How to do wall within a wall? Point me to the references!

[henryrobinett]

This question is so basic I can’t find it in the reference section. I’m sure I’m just being impatient and not looking carefully enough. So when installing two layers of drywall with an air gap, how do I do that? Without nails? It’s ok to use screws, even if they’re metal? Do I use wood studs glued? If I’m usibg just one drywall on top of another with air gap, an I still supposed to use green glue? On what? Isn’t that supposed to stick to the other drywall? Do I just slather it in the backside of a wall?

I listed my credentials on another post. I’m giving this information to a contractor who’s never done anything like this. Cottage/house building as a studio. Going to get started fairly soon. Doing one tracking room. I want to do a wall on top of the existing one with an air gap. And for the ceiling: how do i do that exactly? Build another frame? Is it entire necessary? Lol I can hear Stuart now!

[Soundman2020]

I'm not sure that I understand the question! There is never an air gap between the layers of drywall on a "leaf" in a two-leaf wall. There's a gap between the LEAVES, yes, but no gap between the LAYERS. So there's often two layers of drywall on the outer leaf, and also two layers of drywall on the inner leaf. If you want enhanced isolation, then you would put Green Glue in between those two layers. You could use Green Glue on only one leaf, or you could use it on both leaves, but in each case you would only apply the Green Glue in between the layers of drywall, never on the outsides of the drywall.

So, if you are building the inner-leaf, for example, and using "conventional" construction (not "inside-out" construction), then the sequence would be to first build the stud frame, then nail one layer of drywall onto the studs, then put the panels of drywall for the second layer laying flat on the floor (face down), spread the Green Glue over that drywall on the floor by just squirting it out of the tubes, then lift the drywall into place, pressing it up against the first layer, and nail it in place. So you will end up with the first layer of drywall nailed to the studs, and the second layer also nailed into the studs THROUGH the first layer, with the Green Glue sandwiched in between the two layers, like cheese in a sandwich.

Not sure if that answers your question...


- Stuart -

[henryrobinett]

HM. OK. I've done it before, or had it done, with an air gap between drywall sheets. I think we used little wood studs glued to the wall, or nailed, to create the space. Do I have this completely wrong? Was it never done this way? Using green glue between layers is more effective, though more expensive?

[Soundman2020]

HM. OK. I've done it before, or had it done, with an air gap between drywall sheets. I think we used little wood studs glued to the wall, or nailed, to create the space. Do I have this completely wrong?

Yep, that was wrong. Those bits of wood are usually called "battens", and they are not intended for walls that have to produce good isolation.

Here's the issue: any time you trap air between two surfaces, you have create a resonant cavity. That's how drums work, and many other musical instruments too! Air trapped inside a box, tube, or other hollow object. The air resonates at certain frequencies (which is very good for musical instruments! But very bad for walls). At the frequencies where the air resonates, obviously the "thing" does not isolate at all. If that "thing" happens to be a flute, guitar, or violin, that's great! Wonderful! It produces a loud, clear note for everyone to hear. But if that "thing" happens to be a wall, ... mmm... well, that's definitely NOT great at all, because it produces a loud, clear note for everyone to hear....

So a wall is a resonant "thing", and just like all resonant "things" it does not isolate at any frequency where it resonates. Even worse, it will resonate in sympathy with any note that happens to be similar to one of the frequencies that it is tuned to! If you listen with your ear close to the sound hole of an acoustic guitar while some plays another instrument nearby, you will clearly hear this "sympathetic resonance" going on inside the guitar for some notes, but not for others.

So far, this is easy to understand. Tuning an instrument is also easy to understand: if you want to get a different note on the snare or tom, you do that by tightening or loosening the skin, or change to a heavier or lighter skin. If you want to get a different not on a trombone, you move the slide so that there is a longer or shorter air path inside. The same when you press the keys on a sax or trumpet: you are changing the length of the air path inside, which changes the tuning. And the same happens with your wall!

