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PostPosted: Thu Nov 08, 2012 2:56 am 
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Location: Kearny, NJ, USA
I spoke with a structural engineer last night. He said I have an unusually well constructed attic -- in the 2% range according to him. It has 2 x 8 beams spaced 16" apart with a maximum span of 15 feet. He said they could easily support the weight of my design and the addition of a 1" layer of gypsum concrete.

I thought the house was very solid. You can feel it when you walk. Even jumping up and down yields no give.

So using gypsum concrete is another option for reducing airborne noise transmission through the floor. Right now I've got just a hair over 50 db street noise coming into the attic. Most of that is coming through the uninsulated roof rafters and walls, which I'll treat with hat channels, green glue and double layers of 5/8" sheet rock.

I'm not sure how much noise is coming through the floor, but some is.

In searching the web on gypsum concrete for sound control, most sites specify the gypsum be poured over a sound mat. I'm not concerned with impact noise, only airborne noise. So I'm not certain the sound mat will do much for me.

Also still not certain this is the best approach. Once again, one recommendation was adding two layers of plywood with green glue between the layers.

What do you all think about a floor treatment?

--Nick


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PostPosted: Thu Nov 08, 2012 3:29 am 
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Location: Santiago, Chile
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I spoke with a structural engineer last night. He said I have an unusually well constructed attic -- in the 2% range according to him. It has 2 x 8 beams spaced 16" apart with a maximum span of 15 feet.
something doesn't add up here. Gypsum concrete weighs in at about 1250 kg/m3 or 75 pcf. A one inch layer works out to about 6.5 psf. The rest of the construction is going to be at least another 3 psf, so your dead load is at least 10 psf. Live load I'm guessing at 30 psf, which is likely the minimum allowed anyway.

Plugging those into standard span tables, there is no type of wood that can span 15 feet under those conditions. You would need at least 2x10 if you used best grade structural joists, or 2x12 for ordinary joists.

The maximum span I can find for any type of 2x8 wood under those load conditions, is about 14'5", and most come in at around 14'.

No idea why this doesn't add up, but the tables I'm using don't lie, and are published by the American Wood Council.

And all of the above is just for the gypsum concrete floor, without considering the weight of the inner-leaf walls, which will be bearing on those same joists.

Maybe you should get a second opinion from a different structural engineer, or ask the first guy to re-check his calculations.


- Stuart -

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PostPosted: Thu Nov 08, 2012 3:34 am 
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Location: Santiago, Chile
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Right now I've got just a hair over 50 db street noise coming into the attic.
What is your target noise level inside the room, once all the soundproofing is in place? What number are you aiming for?

Quote:
Most of that is coming through the uninsulated roof rafters and walls, which I'll treat with hat channels, green glue and double layers of 5/8" sheet rock
Hat channel does not decouple the drywall (sheet rock) from the rafters. You should use resilient channel for that, not hat channel. Or if you must use hat channel, then you also need RSIC clips.

Two layers of 5/8" drywall plus Green Glue does seem like a lot of isolation for just reducing ambient noise a bit. Are you sure that you need that much? What are your plans for building doors and windows to the same level of isolation?


- Stuart -

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PostPosted: Thu Nov 08, 2012 4:19 am 
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Location: Kearny, NJ, USA
Soundman2020 wrote:
Quote:
Right now I've got just over 50 db street noise coming into the attic.
What is your target noise level inside the room, once all the soundproofing is in place? What number are you aiming for?

Quote:
Most of that is coming through the uninsulated roof rafters and walls, which I'll treat with hat channels, green glue and double layers of 5/8" sheet rock
Hat channel does not decouple the drywall (sheet rock) from the rafters. You should use resilient channel for that, not hat channel. Or if you must use hat channel, then you also need RSIC clips.

Two layers of 5/8" drywall plus Green Glue does seem like a lot of isolation for just reducing ambient noise a bit. Are you sure that you need that much? What are your plans for building doors and windows to the same level of isolation?


- Stuart -


Hmmm. Well, I certainly don't want to take any chances with a catastrophic collapse.

Engineer did mention that the fact the roof is sloping to the floor makes a difference. My design doesn't put weight on the full 15 foot span. If you look at the floor plan, the bottom 2' 7" is outside my studio wall. From the inside of the studio wall to the supporting wall is about 11 feet. That's the widest area that will support the weight.

Again, I don't know how much sound is coming through the floor. Perhaps I should go ahead and construct the walls and then do a sound measurement to see how much is getting in. I have no doubt, it'll be considerably less than the 50 db that I get from the street now. Then I'll at least know exactly how much sound I'm trying to remove.

I was going to use RSIC clips with the hat channels on the roof.

For the double doors, I was going to use sealed solid-core doors. For the external door, I was going to create a heavier door, following the design for a heavy door in Rod Gervais' book "Home Recording Studio." For the single window, I was going to cover with an additional acoustic window.

For most of what I'll be recording (pop, jazz & rock), I don't need complete silence. However, it's likely I will be recording some classical flute and string ensembles, because two of my kids are classically trained musicians. I will also be recording a cappella singing now and again.

It doesn't need to be perfect, but it should be quiet -- a lot quieter than it is now. I'd rather err on the side of too quiet and wasting some money, within reason.

Maybe gypsum concrete isn't the way to go.

That said, budget IS a concern. I'd love to keep the construction costs to around $25K, including electrical and HVAC work.

And I'm borrowing from my retirement fund to come up with that much. (Not a wealthy man unfortunately.)

I've got good mikes, decent recording equipment and excellent musicians. The problem is I need a good space to record them. I am trying to do this right (again, within reason).

What do you think Stuart?

--Nick


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PostPosted: Fri Nov 09, 2012 10:57 pm 
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Joined: Tue May 15, 2012 10:17 pm
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Location: Kearny, NJ, USA
I'm leaning heavily back in the direction of the acoustic-supply salesman's original suggestion of adding two layers of plywood with green glue in between on top of the existing floor boards.

I'm thinking about sealing all the cracks in the floor between putting on the first layer of plywood but wonder:

a) is there an easier way to seal the floor?
b) is this step really necessary or will the green glue under the first layer of plywood suffice?


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