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My last paragraph was referring to the diff size parameter.  Normally room size affects early reflections, and sometimes it is also wired into the diffusion network, so there is one control that controls both. Sometimes there are seperate controls for them.

Diffusion is more to do with the surface, and how frequencies roll off and reflections propagate. Depending on how this is acheived (and the diffusion block is only one of many mays to acheive this), similar parameters may be called adsorption, and sometimes there is a filter in the feedback path, so there may be a rolloff control.

Size tends to control the space between reflections.  Diffusion tends to control what happens to the “colour” of the reflections.  A large room with reflective surfaces may have a high time, but low diffusion. A small room covered in a highly irregular surface may have a low time but high diffusion.

It’s hard to say how diffusion is used in the particular algorithm. My impression from the documentation (I haven’t used this particular block in any of my algorithms yet) is that diffusion is kind of like a wet/dry for the diffusion network.  There is also a delay amount parameter and a g parameter, which control the delay for each allpass section, and g is the amount of feedback for each of the allpasses.

Your settings above are mostly right, but for a hard surfaced room, your decay will probably be long, as the soundwaves will bounce around for longer.

And as always, use your ears. Sometimes parameters can be counterintuitive when you hear them.  For example for a hard room reverb, you may just use and extend the early reflections set with lots of feedback, and turn off the late diffusion, which will keep things clean and create a better sense of space, but you can also create a sense of space with very little early reflections and a long spacious diffusion.