Can you hang bass hangers inside the air gap?

[TerrorFiend]

Im building a recording room with a 2 leaf double wall system, with non parallel walls and a varying airgap (as you can see below).

Can I just hang bass hangers inside the walls planned airgap? Or should bass trapping be contained in it's own seperate box?

[knightfly]

Depends on how much isolation you need to the outside world - basically, there's no free lunch.

Acoustically, bass trapping works by not letting the bass back into the room (very simplified) - so, if your inner walls let the bass through and you absorb more of it with hangers, the in-room result is less bass (and more even throughout the room) - however, from an isolation standpoint, you've just breached quite a bit of your inner wall leaf's contribution to isolation (the bass has to go SOMEWHERE, and even though the hangers absorb quite a bit, your outer wall leaf still gets hammered) -

If, on the other hand, you build a double-leaf wall system and THEN build trapping inside that, first thing that happens is you need MORE trapping - this, because your outer, double-leaf wall will have to be stiffer and heavier to actually stop the sound from escaping - this, in turn, means that you've CONTAINED more of the bass in the room; so, more trapping is necessary to reach the same acoustic result inside the room.

Say you built a double wall, and managed to get your STC up to 66 dB, with the Transmission Loss (TL, which is NOT the same as STC; TL is rated at specific frequencies, where STC is a weighted AVERAGE of all TL values) - Anyway, say you build this double leaf wall with STC of 66 dB, and a TL @ 50 hZ of around 32 dB (these are realistic figures) - your room would be pretty quiet between you and the world, but wouldn't sound too good without acoustic treatment.

Now, you add bass trapping in corners, above sloped ceilings, wherever - this creates a triple leaf wall condition, because you now have a third layer of mass between you and the world - the good news is, it gets the bass more under control in the room; the bad news is, it has to go somewhere. At least part of it "shorts out" your soundproof wall and escapes, because the third leaf has just changed your wall performance curve. This is made worse at bass frequencies, and usually better at midrange frequencies.

Getting kind of long-winded here; basically, you can do it either way. Your way will work for acoustics and worsen your isolation (possibly by as much as 20 dB or so at low end) but it will be cheaper. The other way, if you need really good isolation AND good acoustics, costs more. You need to build your outer double leaf system EVEN HEAVIER than you thought, because your inner trapping will compromise it while accomplishing a good-sounding room.

Hope that helps, wish this stuff were easier to understand/do/pay for... Steve

[TerrorFiend]

Well seing I've already built three sides of the outer wall, and it's not as heavy as it could be, it looks like I'm stuck with what I've got.

So you're saying that bass hangers in the walls would improve the acoustics but detract from isolation?

What if I left out the hangers, built the wall as it is, and then tried to contain the problem later with the acoustic treatment of the room?

[knightfly]

Given the same conditions and construction of inner and outer leaves, and that both leaves are hermetically sealed, placing hangers between the two leaves likely won't do much either way - if you build your inner leaf with more than one layer of wallboard, that will stiffen the inner leaf and lessen the amount of bass that gets between the two leaves in the first place; less bass rattling around between the walls, less for the hangers to absorb, so less effect.

If your outer leaf isn't very heavy, then the inner one will need to be fairly stout (at least two layers of 15mm wallboard) or you'll lose isolation. Doing it that way (and keeping the two leaves sealed so they still work for isolation) means that those cavities can only be used somewhat as panel traps, not absorptive traps. Using the inner leaf as a panel trap will weaken the wall's isolation somewhat, but I'm not sure just how much. I do know that it will be weakest at the frequency of resonance, which lowers as you use heavier mass in the leaf.

As I said, it's kind of a trade-off - generally, though, hangers work better (by far) when used in a partly (or fully) VENTED trap, where the sound energy can be intentionally vented into the trap (and absorbed) - given that, you might be better off just using a double layer of wallboard on your inner wall frames and filling the entire cavity (or at least stud width each side) with fiberglass insulation. If you fill the entire cavity, you can use standard fluffy fiberglass insulation. IF using just the stud depth, I'd go with either 48 kG rigid fiberglass or mineral wool, and make sure it touches the inner wallboard panels a bit (damping the panels this way improves Transmission Loss for better isolation)

Then, you'd still need to do some bass traps of one kind or another - any space usable in the ceiling, or possibly a few panel type traps in/near the corners -

HTH... Steve

[TerrorFiend]

Thanks man that was very helpful.

