We present a new approach to efficiently compute effects of sound occlusion and diffraction by obstacles in general virtual acoustic environments. Based on the Fresnel-Kirchoff approximation to diffraction, our method uses a 3D model of the environment and computer graphics hardware to evaluate diffraction effects between a sound source and a receiver position. No further hypothesis is needed on the diffracting domain. This method allows us to compute attenuation data and filters which can then be used for auralization. The method can also be extended to indirect sound propagation paths.