Practice studio construction guidance

[GeorgeG]

I have a conversion project where the objective is to convert a 34' x 27' building into 2 room structure with most of it being a single room rock band practice room and the rest a 4' x 34' storage room. The isolation requirements are "make it good enough to keep the neighbors happy"

The construction of the building:

Concrete block walls, painted on the inside
Two small windows
One 36" entrance door
Two roll up aluminum doors on the long wall that will be entrance to the storage area
Monolithic concrete slab, 4" thick
Truss roof made of 2x4" #2 pine with bottom cords 24" OC
Soffits vented
Shingles
Ceiling is one layer of oriented strand board (OSB)

Here are the guidelines that I have learned so far:

Build a 2 leaf M-S-M system
A sealed inner air space is the spring
Mass is my friend on the leafs
Use the existing concrete floor
The existing concrete block outer wall is one leaf

Strategy so far:

Build a room in a room made of a wood bottom plate attached to the common concrete floor, wood studs, and wood framed ceiling

Pink fluffy insulation in the new wood framed wall and ceiling

Multiple layers of sheetrock on the inside surfaces of the walls and ceiling of the new structure

A door on the new wall lined up with the existing outer door in the block

Pass the AC duct through from the osb ceiling to the new sheetrock ceiling with a flexible fabric boot

My questions:

A) What lumber dimensions on the floor and ceiling framing?

B) What seperation on the walls?

C) Any treatment on the inside surface of the block wall?

D) how to connect the two 36" doors that are lined up?

E) What about the long wall the forms the narrow storage area where the roll up doors are? Does that form 3 leafs or is it still only two leafs because the outer wall is leaky from the doors? Do i also use multiple layers of drywall on the storage side of this new wall?

F) Any treatment of the two buried windows?

G) Anti sway coupling with isolated clips or floating?

H) Is three layers of 5/8" sheetrock the right answer?

Thanks,

[Soundman2020]

Hi George, and welcome!

Looks like you already have all of the basics figured out, and are getting into the details!

A) What lumber dimensions on the floor and ceiling framing?

2x4 is fine for the walls, assuming that you are going with two or three layers of 5/8 drywall for the inner leaf. The ceiling is a different story: it depends on the span and the load. There are span tables on the internet that tell you what dimensions you need for certain spans, loads and types of wood, but it would be far better to check your local building code, or better yet ask a structural engineer.

B) What seperation on the walls?

You mean the air gap, between inner leaf and outer leaf? At least 4", but as much as you can. It depends on what level of isolation you need, in terms of real numbers (not guesses). So if you set your bottom plates 2" away from the existing concrete walls, that will give you an air gap of nearly 6", which should be OK for moderate rock band type levels if you have a couple of layers of 5/8, good sealing, and do your doors, windows and HVAC well. If you can afford more space, an 8" air gap is ideal.

Two rules of thumb:
1) More air gap means better isolation (especially in the lows).
2) More layers of 5/8" drywall means better isolation (especially in the lows).

C) Any treatment on the inside surface of the block wall?

You said it is already painted, so as long as the paint covers 100% of the surface, is in good condition, and applied decently, that's fine. You jst need to make sure that the surface is sealed well, and paint does that. If you are not sure, clean up that paint and put another coat on.

D) how to connect the two 36" doors that are lined up?

One method is just to stretch cloth across the gap. Another is to wrap cloth around 703 and put that in the gap. Or you can just leave the gap, if you like a rustic look!

E) What about the long wall the forms the narrow storage area where the roll up doors are? Does that form 3 leafs or is it still only two leafs because the outer wall is leaky from the doors?

Photos? SketchUp diagram? You can see what you are talking about, but we can't! Give us something to go on!!!

F) Any treatment of the two buried windows?

See above.

G) Anti sway coupling with isolated clips or floating?

For the wall tops? Depends on code and construction. If your local code allows it, then go without the sway braces but in that case you should probably replace one layer of 5/8 with plywood, to give the structure more strength.

H) Is three layers of 5/8" sheetrock the right answer?

Depends on the question!!! In other words, you need to provide real-world numbers about how much isolation you need. Get a sound level meter, check your levels (search the forum to see how to do that), post your results, and we can give you a better answer.

But most likely 3 layers of 5/8 is PLENTY! Provided that you seal it well, and build your doors / windows / HVAC / electrical to the same level.


- Stuart -

[GeorgeG]

Thanks so far.

The ceiling will probably be 2x10 beams without anti sway attachments

Lets assume that the 2 entrance doors to the studio are connected with cloth for now

Questions:

A) As a starting point I inserted a new wall on the storage end by the large overhead doors that would serve as the second leaf on the assumption that the leaky outer doors are not a third leaf. Does this work?

B) On the assumption that a std rock band generates about 110 db and 50 db is a good number for generic neighborhood tranquility, is 60 db of isolation rational?

C) What to do about the 2 windows before they get sealed up?


Drawings attached for reference.

GM Acoustic Project.pdf

GM Acoustic Project wall details.pdf

Thanks,

[GeorgeG]

Can someone give me some help with:

1) How to treat the windows in the exterior block wall (B in diagram)

2) How to preserve the 2 leaf system and required isolation at the studio/storage room interface (F in diagram)

Thanks,

[xSpace]

In respect to the windows, they need plugs of the same mass as the walls they will be plugging.


