The book Acoustic Absorbers and Diffusers by Cox and D'Antonio describe frequency domain models of surface scattering for use with stage canopies, reflectors and room acoustic diffusers that exploit the Kirchhoff boundary conditions. In 2015, a JASA Express letter was published that develops new time domain equivalents that can more rapidly produce predictions of impulse responses (http://dx.doi.org/10.1121/1.4921675). The code are examples of both types of models for plane, curved and QRDs. These are wave based predictions that are quicker than BEM models but of course more approximate.
Cox, T. J. (2015). Fast time domain modeling of surface scattering from reflectors and diffusers. The Journal of the Acoustical Society of America, 137(6), EL483-EL489.
I have added greens.m that was needed for the frequency domain models