I know peel-n-seal is popular, but I've not found it in local store here. I've seen the measurements of using standard 30# roofing felt and they are very impressive -20db. I'm thinking this will work well, but wondering if any folks here had tried it.
Also I've got no clue how I'd glue this.
Bracing is important, but the problem is one of energy, not stiffness. You must dissipate the energy into heat.
After much reading and study on the subject I have come to the delusion that a constrained layer and felt approach makes the most sense.
Green glue is the best stuff I can find. It is a special acoustic elastic goo that works magic. I have no idea where you live, but in the US it is available
here.
Green glue never hardens, so building with it can be problematic. You can't make up raw boards, apply green glue, then cut them to size. You must cut each piece to size first.
The best approach would be to build the cabinet from the inside out. Build the cabinet like you normally would with internal bracing. The front baffle should not be constrained, but make it oversized by the thickness of the outer layer plus the thickness layer of the glue (about 1/16"+). I would make the front baffle at least 1.5" thick or more.
Cabinet walls can be two layers .5 to .75" thick each. For big cabinets with monster woofers you may even want to consider thicker yet.
Apply green glue using a caulking gun as per manufacture directions, then apply the outer cabinet layer.
You will need some small wooden shims all along the edges of each cabinet face to secure the outer panel and insure the panel is consistently spaced. Use brad nails to secure the outer panel, but only nail the outside board edges. Keep the center nail-free.
Continue with each side in turn until all five sides are done, nailing only at the edges. You should use wood glue where the outer shell's walls meet each other.
Some purists will try to make the inner cabinet float about the inner cabinet, but I don't think the extra effort is worth the aggravation. While you will be transmitting some vibration from the inner shell to the outer shell, the outer shell is still dampened by the inner elastic layer, so you really should not see much of a loss (if any measurable loos at all).
When the outer shell is completed there should not be any visible openings where you see the constrained layer. Seal any cracks or seams with epoxy or a hard glue.
On the inside walls of the cabinet staple two layers of 30# roofing felt. This step is cheap. Then treat the felt with Acousta-stuff or whatever dampening material you choose.
The system is really two systems in one. The stuffing reduces the internal wavefront and standing waves by absorbing the acoustic wave energy. The constrained layer dissipates the remaining acoustic energy and resonances that are coupled to the cabinet walls and bracing and turns that energy into heat.
When you are done with this you should have an extremely well damped cabinet.
