What insulation/mass infills to use?

Given that the object of the exercise is to build mass into a two leave system (MSM); I'm assuming that the densest and heaviest of these would be the best to use: http://guide.rockwool.co.uk/products/bu ... slabs.aspx However, in the UK most folk go for either RWA45 or RW3. Is there a reason why RW6 isn't or shouldn't be used?

Given that the object of the exercise is to build mass into a two leave system (MSM);

Well, yes, but that's the purpose of the MASS in the system, not the purpose of the SPRING! The mass on each side is supposed to be massive (duh!), the spring isn't: it is supposed to be a damped spring.

I'm assuming that the densest and heaviest of these would be the best to use:

No. The right one to use is the one that has the optimal gas flow resistance for an MSM spring damper. The purpose of insulation in an MSM wall cavity is not to add mass, but to dampen the spring. The air in there is the spring, and the insulation is the damper. It acts sort of like the shock absorber on your car springs: the springs provide the "bounce" to tune the system, and the damper absorbs the energy of the "bounces", stopping oscillation or resonance. So your insulation needs to have the right characteristics for damping resonance in an MSM wall, just like your car shocks need to have the right specs for your car. Just like you cannot use shocks from a 747 or a moped in your car, so you also cannot successfully use the wrong kind of damper (insulation) in your wall. It turns out that the characteristic that determines "how much damping" you need, is called "gas flow resistance" (or perhaps more technically correct "gas flow impedance"). Unfortunately, most manufacturers of insulation do not publish the gas flow resistance figures for their products. But they do publish the density, and fortunately there is a rough correlation between the density of most types of common insulation and their gas flow resistance (although the relationship isn't very linear). So as a general rule the density of a certain type of insulation tells you something about its gas flow resistance characteristics. It also turns out that the best gas flow resistance for MSM walls is found in mineral wool that has a density of roughly 48 kg/m3, or ordinary "pink fluffy" fiberglass insulation that has a density of roughly 32 kg/m3. Of course, those aren't carved in stone, and it probably differs from manufacturer to manufacturer, but as a general guideline, that's roughly what you should be looking for. So to answer your question: No, more mass in the insulation is not a good thing for the wall. nether is less mass. You don't want more mass, and you don't want less; you want the correct density, which should give you the correct gas flow resistance. In fact, using stuff with very high density could be counterproductive, as it could even cause flanking across the wall cavity, which would be very sad... Short answer? Go with either RWA45 or RW3. Both are fine, but the RWA45 is probably better. - Stuart -

Hmmm.....seems I have a lot to learn. :oops: I was thinking that the spring was the air sandwiched between the two leaves and caused by the air and the Residual Bar. So I guess what you are saying is that using say, RW6 is like taking all the "action" out of a shock absorber. IOW's, there is no shock absorber!

I was thinking that the spring was the air sandwiched between the two leaves and caused by the air and the Residual Bar.

The air is the spring, yes. And the insulation is the damper on the spring. But the Resilient Bar is there to decouple, not to be a spring (although just to make things even more confusing, the way it decouples is kind of like a spring...). The air itself is plenty springy, as far as sound waves are concerned.

So I guess what you are saying is that using say, RW6 is like taking all the "action" out of a shock absorber. IOW's, there is no shock absorber!

Basically, yes. It will still work, because it does still have usable gas flow resistance, but it isn't optimal. "Optimal" is important. :) - Stuart -

And I'm guessing that the stiffer the materiel, it affects the high and low frequencies differently with the stiffer material reflecting the lows instead of absorbing?

The highs are typically taken care of with treatments, the lows will either, depending on room size and mass, go right through the wall or buildup in the room. Bigger rooms with rigid walls let longer low frequency waves breath...smaller rooms with rigid walls accumulate low frequency in the pressure points, the corners. Rigidity with mass is a good thing, rigidity alone, is not a goal. The higher the mass the higher the expected rigidity....but everything matters.