de-coupling the two sides of a normal double-leaf (one air cavity) wall is probably the most familiar means of improving sound isolation to many, and the results can be over-whelming.
Resilient channel functions by de-coupling, as do sound clips, and room-within-a-room constructions. the ultimate level of decoupling would be a room floating on springs inside another room.
used in combination with absorbing materials (insulation in the cavity), de-coupling has a great effect. The sound can't go readily through the studs to the other side, so it has to go through the air/insulation, and much of it is absorbed/destroyed/converted to heat. like this
at higher frequencies, de-coupling can be wildly effective at improving sound isolation. the use of a clip (or spring hanger), room within a room, or properly installed channel can yield a greater improvement at many frequencies than almost any feasible amount of added mass.
Also, when de-coupled construction is utilized, the gains from adding mass become far greater than just throwing mass on a normal wall and calling it good. more on that in a minute.
the limitation of decoupling is the air in the cavity. Air acts like a spring, (as do other things, more on that later). At the resonance point the performance of the de-coupled design will fall below that of a single panel of same mass.
so, at the resonance point (and around it), performance will be worse for our trouble - less than coupled, and below it we won't gain anything - below the resonance point the air will simply couple the two sides.
as a general rule, it's a good idea to try to drive the resonance point as low as possible, for as we drive it down in frequency, we push it to frequencies that are less disturbing, and more importantly we drive the frequency where the wall de-couples and isolation goes up dramatically down.
to push the resonance frequency down these basic rules are helpful
1. use as much mass on both sides as you can
2. use as deep an air cavity as you can
3. use insulation in the air cavity
but, unless you have a very deep air cavity, and very heavy walls, you won't realistically be able to de-couple the sub-woofer region. for example, Terry Montlick comments on this general topic in this threadhttp://www.avsforum.com/avs-vb/showt...&highlight=DMF
ok, this is a bit of fun, and i'll try to come back to finish later this week.
take care all,