0 – 0% stereo width in the reverb effect (mono)
0.5 – 50% stereo width in the reverb effect
1 – 100% stereo width in the reverb effect

I don't think this is actually intended here. AFAIK, the range 0-100 exists for historical reasons, because it was the default range of the corresponding shell command. And I just applied that range when turning it into a true setting in #49. In the reverb's code there's the following comment:

There is also a comment about values >1:

I think whether or not a "ton of feedback" is induced also depends on the soundfont. E.g. take this tune, which I just added to the testsuite: MIDI + SF2 - even with width=100 it still sounds quite fine to me.

Yet I agree about the docs. #1460 elaborates on the synth.reverb.width setting, pls. have a look.

PS: Happy new year :)

The update to the documentation states "A value of 100 indicates maximum separation between left and right channels.", but I would argue the maximum separation is already achieved at 1. In fact, FluidSynth's reverb width between 0 and 1 behaves exactly like other reverb engines I've used when setting between 0% and 100% effect stereo width. Going above 1 doesn't really seem to increase the stereo width, at least as I can perceive it, but instead introduces the other attributes we've already discussed (louder, almost a "doubling" type attribute to the sound).

Looks like we need a third opinion. @jjceresa Happy new year to you! Since you've implemented the current FDN reverbator, what's your opinion on the meaning of the "reverb width" range?

First of all happy new year to every one. Thanks for the work done in fluidsynth !
A few points of note related to reverb width meaning.

1. The stereo unit comes directly from the previous reverb (freeverb) with width in the historical range [1..100].
   outR = 0.5(1 + width) inR + 0.5 (1 – width) inL
   outL = 0.5(1 + width) inL + 0.5 (1 – width) inR

2. This unit receives at its input inR and inL which are very uncorrelated signals (i.e out of phase) resulting from FDN reverb.

I would argue the maximum separation is already achieved at 1.

That is right, and the formula in point (1) shows that. So width sub range [0..1] meaning should be perceived as a stereo width effect with 1 being the maximum stereo width.

Going above 1 doesn't really seem to increase the stereo width, at least as I can perceive it.

Yes, it increases the perception of uncorrelated signals inR, inL which aren't stereo channels because the reverb is a monophonic reverb. So width subrange ]1..100] meaning produces other resonnant effect.

As Tom , I noticed that subrange ]1..100] effect is input music material dependant. Sometimes it sounds fine, in other situation it sounds muddy.

It turns out that width has two meaning. As Chris said for documentation clarity, perhaps subrange [0..1] should be the only one prefered for stereo width meaning.