How deep should my walls be? Air Gap and Lfe performance

[Harkon]

I'll try to include all the details needed to answer this but just after a bit of advice.

I will be soon constructing a new room to use as a Home Cinema/Theatre. Room will be 4m x 3.7m before stud work. I stumbled on this forum, where the knowledge for building such things is much greater than most AV forums. I've also read Rod Gervais's book which has been very useful.

Room will be a part of a new extension. The first wall is the existing exterior wall of the house (30cm/12" thick) and two of the other walls be new external walls (30cm/12" thick).
The final wall will be 100mm concrete block sitting between the home cinema and the kitchen area.

Inside the room I plan to have stud work completely isolated from these walls. My question is about the depth of the stud work used.

My original plan was to use 2x4 studs, so 100mm of insulation, and about 2cm gap from stud work to the brick/concrete walls.

If I use smaller stud work framing (wood or steel) but still use the same 2 layers of plasterboard (same mass), how far does this reduce the effectiveness of the wall?

Has anyone produced data showing the STC rating for a 50mm, 75mm and 100mm wall cavity?
I'd like to make an informed choice on what size timbers to spec, considering sound proofing vs maximising room dimensions.

[John Steel]

Hello & howdo' Harkon?
If I was building a project like yours, I would start by measuring the the typical SPL of the equipment (home cinema systems can get very loud as I'm sure you know) with a good quality sound level meter. It's only when you know how much noise you want to stop getting out (and getting into) your insulated room that you can figure out the amount of mass needed to stop it. There are huge cost implications here as achieving transmission loss is partially dependant on the amount of mass required.

wood or steel

If you're self building, I would definitely make your studs out of wood as it's easier to work with.

Has anyone produced data showing the STC rating for a 50mm, 75mm and 100mm wall cavity?

Not that I'm aware of. I wouldn't get too wedded to STC based comparisons myself as it's a poor measurement for applications like this (really intended for the building industry, it only includes a limited range of frequencies). Have you seen Gregwor's transmission loss calculator?http://johnlsayers.com/phpBB2/viewtopic.php?f=1&t=21770 It's a great tool for estimating mass / materials used vs TL. Looking forward to hearing what you decide - good luck and best wishes, John.

[Harkon]

Many thanks John, I'll take a look.