My outer wall is a 70x35 wood stud frame with 2 layers of 13 mm 'soundstop' plasterboard. (Denser than normal drywall and apparently made for use with acoustics). And 75mm rockwool (Bradford 'soundscreen' batts if you know australian companies). This stuff is 30kg/cubic metre which is pretty much the densest affordable insulation i could find around here.

My inner wall will be 90x45 studs with a base layer of 16mm 'firestop' plasterboard and an outer later of 13mm soundstop. This frame will have 115mm rockwool in it, including the ceiling.
The floor will be floated and have 75mm rockwool under2 laters of particleboard.

Thats the best I can come up with. I hope it works!

[John Sayers]

Brad - what are your outer walls for the live room - if you look at this drawing the outer wall (i.e. your mass wall) is the black walls whereas the brown walls are the inner acoustic treatment walls.

I'm confused as to how you have built the brown walls. Normally you would have the hangers in the space between the brown and black walls.

cheers
john

[John Sayers]

Brad - what are your outer walls for the live room - if you look at this drawing the outer wall (i.e. your mass wall) is the black walls whereas the brown walls are the inner acoustic treatment walls.

I'm confused as to how you have built the brown walls. Normally you would have the hangers in the space between the brown and black walls.

cheers
john

[TerrorFiend]

Thanks for replying.

Please look to my diagram below, I'll label it to make it easier. The brown wall (A) is the already standing brick, and theres absolutely no way to seal against it hermetically, and incorporate it into a double wall system.

The only thing I could do was to build wooden stud frames inside of this existing brick wall, and plasterboard the INSIDE of them, making sure of a hermetic seal at the floor and at the ceiling. (The ceiling we have attached under the girders which were already there).

The space (H) is NOT hermetically sealed.

That angled wall you see (B) is the INNER wall of a double leaf system, which has a varying airgap (C).

(D) is two 13mm layers of 'soundstop' plasterboard, and (E) is a base layer of 16mm 'firestop', and a face layer of 13mm soundstop.

The inner frames (B) have 115mm rockwool, while the outer frames (the ones with (D) attached) have 75mm rockwool.

The wall with the window and the door (F) hasn't been built yet, as I wanted to place it under the plasterboard ceiling after we plaster it, and screw it through the plasterboard, to ensure a hermetic seal and build at least one wall as it's supposed to be.

The mixing desk will be at (G).

Yes I'm very aware (now) that this is NOT the ideal way to build a double wall system, and that I won't achieve the maximum STC that a proper double leaf wall would. But when you have what I had to start with, it makes things difficult. And I have to live here as well, so I need all that space on the left for my stuff.

[TerrorFiend]

Here's a few pics.

You can see what a nightmare I had to start with.

[TerrorFiend]

You can see what we did with the roof.

In the bottom picture, I had layed out scrap bits of wood to get a visual on the size of the tracking room.

[John Sayers]

OK - so I assume you intend to drywall the stud frames and the drywall will meet the new ceiling. Inner skin? That's assuming the brick is the outer skin. If the new ceiling drywall extends to meet the brickwork (not touch just caulked) above the new inner wall it will seal off the ceiling cavity and make the space between the new drywall layer and the brick the air space.

Thus you end up with two skins and one air space except for the ceiling where you are relying on the gal iron roof to act as some sort of skin unless you added a skin to the purlins before the extra rafters but I couldn't see anything like that in the pics.

OK now those walls you laid out on the floor become your internal acoustic treatment and create cavities as shown in this drawing. The cavities can be covered with slots or have cloth and heavy 703 of you can hang hangers in them.

cheers
john

[TerrorFiend]

That's a good plan, but unfortunately I can't adapt to it for a few reasons.

Those white squares of fibro you can see stuck on the walls are covering up some vents in the the brickwork. Though these could be filled with mortar if I wanted to go to the trouble.

The second reason is that those stud frames on the side extend right up and attach to the ceiling beams (I'll post a photo). They could be taken down with a lot of effort and put back up once the ceiling is done, but...

The brickwork is actually really uneven and IMO it would prove almost impossible to ensure an airtight seal between it and the edge of the ceiling, no matter how much caulk you used. And there's also those brick piers to contend with. You can see them in the photos.

In designing this, I've basically treated it like the brickwork wasn't there - except for the fact that the wall covering on the outer walls will be on the wrong side of the stud frame. (Except on wall (F)). I just accepted that it wouldn't work quite as well as a normal double wall would. I've been told that it's standard practice to treat brickwork as if it leaks, and to have nothing toching it except things that are water resistant. (Fibro, galvanized steel, etc.).

What do you think of this design? It's basically the same as my other one except that the angles in the room are created by panel absorbers/bass traps etc (A)and are NOT hermetically sealed.