You "tune" your wall by changing the depth of the air cavity inside, just like you "tune" the note of a trombone. If you have more air depth inside the wall (bigger distance across between the drywall on one side and the drywall on the other side) then the note is lower down the scale. If you make the air gap thinner, then the note is higher up the scale. So a wall follows the same concepts as any musical instrument: deeper air inside means deeper note (lower frequency).

So how do you tune a wall to isolate ALL frequencies? Simple! Make the air gap deep enough that the note is lower than anything on the musicl scale! It really is that simple. If the tuned frequency of the wall is lower than any note that your instruments can play, then it will never resonant at any frequency in your room, so it will isolate very well!

OK, so now we get back to the point of the battens: A batten is a thins strip of wood, so if you put a sheet of drywall, then a batten, then another sheet of drywall, you have a thin air gap, which means that the wall is tune to a high note, which means that it does not isolate!

That's why I said that it is wrong to use battens between layers of drywall: it tunes the wall resonance way too high, so the wall will "play along with the music" due to sympathetic resonance.

Therefore, you should only ever have a very deep air gap inside your wall. Never a thin one. You should only ever have one single air gap inside the wall, for the same reason: if you put another layer in the middle of the wall, then you have two thin layers, with high tuned notes! It's much better to have just one very deep air gap, as deep as you can make it, to keep the tuning very low: off the bottom end of the scale.

In summary: thin air gaps in a wall are a bad thing, as they tune the wall to high notes. Big air gaps are good, as they tune the wall to low notes.

In addition to the air gap, you also need a panel on each side, of course, and the panels themselves also resonate. So to get THAT under control, you need to make the panel as massive (heavy) as you can. More mass = lower resonant note. That's why we put two or even three layers of drywall on each side of the wall: because it increases the mass, and that tunes the wall even lower. But you don't want to trap thin layers of air between the layers with battens, for the reason above. So just put the layers right on top of each other, directly nailed all the way through into the studs.

Green Glue is something else you can use. You apply it in between the layers of drywall. The name says "Glue" but in reality it is NOT glue at all! It is a carefully formulated acoustic compound that "damps" the resonance of the drywall itself, which once again drives the resonant frequency down even lower.

And finally, you also need to put some type of insulation in the actual air cavity inside the wall: either fiberglass insulation or mineral wool insulation, of the correct density for this application. That damps the resonances INSIDE the wall cavity, which one more time also drives down the resonant frequency, and improves isolation.

So the ideal studio wall consists of two "leaves" of mass, where each "leaf" can be made up from one or more layers of drywall or something similar, perhaps with Green Glue in between. Each "leaf" is attached to it's own independent stud frame, and the air cavity between the two leaves is filled with suitable insulation. A wall built like that can get you really, really good isolation.

General rules:

- Avoid thin air gaps in your wall
- Only two leaves: never one, never three, never any other number
- Each leaf on it's own separate stud frame, and the frames don't touch each other at all
- Put as much mass on each of those two leaves as you can
- Make the air gap between the leaves as big as possible
- Fill the air gap with suitable insulation
- Each leaf can be made up from one or more layers of drywall
- Optionally use Green Glue in between the layers of a leaf, if you need exceptionally good isolation.
- Seal the wall to be completely air-tight
- Don't build this wall close to another wall! If you do, that's the same as building a three-leaf wall
- If you already have another wall in place, then that counts as a leaf, so you only need to build ONE leaf to complete the 2-leaf system.

There's actual equations you can use to predict how well a wall will isolated, based on the mass of the leaves and the depth of the air cavity, if you want to do that.

Hope that helps!

Was it never done this way?

IT might have been done that way some place you saw, but it was not correct. YouTube, for example, is fully of incorrect videos, where enthusiastic but ignorant people show all kinds of silly, and even dangerous, ways they have built their "studios". You might have seen thin battens used on a you-tube video, but if you did, it was wrong.

Using green glue between layers is more effective, though more expensive?

