What are the mechanics behind the paper filter around drivers?

You all know what I mean here, the white filter paper headphones have around the drivers. Most notably beyerdynamic where the entire baffle assembly is covered. From my experiments with fostex t50rp drivers, and studying the physics behind headphones, it would seem that these are mostly dust covers to allow the backwave from the cup to enter the cup and interact with the direct sound from the driver.

It would make sense, then that the dt880/990 has a ring of felt in the back of the cup surrounding the vent hole, dampening high end. This would prevent the backwave from containing high end information that would likely cause comb filtering and harshness.

I wonder what you could achieve by messing with the filter paper? Add more filtering over it, cover it entirely, etc. I intend to try this out and see what the phase relationship ends up being in different headphones. This could be done with simple math of course, by simply measuring the distance from membrane to cup and back, but actually geting an idea of what full frequency spectrums do in such a setting is harder.

Some thoughts: what could you achieve with diffusion in the cup? Can you raise the reflective floor of the cup by filling it in with a donut shaped ring, and change the phase relationship of the backwave to the direct signal?

I also wonder what is canceled out vs what is reinforced with the different cup depths when using the backwave.

Does anyone have thoughts and experiences with this?