You question in reference to "F" is vague, so I cannot say one thing or the other.

[GeorgeG]

Those two windows are embedded in a std 8" (partially hollow) cement block wall. The cleanest route would be to remove them and insert the identical blocks. In order to avoid the demolition of that approach, can solid concrete plugs of a smaller thickness be inserted with the windows left in place and still maintain the isolation performance?

The question with regards to "F" is simply - is the three wall design at the bottom of the drawing effective?

Thanks,

[Soundman2020]

In order to avoid the demolition of that approach, can solid concrete plugs of a smaller thickness be inserted with the windows left in place and still maintain the isolation performance?

Sure, you can do that if you want, but that will make it really hard to return the room to normal of you ever move out. Solid concrete plugs will work very well, due to the very high mass, but the will need to be cast in place and sealed to the surrounding wall, so the only way to get them out again will be with a jack hammer. If you want to go that route, then yes, it will work, for sure. A couple of inches of concrete is way massive, and I doubt that you'd end up with much of a 3-leaf problem. But we can do some math, if you want: Can you keep at least a 4" air gap between the glass and the concrete plug, and still make the plug at least 4" inches thick?. What type of glass is it, and how thick is the glass?


- Stuart -

[xSpace]

""make it good enough to keep the neighbors happy""

Well, the glass isn't an issue,,,with even a little bit of concrete the low transmission loss goes away.

Problem is, you would have to use the glass as part of the form:) So let's walk away from that quickly.

Your neighbors...never said what kind they are, I'm thinking this is a business district and you intent is to work in the after hours.

Great, still have one thing to look into and that is that pesky roof.


You can do what you want with the windows and double frame the walls...but the roof...it's a hole baby.

So , I agree, maybe taking the windows out is not in your future...but that roof...it is

[GeorgeG]

The glass is single pane non-laminated 3/32" thickness.

It is a residential neighborhood with 3 acre lots.

I am confused about the roof. The trusses have OSB sheating under the bottom cords (the existing ceiling) and the new free standing inner room will have three layers of drywall on the walls and ceiling. Is that not my two leafs or does the non sealed attic/roof act as a third leaf?

Same question about the wall system at the bottom of the diagram. Is the leaky outer wall with the roll up doors a leaf or does the new inner room and wall "F" form a proper two leaf system?

Thanks,

[GeorgeG]

I have a more general way to ask my questions. Its much easier to understand the wall systems in the simple two leaf case of high mass external wall/leaf - 8" of sealed air/spring - high mass internal wall/leaf. The real world is rarely this simple including my current project. Beyond those two leafs there can be roofs above, rooms above, hallways beyond, stairwells, other rooms beyond, etc.

How do I think of the remaining infrastructure of the building that exists beyond the initial two leaf structure? Is there a certain distance (air gap) beyond the initial two leaf system beyond which you ignore any other walls, roof planes, etc? In other words, if my structure (starting from inside the isolated room) is 3 layers of 5/8" drywall, 8" of sealed air, 3 layers of 5/8" drywall and then another wall exists 4 feet away on my way to the outside world, does this last wall come into play or do I ignore it because it does not form a meaningfull third leaf?

If I cannot ignore the remaining building structure beyond the isolated room then what are the rules for two leaf systems that have to exist within buildings that are more complicated than a simple box inside a box seperated by 8" of sealed air?

Thanks,

[Soundman2020]

How do I think of the remaining infrastructure of the building that exists beyond the initial two leaf structure?

Great question! When I first started looking into this whole 2-leaf, 3-leaf MSM thing, I ran into the exact same doubts as you, and it took a while to get to the answer...

Is there a certain distance (air gap) beyond the initial two leaf system beyond which you ignore any other walls, roof planes, etc?

Yes there is, but it isn't a fixed distance. The answer is not "27 and three quarter inches". The answer is: "It depends". Probably not the easy, simple answer you were looking for! But then again, I've learned that in acoustics practically nothing is "easy" or "simple"

In other words, if my structure (starting from inside the isolated room) is 3 layers of 5/8" drywall, 8" of sealed air, 3 layers of 5/8" drywall and then another wall exists 4 feet away on my way to the outside world, does this last wall come into play or do I ignore it because it does not form a meaningfull third leaf?

You can ignore it, because the 3-leaf effect will be so tiny as to be irrelevant.

It works like this: You probably already know that a 2-leaf wall is a resonant system, where the resonance is governed mostly by the mass on each leaf and the distance between the leaves. The idea is to get that resonant frequency down as low as possible, because the wall is actually an amplifier at that frequency (helps the sound to get through the wall, and boosts it along the way!), is neutral at about 1.4 times the RF (resonant frequency), isolates OK at 2 x RF, and isolates well at 3 x RF and above. So, if you can get your RF down to 10 Hz, then the wall is already isolating at 20 Hz, which is generally considered to be the lowest frequency audible to man.

