  We deal with the time-harmonic acoustic waves scattered by a large number of
small holes, of maximal radius $a, a<<1$, arbitrary (i.e. not necessarily
periodically) distributed in a bounded part of a homogeneous background.
  We show that as their number $M$ grows following the law $M:=M(a):=O(a^{-s}),
\; a<<1$, the collection of these holes has one of the following behaviors:
  1. if $s<1$, then the scattered fields tend to vanish as $a$ tends to zero,
i.e. the cluster is a soft one.
  2. if $s=1$, then the cluster behaves as an equivalent medium modeled by a
refraction index, supported in a given bounded domain $\Omega$, which is
described by certain geometry properties of the holes and their local
distribution. The cluster is a moderate (or intermediate) one.
  3. if $s>1$, then the cluster behaves as a totally reflecting extended body,
modeled by a bounded and smooth domain $\Omega$, i.e. the incident waves are
totally reflected by the surface of this extended body. The cluster is a rigid
one. These approximations are provided with explicit error estimates in terms
of $a,\; a<<1$.
