  In this paper we discuss the convergence of state-of-the-art optimized
Schwarz transmission conditions for Helmholtz problems defined on closed
domains (i.e. setups which do not exhibit an outgoing wave condition), as
commonly encountered when modeling cavities. In particular, the impact of
back-propagating waves on the Dirichlet-to-Neumann map is analyzed. Afterwards,
the performance of the well-established optimized 0th-order, evanescent modes
damping, optimized 2nd-order and Pad\'e-localized square-root transmission
conditions is discussed.
