  We analyze the Helmholtz equation in a complex domain. A sound absorbing
structure at a part of the boundary is modelled by a periodic geometry with
periodicity $\varepsilon>0$. A resonator volume of thickness $\varepsilon$ is
connected with thin channels (opening $\varepsilon^3$) with the main part of
the macroscopic domain. For this problem with three different scales we analyze
solutions in the limit $\varepsilon\to 0$ and find that the effective system
can describe sound absorption.
