The model that underlies the BERIM method models the acoustic field in a cavity with one opening. A recent paper describing the method in more detail and demonstrating its application to the acoustic field produced by a horn loudspeaker is given in the following link.
The Fortran (77) subroutine BERIM3 implements the Boundary Element - Rayleigh Integral for general 3D problems: BERIM3 delivers a computational solution to the acoustic field produced by an open cavity. The BERIM3 package include the Fortran source code and a manual.