In Eq.(3.2) we defined the allpass notation by
A look at allpass.h reveals that Freeverb implements
As a result, each of the four Freeverb ``allpass'' sections is really a feedback comb-filter in series with a feedforward comb-filter , where (cf. §2.6)
A true allpass is obtained only for (reciprocal of the ``golden ratio''). The default value used in Freeverb (see revmodel.cpp) is . A detailed discussion of feedforward and feedback comb filters appears in §2.6, and corresponding Schroeder allpass filters are described in §2.8.