  We present an unified approach for the study of idealized gravitational
compact objects like wormholes and horizonless stars, here simulated by the
presence of boundary conditions at a deeply inner wall. At classical level,
namely neglecting quantum effects, the presence of the wall leads to
characteristic echoes following the usual ringdown phase, such that it can
discriminate black holes from other horizonless, and probably exotic, compact
objects. With regard to this issue, an analytical though approximated
expression for the complex frequencies of the quasinormal echoes is found and
discussed. At quantum level, we show that static wormholes do not radiate.
