  The boundary integral method is an efficient approach for solving
time-harmonic obstacle scattering problems by a bounded scatterer. This paper
presents the directional preconditioner for the iterative solution of linear
systems of the boundary integral method. This new preconditioner builds a
data-sparse approximation of the integral operator, transforms it into a sparse
linear system, and computes an approximate inverse with efficient sparse and
hierarchical linear algebra algorithms. This preconditioner is efficient and
results in small and almost frequency-independent iteration counts when
combined with standard iterative solvers. Numerical results are provided to
demonstrate the effectiveness of the new preconditioner.
