A basic 2D version of the Beam Tracing algorithm presented in:

Thomas Funkhouser, Ingrid Carlbom, Gary Elko, Gopal Pingali, Mohan Sondhi, and Jim West. 1998. A beam tracing approach to acoustic modeling for interactive virtual environments. In Proceedings of the 25th annual conference on Computer graphics and interactive techniques (SIGGRAPH '98). ACM, New York, NY, USA, 21-32. DOI=http://dx.doi.org/10.1145/280814.280818

The environment is represented as a set of lines. Red lines represent walls and green lines represent doors. For each test case the ray emanate from the same source (represented as a white dot) and the listener is somewhere inside the room R. The maximum number of reflection/transmission allowed is showed in the upper left part of each image.