  The scattering and transmission of harmonic acoustic waves at a penetrable
material are commonly modelled by a set of Helmholtz equations. This system of
partial differential equations can be rewritten into boundary integral
equations defined at the surface of the objects and solved with the boundary
element method (BEM). High frequencies or geometrical details require a fine
surface mesh, which increases the number of degrees of freedom in the weak
formulation. Then, matrix compression techniques need to be combined with
iterative linear solvers to limit the computational footprint. Moreover, the
convergence of the iterative linear solvers often depends on the frequency of
the wave field and the objects' characteristic size. Here, the robust PMCHWT
formulation is used to solve the acoustic transmission problem. An operator
preconditioner based on on-surface radiation conditions (OSRC) is designed that
yields frequency-robust convergence characteristics. Computational benchmarks
compare the performance of this novel preconditioned formulation with other
preconditioners and boundary integral formulations. The OSRC preconditioned
PMCHWT formulation effectively simulates large-scale problems of engineering
interest, such as focused ultrasound treatment of osteoid osteoma.
