The Room Acoustic Rendering Equation introduced in [1] formalizes a variety of room acoustics modeling algorithms. One key concept in the equation is the Acoustic Bidirectional Reflectance Distribution Function (A-BRDF) which is the term that models sound reflections. In this paper, we present a method to compute analytically the A-BRDF in cases with diffuse reflections parametrized by random variables. As an example, analytical A-BRDFs are obtained for the Vector Based Scattering Model, and are validated against numerical Monte Carlo experiments. The analytical computation of A-BRDFs can be added to a standard acoustic ray tracing engine to obtain valuable data from each ray collision thus reducing significantly the computational cost of generating impulse responses.
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