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Just curious, Armaegis. Do you think that vibration damping the cup is ineffective? (From my own experience, I found that plasticine in the baffles and getting a good seal with the pads made the most significant contribution to my sound quality.)
I would say inefficient... dynamat is not really designed for absorbing pressure waves. It it meant for absorbing mechancial vibrations, which will thusly reduce the sound emanating from those vibrating surfaces. It primarily does so by converting vibrational energy into heat via shear (sort of like sliding) of the goop between the two layers (which is why you have to leave the foil there), but it also does so via simple mass loading since the heavier a body is, the less it wants to vibrate.
The thing is, absorbing the vibration in the cup doesn't stop the driver/baffle from generating those vibrations, which is where you really need it if you want to get the best driver response (especially in the bass, due to inertia).
You want the driver and connected baffle plate to be as stable as possible, so that all the energy goes purely into producing pressure waves instead of vibrating the driver assembly. Since bolting the driver directly to your skull is not an option most people will choose, nor is a strong mechanical support (ie: clamp like a mofo), your best bet to ensure driver stability is to mass load it. The heavier the assembly is relative to the diaphragm/cone/whatever, the better the driver response will be. This is basically physics and conservation of momentum, simplified of course as we are neglecting the elastic and damping properties (both electrical and physical) of the driver.
Now once you've minimized the resultant vibrations, you still want to reduce the remaining ones from going any further or reflecting around the structure (and we're still talking purely mechanical vibrations, nothing acoustic yet). This is where the vibration dampers like dynamat come into play. Ideally in this case, you would put them near the point of incident or at the point of maximum flexure. Since vibration dampers are also generally heavy, they can also serve as mass loaders.
Now in terms of acoustic damping, you've got the backwave from the driver, and the secondary wave emanating from surfaces due to vibration of the enclosure. There's also vibration generated in the enclosure due to backwave from the driver hitting the cup, which could conceivably go all the way back down and affect driver response, let's call this tertiary wave for lack of a better term. The backwave should be controlled from your cup filler and felt/foam/magicpixiedust lining in the rear cup. The secondary wave should be minimized in the first place from the mass loading and vibration damping of the driver/baffle. Tertiary wave should more or less be negligible assuming you did everything else right.
For acoustic damping, all materials have their own reflective and absorptive indexes. In general, softer and more porous materials have better absorption, and harder and more dense materals reflect more. Dispersion is sort of an in between factor. For general damping, you want pressure waves to pass through progressively denser materials for maximum absorption efficiency. No need to go crazy though; less is more oftentimes. Shapes and specific arrangements shouldn't do too much; it's more the overall percentage and/or combined absorptive/reflective value that matter. The notable exception here is damping that goes between the driver and your ears, as that specifically tunes the sound reaching your ears. Arrangements and shapes here can make a difference as it affects the sound waves that touche different parts of your ear.
Then there's the specific layering directly behind the driver itself. This is impulse control, as it's sort of like immediate backwave reflection that affects driver response. I assume (optimistically) that the manufacturers have already figured this part out in the design of the driver, so I generally don't mess with it. If I ever put anything behind it, it's light and porous so I don't affect the driver too much. Either that or I'm replacing the default filter because I accidentally removed it.
Jebus I'm rambling now. Anyhow, here's the tl;dr version.
1) mass load the driver assembly - make it heavy, add vibration dampeners nearby if possible
2) acoustically damp the chamber by removing hard reflective surfaces and/or putting some filler in; 100% coverage isn't really necessary; minimally just the area behind the driver would be ideal
All of this is so that you hear just the driver response as cleanly as possible. Keep it simple, keep it clean.
/soapbox
