  Feedback delay networks (FDNs) are recursive filters, which are widely used
for artificial reverberation and decorrelation. One central challenge in the
design of FDNs is the generation of sufficient echo density in the impulse
response without compromising the computational efficiency. In a previous
contribution, we have demonstrated that the echo density of an FDN can be
increased by introducing so-called delay feedback matrices where each matrix
entry is a scalar gain and a delay. In this contribution, we generalize the
feedback matrix to arbitrary lossless filter feedback matrices (FFMs). As a
special case, we propose the velvet feedback matrix, which can create dense
impulse responses at a minimal computational cost. Further, FFMs can be used to
emulate the scattering effects of non-specular reflections. We demonstrate the
effectiveness of FFMs in terms of echo density and modal distribution.
