Framing with concrete exterior wall

[The_Nimaj]

Hi everyone, as this is my first post I'll start with a little overview. I'm attempting to design a studio that will be used as a live streaming / mixing room for a church. We want to have some sense of isolation, but as the location in the building we're using has some distance between the main sound source (the sanctuary), it's only critical to the point of being able to monitor relatively accurately when it's noisy, then having a decent (maybe 20 - 30dB) noise floor when it's quiet. As a reference, when producing sound levels of about 110dB from the sanctuary, we measured a 72.3dB (C weighted, slow) maximum peak and 68.1dB average in the area we plan to build.

The space is a corner of an unused part of the building measuring 14' x 21' x 18', but due to limitations/guidelines we're restricted to about 10' for our structure. Three of the walls are concrete (specific material still being determined the ) but it measures about 6" thick while we'd be creating a fourth wall with timber framing. I'm still in the planning/research phase and am considering going with a double wall approach and using the existing concrete outer wall as a single leaf in the system, a 1" gap between framing, then using 2 layers of 5/8" gypsum on the inner framing facing the studio.

I'm no designer or architect and I have very limited knowledge on building fundamentals (the build will NOT be DIY, we plan to hire professionals for this) but the bulk of the knowledge I do have has come from my studies in Music Tech in University, Rod Gervais's book, Youtube, this forum and others. So, the questions:

1. Is this general design viable (Using the existing outer wall as a single leaf)? If so, could I simply add framing to that part of the structure and then add the inner framing.
2. Do I need to fasten the outer framing to the concrete structure or can it be "free standing"
3. Assuming the above is possible, how does one go about framing the ceiling? Do I need to create a ceiling for both the inner and outer structures since we're not going to the actual building's roof?

Here are a couple of photos from sketchup of my idea. I can also upload the .skp file if that would be better.

Thanks in advance

Bryce

FramingIso.jpg

Framing TopView.jpg

[Soundman2020]

Hi Bryce, and Welcome!

It seems you have slightly misunderstood the concept of "room-in-a-room", also called "two-leaf fully decoupled MSM isolation". You only need A TOTAL of two leaves, and that INCLUDES what you already have.

In other words, the concrete walls ARE your outer leaf on three sides. That's it. You don't need any more outer leaf for those three sides. What you do need, is an "outer leaf" that closes off the fourth side, to complete the shell, and since it seems that you also have to lower your ceiling considerably, you will need an outer-leaf ceiling at whatever the maximum allowable height is. Of course, your fourth-side outer-leaf wall will go all the way up to, and will support, that outer-leaf ceiling.

When that is done, completing your outer-leaf "envelope", all you need to do then is to build your inner-leaf within that shell, and you are done! The inner leaf will consist of for single-leaf walls with a single-leaf ceiling on top. That's it. In other words, your stud framing all around, then drywall on only ONE side of the framing (not both sides!). And also with insulation completely filling the cavity between the two leaves.

This is the concept, albeit with stud framed outer walls, not concrete, but you can still understand how it works:

MSM-two-leaf-WallChunk-conventional--NOT-inside-out--one-room--S06.png

In the above case, there were only two existing walls, so two more were needed to complete the outer-leaf shell, then the inner leaf was built within that shell.

So that's how you would do it.

Now for some more general comments:

then having a decent (maybe 20 - 30dB) noise floor

If you are talking dBC as seen on a meter, that is very, very quiet! That's something like NR-20, which is what good pro studios shoot for. And since you are starting with dangerously loud contemporary music (110 dBC), getting isolation to 20 dBC is totally out of the question. That implies isolation of 90 dB, which is extremely extreme. To the extreme. But fortunately, you don't need that much, because you said that currently you are getting about 70 - 75 dBC in that exact location when the levels are at 110 in the sanctuary. Thus, if you really do want 20 dBC inside when it is 75 dBC outside, you "only" need 55 dB of isolation. That's still very high! Just not as extreme as before. That's in the ballpark of the level most high end pro studios are isolated for. Home studio builders are very happy if they can get 50 dB isolation, and ecstatic if they get 55.

That is doable, but you are going to need a pretty decent budget to do that. It can indeed be achieved with 2-leaf MSM, but once again, it's not going to be easy. 55 dB isolation is a tall order.