There are two ways to reduce the RF of an MSM wall: 1) add more mass to each leaf. 2) increase the air gap. For argument's sake, you can say that a wall with 3 layers of 16mm drywall on each side of a 12" gap gets you pretty close to that ideal situation.

Now for 3-leaf. If you add a third leaf to that wall, say by putting a single sheet of drywall in the middle of the air gap, what you did is to re-tune the wall to a much higher frequency. You now have two resonant circuits interacting with each other. Even though you added mass, you also cut the air gap in half on each side of that new leaf: The RF for each half went way up. You split up your single low-pass tuned circuit into a pair of series-connected low-pass tuned circuits, each of which has a much higher cut-off point (resonant frequency) than the original.

In other words, you now have two lousy walls together, instead of one good one. But now it isn't just a matter of figuring out the MSM of each wall and adding/subtracting/dividing/multiplying the RFs and the isolation. Rather, you have to treat that wall as a new tuned network, where all the components take part together. It's like opening up your car radio and sticking an extra capacitor into the tuner circuit: you re-tune the entire thing. The end result is a wall that has much higher overall RF, and will no longer isolate well at the low end.

OK, that doesn't answer your question, since I specified that this hypothetical 3rd leaf goes inside the wall, cutting the space in two. But the same rule applies if you add a leaf outside that wall: you are still re-tuning the entire system, and you still need to treat it as a tuned network. The same concepts apply here as with a 2-leaf: as you add mass to the third leaf, the overall RF goes down. And as you increase the air gap, the overall RF also goes down. Obviously, if you put the third leaf a mile away, it ain't gonna do nuttin! And if you put it an inch away, it sure will do sumtin! So obviously the 3-leaf effect tapers off over distance. On the other hand, if your third leaf is an immensely massive six-foot-thick lead block, it ain't go do nuttin, even one inch away! But if it is a 6mm sheet of plywood, it sure will do sumtin! Once again, obviously mass plays a part.

OK, to get down to nitty-gritty: How far away does that third leaf have to be before it is not really a third leaf any more?

Mathematically, it gets a bit complex to figure this out, but as a very rough ball-park aproximation, you can kind of consider your original MSM wall as though it were just one single massive leaf. Yeah, that isn't valid, two filters connected, blah-blah-blah, but for a reasonably well designed, reasonably massive wall, it can give you a ball-park idea. So, on that (flawed) basis, if you add a single sheet of 16mm drywall 4 feet away from your MSM wall, then the resultant RF of this new wall, would be around 11 Hz. Plenty low enough to not be an issue. Put that same sheet of drywall one inch away, and the RF is is about 80 Hz! BIG Problem! (wall only starts isolating reasonably above 160 Hz!). If the drywall is 8 inches away: about 30Hz; Problem! (wall isolates above 60 Hz) 16 inches=29 Hz.=minor problem (wall isolates above 40Hz). 2 feet=15 Hz=acceptable (wall isolates above 32 Hz). 3 feet=13 Hz= fine (wall isolates at 26), etc.

So the question is: where do you draw the line? Once again, that depends. Are you recording heavy metal with massive bass guitars, and gigantic drums? Or are you recording the piccolo? For a piccolo, even with your 3rd leaf at one inch there is no problem. And for your heavy metal even at 2 feet you might have a problem. It all depends.... But at four feet, you don't have a problem (unless maybe you are firing canons and hand grenades in there!).

OK, that's a rather simplified (and arguably not mathematically valid) way of looking at it, but at least you get the idea: there is no hard and fast rule as to "how far is far enough?". The only answer is "it depends". Depends on the total mass of each leaf, the distance between the leaves, and the purpose of putting the isolation there in the first place. Like Einstein said: there are no absolutes, everything is relative to the observer.

Bottom line: But for a wall like yours, having a third leaf 4 feet way is not going to be an issue. The final RF is going to be so low, and you already have so much mass in there, that there just isn't any effect at all.

- Stuart -

[BriHar]

Hats off...

That's got to be one of the most understandable and clearest explanations of this effect I've ever seen.

Great work Stuart.

[Soundman2020]

Blush!

Thanks, Brian! It took me an awful long time to get my head around it, and I'm still not 100% clear on all of it, but I hope that helps others to get where I am on this issue!

- Stuart -

[Ro]

now, where's the "like" button ?

[GeorgeG]

Stuart

Fantastic! Thank you for taking the time. Very good tutorial.

My design presently has an 8" air gap between the 2 wall leafs (of 3X 16mm drywall each). Should I increase it to 12" if I have the room? In what way is 8" ideal in your earlier post vs the 12" gap discussed here?

The ceiling has some of the same issues because of the pesky pitched , vented roof. The plan is three layers on the underside of the truss cords (2x4, 24" O.C.). The inner acoustic room will probably have 2 x 10" ceiling beams (still working on the weight and span calculations) with 3 X 16 mm drywall on the bottom of the beams, creating once again a 2 leaf system with at least 12" of sealed air gap. Above all of that is of course the roof plane. It varies from a few inches away near the perimeter of the building to 6 feet above the ceiling in the middle. Can I also ignore the roof and just depend on the 2 leaf system below? This one is even harder to model because of the 6/12 roof pitch.

Thanks,