Just to refine a point or two. We have the following elements to help us isolate sound:
Decoupling- Staggered studs, double studs, resilient clips&channel, and resilient channel are all common methods to decouple walls and ceilings.
Absorption- Some material in a sealed air cavity (in our case) that increases the resistance to sound wave movement, thereby robbing the waves of energy. A very large volume of air can accomplish this, but our walls are limited in depth. We add a low-medium density insulation to increase the resistance without introducing conduction. This is a thermodynamic process, whereby the introduction of open-fiber insulation essentially mimics a system with a much bigger air cavity.
Mass- This is primarily the job of materials like drywall and plywood. Could be masonry if wer're looking at exterior walls. Wall / ceiling framing and insulation technically contribute to the overall mass of the partition as well as the resonance and TL calculations, however we don't look to increase the mass of either the framing or the insulation. This is a key point.
Damping- The panels used to build partitions are very resonant. This resonance facilitates the propogation (re-creation) of the sound wave. Effectively damping the panels increases TL dramatically.
So in general, as previously posted, mass is king, but it's what we do with the mass that allows us to exceed pure Mass Law calculations. By decoupling the massive leaves, introducing a larger air cavity and adding prudent insulation, we exceed Mass Law calculated performance in the mid and upper frequencies. Again, the answer is not simply mass.
The value of insulation isn't its contribution to the mass of the assembly. Lightweight insulation isn't contributing anything meaningful to the mass of the system, rather it offers absorption within the sealed air cavity. This is why low / medium density insulation performs better in the more problematic low frequencies. Refer to the NRC data as pointed out already. Massive / dense insulation is not what we want at all.
The value of framing isn't its contribution to the mass of the assembly. Indeed, less massive steel studs perform better than wood in a coupled assembly by allowing the panels to flex more. Similarly, 24" OC studs perform better than 16", which are better than 12". Obviously having studs placed 12" OC would yield the heaviest wall, yet the worst performance. Again, it's not all about the mass.
Another example discussed here:
The Significance of Air Cavity Depth & the Triple Leaf Effect | Soundproofing Company demonstrates that adding a massive leaf in the middle of a decoupled system air cavity increases the mass of the partition significantly, yet TL performance essentially does not increase.
To Sum: Mass is a critical component of any sound isolating system, however it is the arreangement and deployment of the mass that yields increased results. Stick to a low / medium density insulation, typically exactly what you would use for thermal insulation (Home Depot). THe data says this is best.
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