  We present a FEM-BEM coupling strategy for time-harmonic acoustic scattering
in media with variable sound speed. The coupling is realized with the aid of a
mortar variable that is an impedance trace on the coupling boundary. The
resulting sesquilinear form is shown to satisfy a Garding inequality.
Quasi-optimal convergence is shown for sufficiently fine meshes. Numerical
examples confirm the theoretical convergence results.