To understand why, here's some perspective:

The issue is that we measure sound levels on the Decibel scale, and that happens to be exponential. Logarithmic, actually. It's not a linear increase, as most people expect, but logarithmic, and it it's not so easy ot get your head around that. For example, if a car is going at 40 MPH, and then it accelerates to 80 MPH, it's going twice as fast. If you have 40 gallons of water in your bathtub, and you add another 40, then you have twice as much. But sound is not like that. If you are listening to music at a level of 40 dB, and then you turn it up to 80 dB, you did not make it twice as loud. Subjectively, you would say that it is now 16 times louder, because each time you go up by ten dB, we humans judge that to be about twice as loud. But in terms of actual sound intensity, the 80 dB is not double 40 dB. It is not even sixteen times 40 dB. It is actually ten thousand times more intense! I kid you not. Even though your ears tell you that a sound is twice as loud when it goes up by 10 dB, in actual real-world terms, it is ten times more intense (in other words, you need ten time more power to produce that sound). So, each time you go up by 10 dB, that's a factor of ten. From 40 to 50 is a factor of ten. from 50 to 60 is a factor of ten. From 60 to 70 is a factor of ten. From 70 to 80 is a factor of ten. That's four "factors of ten". 10 x 10 x 10 x 10 = 10,000. So going from 40 dB to 80 dB means the sound is 10,000 times more intense, even though our ears judge that it is only 16 times more intense.

The same happens for isolation: each 10 dB decrease in sound means that you have to block ten times more sound, and the subjective effect is that you hear it as being half as loud.

So let's do the above with your situation: To simplify, let's say that you have 75 dBC and you want to get down to 25 dBC, so that's a difference of 50 dB (note: Te difference is NOT 50 dBC! It is 50 dB.... Complicated to explain... ). 50 decibels means that you have 5 steps of ten (5*10=50), and each step of ten means "ten times the power" or "ten times reduction in power". So, that is 10*10*10*10*10 = 100,000 times. You need to reduce the sound intensity by a factor or one hundred thousand. Or state another way; you need to block one hundred thousand times more sound than you are blocking right now.

Like I said: it's a tall order. Not impossible, but not simple either.

More perspective: a typical house wall consisting of a 2x4 stud frame with drywall in each side and no insulation, produces about 25 dB of isolation. You need 55 dB, which is three "steps of ten". So you need to block one thousand times more sound than a typical house wall.

That's the reality of what you are asking for. Possible? Yes. Simple? No. Cheap? No.

To be very honest, unless you have deep pockets, I think you are going to need to lower your sights a little. It would be great to have a room that is quieter than the middle of the desert at midnight with no wind (that's what you are asking for, with a 20 dB noise floor), but it probably isn't a realistic goal. If you were in that situation, and somebody dropped a pin on the floor three feet away from you, from a height of just two inches above the floor, the sound that it makes would be about 25 dBC. That's the level of silence we are talking about. If that same person were to whisper something to you very softly, that would be about 40 dB.

So my question is: do you REALLY need a noise floor of 20-30 dBC? You did say that you only need that when there's nothing loud going on in the sanctuary, but to get that low noise floor you would still need about the same total isolation to get that. NR-20 requires pretty serious isolation. Here's the actual NR curve graphs:

nr-curves.png

Each curved is named from the point where it cross the 1 kHz frequency, since that's the middle of the spectrum. The NR-10 curve is just slight above the absolute threshold of hearing, and your NR-20 curve is one step up from there. I would suggest that NR-40 would be a more realistic goal to shoot for.

Assuming NR-40, and levels of 75 dB outside, you would need roughly 40 dB of isolation (no, it's not just simple subtraction... it's more complex). 40 dB is entirely possible, not too complex, and not too expensive. 40 dB isolation with 75 dBC outside would give you levels of around 35 dBC inside, which is practically inaudible for most people.

I'd suggest setting a goal of 40 dB isolation for your room.

Three of the walls are concrete

What about the floor? Is that also concrete? Is there something below that, or just plain old planet earth? If there's another room below that, then you have an additional problem. If the floor is concrete and is resting on the ground, then you are fine, as far as the floor is concerned.

Also, what is on the other side of those three walls? Other rooms? The outside world? There might be other factors involved.

I think I already answers the first two of your three questions, indirectly, so that leaves:

3. Assuming the above is possible, how does one go about framing the ceiling? Do I need to create a ceiling for both the inner and outer structures since we're not going to the actual building's roof?

I sort of covered this, but not in enough detail, so....

You say that you can only use 10' of the available 18' height, implying that there's 8' up there that is out-of-bounds for you. Therefore you do need to close that off with a new outer-leaf ceiling. That will tie in to your one extra outer-leaf wall, to complete the shell. That shell will need to be fairly high mass, and both the ceiling and the wall will need to have roughly the same surface density, in order to create a consistent shell. Thus, to clarify again: You have three concrete walls and some type of roof. You will build one single stud frame across the fourth side of that area and put drywall on only ONE side of that frame, then you will also build ceiling framing across the top of that, connecting to all four walls (the three original concrete walls and the new wall that you just built), and that will have the same thickness of drywall on it as the wall. All of that will be sealed absolutely air-tight, with not even a slight crack or gap.

