In an FDN, such as the one shown in Fig.3.10, delays
appear
in long delay-line chains
. Therefore, the filter needed at
the output (or input) of the
th delay line is
(replace
by
in Fig.3.10).4.16 However, because
is so close to
in magnitude, and because it varies so weakly
across the frequency axis, we can design a much lower-order filter
that provides the desired attenuation
versus frequency to within psychoacoustic resolution. In fact,
perfectly nice reverberators can be designed in which
is
merely first order for each
[317,218].