John Sayers' Recording Studio Design Forum

Hi All,

I am about to start construction of my second studio in the end of a two car garage.

I have a good understanding of the basic principals, cost and requirements of soundproofing a room from building the first studio. I have also brought and read Rod G's book.

I have a basic plan for the walls, which will be beef up the existing walls (external are brick, internal are timber and drywall) and then build a new wall with multiple layers off plasterboard. Mass-air-Mass.

However, I am struggling to come up with a decent plan for the ceiling.

I am only converting one end of the garage, a room about 3m wide and 6m long with a 2.4m stud. This room was an old workshop. Unfortunatly I need the rest of the garage to remain a garage.

I have had a builder inspect the trusses and he tells me that because of the design and where they are in relation to the rest of the structure I am unable to change the design from a flat bottom chord truss to a sissor type truss to gain some extra height to build a new internal ceiling.

I can't beef up the exising ceiling and then build a room within this room as the ceiling height of 2.4m already tight and I would need atleast 200mm to run new joists to hold up two layers of 16mm drywall. That would drop the ceiling height to 2.2m at the most.

Can anyone give me some suggestions how to tackle this ceiling to keep up with the walls.

I have attached some photos that show the room and the existing ceiling truss design.

Thanks for your help.

JohnG

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Looking at my photos i suddenly thought why not do a inside out ceiling?

I could add another layer of plasterboard the the existing ceiling and then build new walls and slide up ceiling sections on top of the new walls inside out, that is plasterboard on the back.

I could use a smaller joist, maybe 90mm rather than 140 mm, and just add more of them to carry the downward load of the drywall thus gaining a few more inches of ceiling.

Only issue would be the small air gap, maybe only 25-50mm between the two layers of mass but at least it would be totally decoupled from the existing structure and pretty easy to build.

Would the coregatted roof iron be considered a third layer or could I count that out within reason?

What would be everyone's guess on the TL performance of this ceiling for drums?

JohnG

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Anyone have any input on this?
As you can see the area where the roofline meets the ceiling in the corners is very tight.
I got up there last night and it's very hard to move, let alone drag up plasterboard sheets to beef up the underside of the roof.
JohnG

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A couple of questions/comments on that:

How do you plan to beef up the brick walls?

When you say "external" and "internal" walls, are you talking about walls that are right next to each other and parallel? If so, then it sounds like you might be setting yourself up for a 3-leaf wall.

In that situation, one option would be to remove the existing drywall, put RC across the existing joists, then hang new drywall from that. Of course, it all depends on how much isolation you need, and how well sealed/massive the roof is.

Another option might be to do it as a three-leaf system, if you can afford the extra space.

I don't think you have enough height to do that: You say the existing ceiling is at 2.4 m, so allowing for your air gap and new drywall you'd be losing about 15 cm, give or take. Even assuming you can get away with only 2x4 joists for supporting that (unlikely, given the weight and span), that takes out another 10cm, so the bottom of the visible joists would be at about 2.15m.

Did you check that on span tables and/or consult a structural engineer? It doesn't sound safe to me: you need to span 3 m, with a dead load of well over 24kg/m3. What deflection are you allowing for, and what type of wood?

... and it would also drive up the resonant frequency drastically! You need at least 100mm of air gap to get your F0 reasonably low. But you also have the added disadvantage of the existing ceiling probably only being 10mm drywall, so you are losing seriously there too. Rough calculation: MSM F0 would be waaaaay up high at around 142 Hz: That ceiling will not even start isolating until 200 Hz, and won't isolate well until about 430 Hz

Yes, to a certain extent. And for a three-leaf structure, the optimum way to build it is with most of the mass on the MIDDLE leaf. Theoretically, you should have M - 2xM - M. In other words, the mass of the middle leaf should be the same as the mass of the other two leaves combined, and the air gaps should be the same size. Your case, as you outlined above, would be the worst possible case: Most of the mass on one leaf, less on the middle (with a tiny air gap), then more on the far side, over a much bigger air gap.

That said, the distance from the middle leaf (existing ceiling) to the outer leaf (corrugated iron) is pretty large, and you do have insulation in there, so that plays in your favor. But unless you increase that tiny air gap between the middle leaf and inner leaf, as well as adding more mass to the middle leaf, your outcome is not going to be good.

That's pretty hard to predict accurately, since there are so many unknowns, but it would not be good. At a rough guess, if you are lucky you'd be getting something like 25-30 dB of isolation at 100 Hz (kick drum territory), less than that lower down (bass guitar territory), improving to maybe 40dB or so at 300 Hz, then rising at maybe 18 dB per octave from there on up. But its hard to say where your coincidence dips would fall, so even that might not be accurate.


