TL;DW: Smoothest saturation or antisaturation, plus highpass.
This one started a lot! The algorithm used here has echoed through many other Airwindows plugins. It’s literally the smoothest saturation you can have in a plugin: the transfer function’s a sine. This is what’s in Channel, too: there are many ways to adapt such a simple mathematical function.
But there’s more! Because Density runs multiple stages, allowing it to bulk up the tone into an overblown, insanely fat and saturated distort-fest. And then you can highpass just the distorted stuff alone, and trim its output gain, and mix it with the unfiltered dry to produce lots of tonal possibilities. And then there’s the spatial positioning factor: saturating stuff this way brings it forward in the mix. You can also isolate midrangey elements and bring them forward using that trick.
And then there’s the negative values: if you UNsaturate, you get a thinned out lean tone and it drops back instead of pushing forward. And you can blend that too.
Density’s one of the better utility plugins. It’s there to reshape tones in myriad ways, mostly having to do with fatness or thinness, also having to do with upfrontness or recedingness. It can also give articulation to sounds that are murky, or simply produce the hugest fattest roaring wall of grunge you ever heard.
This sort of thing is supported by my Patreon, so if this or the many other plugins I’ve produced prove useful to you, please chip in. I’ve got a lot more work to do before my plugin library is ported, and even when that’s done, I’ll still be creating new plugins, with your support :)