  Effective non-Hermitian Hamiltonians are obtained to describe coherent
perfect absorbing and lasing boundary conditions. PT -symmetry of the
Hamiltonians enables to design configurations which perfectly absorb at
multiple frequencies. Broadened and flat perfect absorption is predicted at the
exceptional point of PT -symmetry breaking while, for a particular case,
absorption is enhanced with the use of gain. The aforementioned phenomena are
illustrated for acoustic scattering through Helmholtz resonators revealing how
tailoring the non-Hermiticity of acoustic metamaterials leads to novel
mechanisms for enhanced absorption.
