Yeah, I know limp mass works well as stated - however, for purposes of sound proofing, you need two, not one or three, centers of mass that are HERMETICALLY SEALED.
Material definitions as I'm aware of them, so we know if we're all on the same page -
15#, 30# roofing felt - asphalt-impregnated paper, in two different weights, light and heavy - normally used as a vapor barrier in roofs, placed over plywood or waferwood sheathing before the final roofing material is attached.
90# roll roofing - basically one long, approximately 3 foot wide, asphalt shingle, rolled up for shipping/handling - available with (and I believe without) wear enhancing gritty stuff. This and the lighter felts are labeled by their weight in 100 square feet, or as roofers call it, 1 square. Roll roofing is normally used in budget roof construction - it looks cheap, but goes on faster and easier than separate 3-tab asphalt shingles.
Plywood - hopefully self-explanatory. Lighter and less effective sound blocking per unit thickness than several cheaper alternatives, among them -
Particle board, a sheet of wood-based material made of sawdust and glue, mixed into a slurry and run through a multi-ton roller press at high temperatures to form a dense but structurally weak sheet - typically 4x8 feet, in thicknesses from 1/4" up to at least 1" - most reasonably priced particle board is made with aldehyde binders, and tends to outgas for long periods of time making it not a good choice for inside layers in a building. There are reports linking aldehydes to cancer. It can be used as an intermediate, dissimilar layer in sound walls but should be sealed with either special particle board sealer or two coats of urethane varnish. Not usable outside, even with primer and paint. Swells up with moisture and crumbles.
Wafer wood - made similar to particle board, but I don't believe they use the same types of glue - I've not heard of any outgassing problems (doens't mean there are none) Waferwood has larger "chunks" of wood used in it, and looks like someone took a bunch of wood scraps and ran them through the same big press (they did) to make a flat sheet. Used as sheathing on most new houses for both walls and roof. Not as dense, nor quite as good for sound attenuation, as particle board. Can be used for outer layers if primed and painted, or wrapped with vapor barrier before siding is attached.
MDF, or Medium Density Fiberboard - like particleboard, but denser, more expensive, and I'm not sure about the glue/binders used. Really good for sound proofing layer. Also used for the majority of speaker boxes, unless they are made with portability i mind, then they're usually made with plywood for more strength, less weight. Not good for outer layers, even when primed and painted.
OSB, or Oriented Strand Board - kind of a cross between particle board and Wafer wood - larger chunks (oriented, so it has sort of a grain structure) than particle board, but smaller and more directional than Wafer wood. About the same density as Wafer wood, maybe slightly more? Can be used as outer layer if primed and painted - not very attractive though, best used out of sight.
Sheet rock - compressed gypsum contained between two paper layers - some types are fire retardant (called fire code) for use between garages or other sources of possible fire, and living/sleeping quarters. The thicker the layer, the longer the fire rating. Low cost, excellent sound proofing material with limited structural strength and can't be used as an outer layer because it will soak up water and crumble. There are versions made for relatively wet environments, such as behind tile in a bathroom, but they are more expensive and still have little structural strength compared to plywood, etc.
Durock, or Wonder Board - used behind tile in tub areas mostly - made of cement with fiberglas matting on both sides to hold it together, it is heavy, dense, expensive, and probably really good for sound proofing because of all those qualities (except the expensive part) comes usually in 3x5 sheets (@%$@#$@???) which would make it a pain in the ass to fit to most stud spacings, but just about perfect for a 5 foot bath tub...
There are probably several materials I left out, but those are the most commonly available with any useful sound proofing application that someone other than Bill Gates can afford.
Quiet Zone between floor joists or wall studs should work fine - if I remember correctly, it's about the same mass as rockwool, maybe a little less.
As to the idea of "adding another wall" if necessary, you'd be better off using better isolation in the first place - you REALLY need to understand how little help a third center of mass, with it's additional air space, helps sound proofing. You're much better off putting another layer of mass in one of the two main leaves of the wall instead. There is documented PROOF that the PLACEMENT of materials can make up to 23 dB of difference in wall performance. That's the same as moving the noise source FOUR TIMES as far away!
I just ran a quick calculation of material costs using Waferwood outside, sheet rock inside, including trusses, roofing, double 5/8 sheet rock inside, with the floor as I drew it - not including siding, doors, windows or acoustic treatment you're under $4k by a small amount. Adding staggered studs and RC for a SERIOUS improvement in wall performance would only add about $400 to that cost. If you don't want to be disappointed in the result, I really wouldn't recommend trying to save money on the basics because you can NEVER get it back by add-ons later. I can't even count the number of people who tried to save a little, and ended up either spending LOTS MORE for very little improvement, or else RIPPING it all out and starting over.
I'm only trying to save you some grief here, but in the end it's your choice. I hope you make the right one for you... Steve
