Bound on induced gravitational wave background from primordial black
  holes
  The today's energy density of the induced (second order) gravitational wave
background in the frequency region $\sim 10^{-3} - 10^3$ Hz is constrained
using the existing limits on primordial black hole production in the early
Universe. It is shown, in particular, that at frequencies near $\sim 40$ Hz
(which is the region explored by LIGO detector), the value of the induced part
of $\Omega_{GW}$ cannot exceed $(1-3)\times 10^{-7}$. The spread of values of
the bound is caused by the uncertainty in parameters of the gravitational
collapse of black holes.
