  We introduce a new additive sweeping preconditioner for the Helmholtz
equation based on the perfect matched layer (PML). This method divides the
domain of interest into thin layers and proposes a new transmission condition
between the subdomains where the emphasis is on the boundary values of the
intermediate waves. This approach can be viewed as an effective approximation
of an additive decomposition of the solution operator. When combined with the
standard GMRES solver, the iteration number is essentially independent of the
frequency. Several numerical examples are tested to show the efficiency of this
new approach.
