  For 6-DOF (degrees of freedom) interactive virtual acoustic environments
(VAEs), the spatial rendering of diffuse late reverberation in addition to
early (specular) reflections is important. In the interest of computational
efficiency, the acoustic simulation of the late reverberation can be simplified
by using a limited number of spatially distributed virtual reverb sources (VRS)
each radiating incoherent signals. A sufficient number of VRS is needed to
approximate spatially anisotropic late reverberation, e.g., in a room with
inhomogeneous distribution of absorption at the boundaries. Here, a highly
efficient and perceptually plausible method to generate and spatially render
late reverberation is suggested, extending the room acoustics simulator RAZR
[Wendt et al., J. Audio Eng. Soc., 62, 11 (2014)]. The room dimensions and
frequency-dependent absorption coefficients at the wall boundaries are used to
determine the parameters of a physically-based feedback delay network (FDN) to
generate the incoherent VRS signals. The VRS are spatially distributed around
the listener with weighting factors representing the spatially subsampled
distribution of absorption coefficients on the wall boundaries. The minimum
number of VRS required to be perceptually distinguishable from the maximum
(reference) number of 96 VRS was assessed in a listening test conducted with a
spherical loudspeaker array within an anechoic room. For the resulting low
numbers of VRS suited for spatial rendering, optimal physically-based parameter
choices for the FDN are discussed.