On the other hand, beef up the middle leaf with an extra layer of 16mm drywall plus Green Glue, increase your air gap to 100mm, put 2 layers of 16mm on the inner leaf (also with GG), fill the cavity between the middle and inner leafs with good insulation, and you could improve that considerably. Your lowest resonance would be down to around 25 Hz, so the ceiling would isolate from 50 Hz upwards, and isolate well from about 70 Hz.

- Stuart -

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I want this studio to amaze people. "That'll do" doesn't amaze people.

Hi Stuart,
Thanks for your message.
I thought it was in the to hard basket for everyone.
To answer your questions:
(1) Not sure about beefing up the brick walls now. I cut a hole in the plasterboard and found that the brick sits 25mm from the existing timber frame (that I will remove plasterboard from) but is connected to this frame with sway braces on each stud. You can put your hand around the back of the frame inbetween this and the brick. The brick is 70mm thick, is a beef up really required? surely thats the same approx mass as 2x16mm plasterboards? maybe not? I will be building a new wall inside this old frame (with plasterboard removed of course) so the walls will be 70mm brick, 250+mm airgap, new wall with 2x16mm plasterboard.
(2) I will be adding at least one layer of 10mm plasterboard and one layer of 16mm plasterboard to the existing ceiling making it 36mm thick.
(3) I will consult an engineer, or masterbuilder, on the joist span of new inside out ceiling, but wood is cheap here and I can run 90mm joists at 300 centres if required to support 26kg/m3 of mass above it. Could even use 90x70mm.
(4) Yep head room is an issue but i don't know how to get around it -I calculate it at 2.184mm with a 75mm airgap. It would be alot easier if i could just run isolation connectors and two layers of 16mm plasterboard from the existing joists but I dont think this will stop 125db of drums. The roof is only 6mm corregated iron and not sealed well at all. Whats the TL of a one leaf ceiling with 2x16mm plasterboard?
Thanks so much for your help.
JohnG

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www.peelproductions.net.nz

Also some more info that may mean a simple one leaf (2x16mm) new ceiling on RC on existing joists maybe OK?

I ran some tests in the existing room over the weekend with an SPL meter, C weight, fast setting. VERY VERY hard drumming lots of kick and floor toom. The maximum reading results were.

Inside Room 125db
Direct outside room 92db
In kids bedrooms 3m across from studio 66db
At all boundaries with neighbours 73db

So with the room just as it is I am getting 33db reduction to just outside, about a 50db to neighbours and about 60db into the house.

Any more direction would be great.

JohnG

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Studio Gallery at:
www.peelproductions.net.nz

I'm a little confused about some of those readings, and I think you need to get your sound level meter calibrated!

I've heard (and measured) some loud drums in my time, but that takes the cake! According to OHSA data, you'd suffer irreversible hearing damage in 90 seconds at a level of 125 dB. If that really were the level in your room when you did the measurements, both you and your drummer would be deaf. That's way louder than a jack hammer, about the same level as a 747 at maximum take-off thrust... Stand directly in front of the huge speaker stack at a Led Zepplin concert, only ten meters away, and you just might manage to measure 125 dB...

I often measure live acoustic drums at around 110dB to 115dB, occasionally I've seen 117 or 118, and rarely 119-120. But you say that yours are putting out four times more energy than that? Either you have the most enthusiastic drummer and loudest kit I ever heard of, or your meter is not calibrated correctly! I'd get that checked. Also, switch to "slow" response: You might just have been measuring transients from snare or cymbal hits, rather than true sound pressure levels.

Anyway, on to the other questions:

Are those proper acoustically isolated sway braces? If so, you might be OK. If not, the wall will need modification.

No beef-up necessary. The density of brick is about 3 to 4 TIMES the density of drywal. Brick is around 2,000 to 3,000 kg/m3, drywall is around 600 to 800 kg/m3. So 70mm of brick is roughly equivalent to the mass of about seventeen layers of drywall! You'd need an awful lot of "beef" to make any appreciable difference...

However, you do need to seal that brick: a layer of plaster would do the job well, or even painting it with a good masonry sealant. Brick is very porous, so sealing the surface will gain you a few dB of isolation.

That should be pretty good for isolation.

10mm isn't very useful in isolation. Too thin, too flexible, low mass, high resonant frequency. Just go with 2 layers of 16mm.

