### Lowpass-Feedback Comb Filter

Inspection of `comb.h` in the Freeverb source shows that
Freeverb's ``comb'' filter is more specifically a
*lowpass-feedback-comb filter*
(LBCF^{4.11}--§2.6.5). It is constructed
using a delay line whose output is lowpass-filtered and summed with
the delay-line's input. The particular lowpass used in Freeverb is a
*unity-gain one-pole lowpass* having the transfer function

In Freeverb's `comb` section (`comb.h` and
`comb.cpp`), the ``damping'' is set initially to

`feedback`in the source, and it is set initially to

Increasing the `roomsize` parameter (typically brought out to a
GUI slider) increases and hence the reverberation time. Since
is required for dc stability, the `roomsize` must be less
than 1.0714, and so the GUI slider max is typically 1 ().

The `feedback` variable mainly determines reverberation
time at low-frequencies at which the feedback lowpass has negligible
effect. The feedback lowpass causes the reverberation time to
decrease with frequency, which is natural. At very high
frequencies--those for which the lowpass gain times is much less
than 0.5--the reverberation time becomes dominated by the diffusion
allpass filters (which have a fixed feedback coefficient of ).
Thus, in Freeverb, the ``room size'' parameter can be interpreted as
setting the low-frequency T60 (time to decay 60 dB), while the
``damping'' parameter controls how rapidly T60 shortens as a function
of increasing frequency. A lower-limit on T60 is given by the four
diffusion allpass filters.

In terms of the physical interpretation of the filtered-feedback
comb-filter discussed in §2.6.5, Freeverb's
`roomsize` parameter can be interpreted as the square-root of the
low-frequency reflection-coefficient of each wall. That is, when a
planewave bounces back and forth between two walls, the attenuation
coefficient is `roomsize` after one round trip (two wall
reflections). Therefore, a better name in this interpretation would be
`liveness` or `reflectivity`. Since the round-trip delay
is given in samples by the delay-line length, changing the roomsize
requires changing the delay-line lengths in this interpretation.

**Next Section:**

Freeverb Allpass Approximation

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Freeverb Main Loop