John Sayers' Recording Studio Design Forum Archive

Hi, hope I'm in line with all the rules

This is the double wall between my control room and live room.
Live room side... 6" solid concrete block wall, then, - 6" airspace, then, - 2x4 wall with 12mm drywall and one layer of 6mm Fiber Cement Board with fluffy fiberglass touching the drywall side (control room side)

Trying to figure out what thickness of glass to put on each wall.
(laminated is not available in my area)


Can someone please help me find the mass for each wall ?

Wall #1- 4" thick concrete block wall (all holes filled with mortar) and 1" plaster on both sides.. 6" total thickness.

Wall #2- One 12mm layer drywall and One layer 6mm Fiber Cement Board on a 2x4 wood stud wall.

Wall #1- 4" thick concrete block wall (all holes filled with mortar) and 1" plaster on both sides.. 6" total thickness.

The density of concrete is roughly 2400 kg/m3, so 4" (roughly 10cm) is going to have a surface density of around 240kg/m2.
Plaster is a bit lighter, around 1900 kg/m3, so 2" of that (roughly 5cm) would be around 95kg/m2.
So, average surface density of your wall will be around 335 kg/m2

Glass has a density of around 2500kg/m3, so if you really wanted to have the same surface density in your glass, you'd need glass a bit more than 5" thick! But that isn't necessary (unless you are aiming for extreme isolation)....

Wall #2- One 12mm layer drywall and One layer 6mm Fiber Cement Board.

Drywall density is around 685 kg/m3, so 12mm is roughly 8 kg/m2.
Fibercement density is around 1550 kg/m3, so 6mm is roughly 7 kg/m2.
So total average density is around 15 kg/m2.
To get the same surface density in glass, it would have to be 6mm thick.

So in theory, you would need one pane of glass 5" thick (127mm), and one pane of glass 1/4" thick (6.35mm).

However, that would be silly! This is one of those cases where "rule of thumb" theory doesn't jib with common sense reality. The problem is that you have one very high density leaf, and one very low density leaf, relatively speaking. Your inner-leaf wall has a surface density of only around 5% of the outer leaf, but the equations for MSM resonance assume that the masses are reasonably similar. In fact, optimum isolation is when both walls have approximately the same surface density. (m1=m2).

In the real world, it makes no sense to have glass 5" thick in your wall! Apart from anything else, the cost would be massively high, and I don't even know where you would buy such glass! It also makes no sense to have 6mm glass in a studio wall, as that is rather thin!

So the solution is to go for a reasonable but mathematically solid solution that will give you the isolation you need. You didn't say what that is, but assuming that you are facing the same flanking limits as everyone else, and taking into account that you don't seem to need high isolation anyway (your inner-leaf has low mass), let's assume that you are looking for around 45 - 50 dB of isolation, going down to about 40 Hz.

If you make your outer-leaf glass 19mm thick (3/4") and your inner-leaf 10mm thick (3/8"), and you have an air gap of at least 16cm (6") between them, then in theory you'd be getting close to 50 dB of isolation going down to roughly 40 Hz.


- Stuart -

Hi Stuart, thanks for the quick and detailed reply. Sorry I didn't say what isolation I was aiming for... 50db is plenty for my needs.

I know I can get 12mm and 10 mm glass here but not sure about the 19mm (I will ask the glass guys if its available)

I was also thinking I might need to add another layer of drywall to the low mass inner leaf (the one with 12mm drywall and 6mm cement board) But if I can get 50db with this setup I'm happy with that !

Thanks again,
Steve

But if I can get 50db with this setup I'm happy with that !

Please note! I did say "In theory!" The real world might be different...

As the commercial for wonderful cars and special offers say; "Actual mileage may vary. Void where prohibited by law. Not available in any of the follow places: planet Earth"...

To actually get that theoretic 50 dB level in practice, you'd need to pay great care to getting all your seals perfect, building your walls, doors, windows, etc. very consistently and conscientiously, top quality materials, attention to detail... Not impossible by any means, but certainly above what the average builder would be bothered with.

- Stuart -

yes, I understand.. "in theory"

I just started to build the 2x4 wall and I increased the airspace to 8" (the cement wall is already in place)

Thanks again for your time and help

I just started to build the 2x4 wall and I increased the airspace to 8" (the cement wall is already in place)

So, in other words, you left a gap of 4 1/2" inches between the cement and the new stud frame? Right?

- Stuart -

Soundman2020 wrote:

I just started to build the 2x4 wall and I increased the airspace to 8" (the cement wall is already in place)

So, in other words, you left a gap of 4 1/2" inches between the cement and the new stud frame? Right?

- Stuart -

Hi Stuart, Yes that is correct.