That completes your outer-leaf.

Then you build your new inner-leaf within that shell. The new inner-leaf will consist of framing for the four walls and also the new inner-leaf ceiling, and all of that framing will have drywall on just ONE side of it, not both sides. That leaf will also be sealed completely air-tight. The cavity will be filled with suitable insulation.

So that's the general plan, but there's something rather important you are forgetting here: HVAC. You are going to have people working inside that room, so you need HVAC. People have this silly habit: we like to breathe a lot! We like to inhale oxygen and exhale CO2 and other gasses, along with a lot of humidity. We also sweat, and emit various types of odors. You do not want that happening inside a room that has been perfectly sealed, totally air tight, twice over! You need HVAC. And since HVAC involves poking large air ducts through your walls, that absolutely destroys all the isolation you just obtained. The huge gaping hole that you need for each duct is way, way bigger than what is needed to totally trash your isolation. So you have a problem: you need to build "silencer boxes" to put on each point where a duct passes through a leaf, and each silencer needs to be designed specifically to allow the right amount of air to get through, while still blocking the sound at the same level as the wall. It can be done, of course, but it takes careful design, and careful construction.

The same applies to your electrical system: you can't just chop holes in your walls to put in the outlets, switch boxes, and light fittings like you would in a normal house, office, shop, or whatever. Once again, you can't have holes in your wall, because even a tiny hole will destroy your isolation. There are ways of dealing with that too, and that has to be taken into account in the room design.

So, there's a lot of stuff here for you to be thinking about! Hopefully I've covered the important things to get you on the right track.


- Stuart -

PS. If it helps at all, I head up the sound team at the church I attend, and I also run live sound for church events all the time, so I'm rather familiar with what you will be facing, in terms of trying to get approval to do what needs to be done! Church budgets are tight, and there are usually "more important" things than sound and acoustics to think of. You and I both know that sound and acoustics are every bit as important as many of those "other things", but it's a tough job trying to convince the pastor and elders of that! Been there, done that... and I'm one of the elders too!

[The_Nimaj]

Firstly, thank you very much for the response! Simply posting here and getting a response is making this a bit more of a reality for me.

In other words, the concrete walls ARE your outer leaf on three sides. That's it.

Okay, perfect this is what I expected. In the picture, I have the outer framing on the concrete walls purely for adding insulation, but this isn't necessary I gather? Great, saves cost.

If you are talking dBC as seen on a meter, that is very, very quiet! That's something like NR-20, which is what good pro studios shoot for.

Yes! I know it's a bit of a lofty goal, and it's ONLY for when it's quiet, not during service. But nonetheless, this is not a big factor. "Reasonably quiet" would suffice here. So your 40dB target seems more reasonable.

The issue is that we measure sound levels on the Decibel scale, and that happens to be exponential.

Fortunately, (and perhaps not, at the same time ) I'm well studied on the basic physics of sound and speech and I know first hand the craziness involved with decibels and human hearing. It's really quite fascinating.

What about the floor? Is that also concrete? Is there something below that, or just plain old planet earth?

It's on an upper floor (not quite sure of the construction currently, but I am working to get these kinds of details; I realize this is very important as we have to absolutely ensure that the structure can support the extra weight). I know that much of the structure is masonry though. As for how that works towards isolation, I should be clear that isolation will be nice, but is not the primary focus/objective of this build. What's below is a stairwell, but have been told that isolation there is not a concern. The outer walls face outdoors.

Therefore you do need to close that off with a new outer-leaf ceiling. That will tie in to your one extra outer-leaf wall, to complete the shell.

Okay, this makes sense. The above space isn't necessarily "off limits" in that I'm sure we are able to utilize the space for other needs, such as HVAC. 10' was simply the stipulation we were given for this project, and I'm going ahead and inferring the to mean 10' for the inner structure. I believe I understand that this means the framed fourth outer wall will need to be a bit higher than 10' to accommodate some air gap between the inner and outer ceilings, yes?
On the subject of HVAC, it's fortunately something that i've kept in the back of my mind for this process and we're lucky to have both a go-to HVAC guy and Electrician to assist us. But from my reading and research i've come across some of the hurdles I need to be prepared for in this arena.

PS. If it helps at all, I head up the sound team at the church I attend...

This, 1000%. With the purpose of the room in mind, I understand that it won't be a world class room and that it's there to serve that specific purpose. But like you said, we both know how important these things are and so here I am, trying to make the best of what we are afforded. So it definitely helps, and I thank you for that.