simulations of
structure formation in cubes of size 100 and 256 Mpc/h, with resolutions 256^3
and 512^3 particles and cells. To understand the role of density perturbations
of various scale we cut power spectra at scales from 8 to 128 Mpc/h, using in
all series identical initial random realisations. We find that small haloes and
short filaments form all over the simulation box, if perturbations only up to
scale 8 Mpc/h are present. The phenomenon of large multi-scale voids in the
cosmic web requires the presence of an extended spectrum of primordial density
perturbations. The void phenomenon is due to the action of two processes: the
synchronisation of density perturbations of medium and large scales, and the
suppression of galaxy formation in low-density regions by the combined action
of negative sections of medium- and large-scale density perturbations, so that
their densities are less than the mean density, and thus during the evolution
their densities decrease.