Yes, that's correct. If you need very good isolation, especially for low frequencies, then using Green Glue can help. Yes it is expensive. No there are no substitutes. You might find this thread interesting: http://www.johnlsayers.com/phpBB2/viewt ... =2&t=21488 That just happened yesterday and today, where somebody pointed out a YouTube video that is totally wrong: a guy tried to use carpet adhesive, instead of Green Glue! Believe it or not... In that thread, I explain exactly how Green Glue works, when to use it, when not to use it, and a few other things that might help you with your studio.


- Stuart -

[henryrobinett]

Thank you so much. Let me just clarify and simply this for myself. If I use the Green Glue, I don't have to put an air gap? I'd like to save space, if possible. Big air gap isn't desirable and neither are 3 drywall panels. But the added expense of just GG between two layers of drywall and no gap sounds great

[Gregwor]

Thank you so much. Let me just clarify and simply this for myself. If I use the Green Glue, I don't have to put an air gap? I'd like to save space, if possible. Big air gap isn't desirable and neither are 3 drywall panels. But the added expense of just GG between two layers of drywall and no gap sounds great

If you want medicore isolation at best, then yes, you can build a normal wall and only use green glue. But let's get this straight - even a "normal" wall typically has drywall on each side of the stud. The space between the drywall is the "air gap". The problem with these walls is that the mass on each side of the stud is fully flanking/touching one another through the stud. So the only way to semi-decouple them is with green glue and/or resilient channel. It's not a real solution to achieve good isolation though. That's why people build rooms in rooms. Now, the concept of mass spring(air) mass is the same for a room in a room. The difference and benefit to this method is that the walls are fully decoupled from one another. Applying green glue to between your layers of drywall will further improve this designs performance.

I hope that clarifies the situation. Of course we all want to save space. Of course big air gaps is not desirable as it eats up more space and uses more material. The same goes for all of your other points. But, if you want isolation, that's how you do it. There have been countless threads on here (and stories of my friends) who have built it the "cheaper and easier" way. Every time, the rooms don't isolate enough and they end up tearing it all down and doing it all over, but this time, the correct way. Save yourself time and money and put in the effort and money the first time.

Greg

[Soundman2020]

Greg is absolutely correct. If you want decent isolation, a proper fully decoupled 2-leaf wall is the best way to go. It's the ONLY way to go on a limited budget.

I'd like to save space, if possible. Big air gap isn't desirable and neither are 3 drywall panels.

Then use more mass on the leaves! Instead of drywall, use something that has much higher density, such as concrete, aluminium, steel or even lead. It's much more expensive, of course, but it works. It allows you to have a smaller air gap.

The equations that govern MSM isolation are fairly simple, and the two major variables that you can control are the mass and the air gap. You can increase either one of those to get the desired effect. If you increase one of them enough, then you can decrease the other. So if you have plenty of space available, then you can increase the air gap and use thinner, lighter materials, saving money. But if space is tight, as in your case, then you can make your walls thinner, with a smaller air gap and thinner sheathing materials, as long as those materials are much more dense. For example, fiber cement board is a bit more than twice the density of drywall, so you could have just one layer of 5/8" fiberboard, instead of three layers of 5/8", and still get the same result. Or use two layers of fiber cement board and reduce your air gap by about 20%: same result.

Going more up-scale, you could use steel plate for your sheathing: it is about ten times the density of drywall, so a single sheet of steel plate 3/16" thick will give you about the same mass as three layers of drywall. Or using 1/2" steel plate would allow you to reduce your air gap by about 40%: Expensive! But the math works.

And if you REALLY want to go to the top, use lead sheeting: it is 18 times the density of drywall, so a thin sheet just 3/32" thick is the same mass as your three layers of drywall. Or use a sheet 1/4" thick, and you can halve the depth of the air cavity. Lead also has the advantage that is is "limp mass", and self-damping, to a certain extent, so you get the extra benefit from that. But the cost is a little high!