Supporting the ceiling correctly is more than just a matter of cheap wood spaces close together: there are several factors that need to be considered in calculating the safe span: the type of wood is the first one: different types of wood have different strengths, flexibility, and therefor load-carrying capability. Spruce, pine, oak, maple, etc all have different span capability for the same load and same deflection. You need to know the species of your joists in order to do the calculations, as well as the dead load, live load, span, spacing and deflection. It's a bit more complex than most people realize, and having a few tons of drywall hanging over your head makes it important to get it right!

Practically nothing will stop 125 dB of drums! But I don't think your drums are anywhere near that loud, in reality. Probably more like 115-118.

A single-leaf with mass of about 24 kg/m3 will have TL of somewhere in the region of high 20's to low 30's. No better than about 35 dB, maximum, under best conditions. Single leaf is not a good option: Mass law is against you (and so is Murphy's law, which overrules all other know laws of physics!!! )



- Stuart -

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I want this studio to amaze people. "That'll do" doesn't amaze people.

Hi Stuart,
Thanks for your reply again. To answer your questions:

(1) the existing room is concrete floor, and plasterboard walls and ceilings, my drums were insanely loud in there! Especially my snare! The SPL meter was calibrated but set to max reading, so that was the top peak, average was probably like you say, about 115db.

(2)I checked the wall again, what I thought was a sway brace was actually a large staple holding building paper to the studs. So it's, 70mm brick-25mm air gap-building paper-timber stud. It looks like the brick wall is not connected to the existing timber stud, either way this existing wall will not have plaster board on it so it really discounted anyway.

(3) I'll have to rip this building paper down to seal these bricks from the inside with paint, is that ok to do?

(4) The reason for adding 10 mm plasterboard and then 16 mm to the ceiling is that there is already a layer of 10mm there, that will give me a total of 36mm for that leaf.

(5) I checked the NewZealand building code today regarding the span of the 90 mm timber I am using for the inside out ceiling. Its structurally rated wood and can span 3.0m at .600mm centres if used for rafters in a high wind and snow loading. I checked with a designer and she tells me that a consistent load of 26 kg / m2 is totally ok for this timber at this span. I am going to close in centres to .400mm to add another degree of caution.

(6) Looks like I need a separate ceiling to get the isolation I need, I suppose I'll just have to balance air gap and ceiling height verse isolation. What would 36mm plasterboard, 75mm air gap, 32mm plasterboard get me roughly?

Thanks,

JohnG

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Studio Gallery at:
www.peelproductions.net.nz

!That makes sense, considering the construction: probably lots of reflections, reverberation and resonance going on in there.

I'm wondering if that building paper might be intended as a vapor barrier, Is this a very old house? Most vapor barriers these days are plastic. So if it is a vapor barrier then you'll need to install a new one if you take it out. But that might not be a bad idea anyway, if it is old. Plus, you should not have two vapor barriers in your wall, for obvious reasons.

Should be OK, provided that you then put something else in to perform the same function, assuming that it is a vapor barrier. You might want to check with local builders to find out what the purpose of that paper is.

OK, but how about the joists above? Can they handle the extra load? 16mm drywall weighs around 12 kg/m2, so you will be adding many dozens, maybe hundreds, of kg load to the ceiling. You should check with a structural engineer to make sure that you can do that safely. What size joists are up there, what distance are they spanning, and what is the spacing between them?

Around 50 dB of isolation (TL), but only above around 72 Hz, since the MSM resonance of that would be roughly 37 Hz. It's the small air gap that is working against you here. Once you get below about 100 mm, you are on a very steep part of the curve, and going down fast. Increase that gap to 100mm, fill it with insulation, and it starts isolating well at about 45 Hz.

Of course, those are all theoretical numbers, which may or may not match reality.


- Stuart -

_________________
I want this studio to amaze people. "That'll do" doesn't amaze people.

Thanks Stuart,

(1) I'll check with the master builder on adding mass to the existing ceiling, the bottom chord of the roof trusses are made from 90x 45mm timber and spaced at .600mm centers. We also have a truss designer at work so I'll get him to run the calculation through his program.

(2) yes, the building paper is a vapor barrier, no problem to replace this once the brick wall is seled up.

(3) understand on the importance of the air gap for the ceiling, I'll have to check the height once the original ceiling is beefed up and see how it looks for height, hopefully I squeeze it all in and still make it workable, my last room had a 2.250mm height and it worked out surprisingly well. I can use 25mm insulation in the inside out cavities so even though the bottom of joist is lower in the room the illusion might be better. Also if you paint it or cover it with dark colour it creates a feeling of height.

Thanks again, and I'll keep you posted.

JohnG

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Studio Gallery at:
www.peelproductions.net.nz