Decoupled Floor and other studio building questions

[Soundman2020]

since the whole air gap gets filled with fibre, we cant really talk about a mass per wall anymore, or not the same way i imagined. any insulations thats between the studs actually filling where the 'air gap' should have been. can we divide that insulations amss and add half to each wall mass??

No, because it isn't mass like in the leaves, and it isn't that massive anyway. Leaves are leaves, and insulation is insulation. They serve two entirely different purposes acoustically. Insulation does not act like solid mass at all. Drywall has a density of nearly 700 kg/m3, fiberglass insulation has a density equivalent to just 4% of that (28 kg/m3). Drywall is rigid, solid, air-tight. Insulation is soft, fluffy, porous and full of air. The tiny amount of mass that the insulation adds to that is negligible. The material itself is nothing at all like drywall, MDF, OSB or plywood. All of those act like either membranes or pistons when a sound wave strikes them, and energy is transferred from the sound wave into motion of the entire leaf. When a sound wave strikes insulation, it goes right through and comes out the other side, suffering a bit of attenuation along the way, but it does not cause the entire piece of insulation to vibrate as a whole unit.

It's just not the same in so many ways. It is not necessary to try to account for the mass of the insulation in you MSM calculations.

could i add more mass and air gap only to the wall facing the other studio wall, and that would help situation? or it wouldnt make sense to have 15Hz on that wall and 18Hz on rest??

It doesn't work that way either!

As I mentioned way back some place, any time you have massive leaves with an air gap between them, you have a resonant system. Two leaves plus one air gap is a two-leaf MSM system, so you use the equations for a 2-leaf system. If there's another leaf and another air gap, then you don't have a 2-leaf MSM system any more: you have a 3-leaf MSMSM system, so you need to use the equations for 3-leaf systems, not 2-leaf systems. It's the bottom equation here:

double-triple-leaf-equations.jpg

There is also a second fundamental resonance for the triple-leaf, and it will be at:

f- = f+ / ( SQRT(2) )

That assumes that the following two conditions are both true:

m1 = m3 = 1/2 m2
d1 = d2


In other words, with a three-leaf system you have two resonant frequencies.


- Stuart -

[Johan Hugo]

amazing thanks...

so are the studs in the walls in each leaf calculated into mass?? theres a lot of mass in all the wood in each leaf which we have use in calculation too..

J

[Soundman2020]

so are the studs in the walls in each leaf calculated into mass?? theres a lot of mass in all the wood in each leaf which we have use in calculation too..

There are equations that take the studs into account as well, but they don't actually change the outcome very much.

Yes, there is a lot of mass in the studs, but it's not in the right place. The thing is, you can't just add the mass of the studs to the mass of the drywall, because the "stud mass" only occurs at a few points along the wall, so it only increases the mass for a small percentage of the total wall area. Isolation is only as good as the weakest part, not the strongest part, so even though things are doing great at the places where the studs are, there's no increase for the rest of the wall, where the studs aren't. Most of the wall is just drywall area, and that's what sets the overall isolation.

The studs do have an affect in other ways, such as increasing the stiffness of the wall (which reduces low-frequency isolation), and making the air gap thinner at those points, and not allowing for the entire space between leaves to be filled with insulation at those points, and a few other things, but overall they don't have a huge effect on isolation. There are equations for dealing with all of that, but it's really not worthwhile taking them into account for the purposes of home studio design.


- Stuart -

[Johan Hugo]

hey stuart, lots of progress on my end but still some questions coming up. if i was to use one layer of MDF (for example 18mm) which is much heavier than the plasterboard, to get the mass of my leafs up more. is there a prefered place for the MDF? like 12mm plasterboard then mdf then 12mm plasterboard. or should the mdf go first (against the studs) and then 2 layers of plasterboard onto of that??? basically, is there a good rule to layering these things??

J

[Johan Hugo]

one more thing..

when we have really gone through maths, dimensions, space, mass etc we seem to find 2 options for our RF and its either 18.2Hz or 19.7Hz. theres a fair amount of cost involved in going from 19.7Hz to 18.2Hz, and i have no idea how much difference it makes in reality. maybe it's something you can't even answer. there seem to be about £1500 difference in price between the 2 results.

basically getting it down to 18.2 requires 1 layer of MDF (18mm) in each leaf, aswell as 2 layers of 12.5mm plasterboard, and a air gap of 35cm (with insulation).

19.7Hz requires 3 layers of 12.5mm plasterboard in each leaf and 30cm air gap (with insulation).

we can't really get any lower within the space we have considering cost. any thoughts on this?

[Soundman2020]

when we have really gone through maths, dimensions, space, mass etc we seem to find 2 options for our RF and its either 18.2Hz or 19.7Hz.

You'd be fine either way. In both cases you'd get pretty decent isolation down to very low frequencies.

or should the mdf go first (against the studs) and then 2 layers of plasterboard onto of that??? basically, is there a good rule to layering these things??

Do the structural layer first, on the studs. That provides sheer strength to the wall, so it needs to go on the studs directly. OSB would be good for that.

- Stuart -