  We consider interior penalty discontinuous Galerkin discretizations of
time-harmonic wave propagation problems modeled by the Helmholtz equation, and
derive novel a priori and a posteriori estimates. Our analysis classically
relies on duality arguments of Aubin-Nitsche type, and its originality is that
it applies under minimal regularity assumptions. The estimates we obtain
directly generalize known results for conforming discretizations, namely that
the discrete solution is optimal in a suitable energy norm and that the error
can be explicitly controlled by a posteriori estimators, provided the mesh is
sufficiently fine.