So you do have options. Do the math, use the equations, price the products, and see what works for you.

- Stuart -

[henryrobinett]

I’m trying to get my contractor to read some of these threads. As I said in another thread, absolutel sound proofing is not mandatory. This is going to be a tracking room in a cottage in my backyard somewhat removed. 100% isolation is not the aim. So if GG works well without using an air gap, even though a gap is the superior method I might opt for that. I just need to make certain I’m comprehending correctly.

[Gregwor]

100% isolation is not the aim. So if GG works well without using an air gap, even though a gap is the superior method I might opt for that.

100% is impossible. Even with a room in a room design, we are still only getting decent isolation. Sadly, if you just use the GG solution, you will get pretty poor results... only marginally better than a normal wall, especially at low frequencies where the problems always live.

Let's do this. Get a meter, take measurements. Look up GG's charts and see how much isolation you'll get with PERFECT construction and see if you think it will suffice. I doubt it will.

Greg

[henryrobinett]

Well then, that’s awesome. I don’t need to use GG at all. I can save some money and back to my original plan to do a wall in a wall. Tuning notwithstanding.

[Soundman2020]

100% is impossible. Even with a room in a room design, we are still only getting decent isolation.

Right! Spot on. There is no such thing as perfect "soundproofing", which is why many acousticians prefer not to use the word "soundproofing", since it is impossible. Instead, words like "isolation" and "transmission loss" are more common.

Sadly, if you just use the GG solution, you will get pretty poor results... only marginally better than a normal wall, especially at low frequencies where the problems always live.

Exactly.

If you have just one single leaf as your isolation, then you are limited by something called "Mass Law". That's the principle of physics that describes how isolation works for a single leaf barrier. Mass Law is not very friendly: this is how it goes:

TL = 14.5 log (Ms * 0.205) + 23 dB

Where:
TL = Transmission loss (how much isolation you get, in decibels)
M = Surface Mass in kg/m2 (how much each square meter of your wall weighs)

If you do the math, you'll see that you need very large amounts of mass to get even mediocre isolation, and obscenely huge amounts to get good isolation. If you take a close look, you'll see that this equation implies that each time you DOUBLE the mass, you only get a tiny increase of 6 dB in isolation. So to get a 12 dB increase, you need to double the mass, then double it again. And to get an 18 dB increase, you'd need to double it, double it again, and double it one more time. So for example, if you started out with a single layer of drywall, then to get an 18 dB increase, you would need to have EIGHT layers of drywall! Yup. I kid you not. This is what Greg is talking about: Even with very massive walls, you don't get good isolation for single leaf.

On the other hand, if you have a two-leaf wall, then it's a different set of equations, that are very much more friendly. These "MSM" equations predict that each time you double the mass, you get an 18 dB increase in isolation! That's a MAJOR jump. In real life home-built studios, it's not quite that high, but you still should get 15 or 16 dB with each mass doubling.

In other words, with a two leaf wall, you need much, much MUCH less materials than for a single leaf wall, to get the same isolation. Different principles of physics, different equations, different outcome.


- Stuart -

[henryrobinett]

Excellent. Remember I had said 100% isolation was NOT what I was trying to achieve. But another thing, I think Stuart, you had said something about nailing the drywall. It was my understanding you wanted to avoid nails at all cost because they transmit sound? Is that right or wrong?

[Soundman2020]

I think Stuart, you had said something about nailing the drywall. It was my understanding you wanted to avoid nails at all cost because they transmit sound? Is that right or wrong?

How else would you hold it up? You can't glue it (it will de-laminate eventually if you try)...

If you are building a proper fully-decoupled two-leaf MSM isolation system, then nails (or screws if you prefer) don't matter at all: the stud frame itself is decoupled, so it does not matter how you attach the drywall to the studs: you could use 1" diameter bolts, if you really wanted to, and that would be fine. But nails and screws are a lot cheaper...

- Stuart -