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Stc- oitc- why can't I find etched rating for exterior walls?

Discussion in 'Room Acoustics / Isolation / Treatment' started by Griff, Jan 12, 2015.

  1. Griff

    Griff Active Member

    I'm permitting a "garage" right now to put on my property. It will be 30x40 with a 10' plate and scissor trusses going up to about 12.5' at the peak. It will have my 72 Bronco it in and my tools etc, but I figure there should be enough space for a home studio too.

    For the exterior wall my plan is to put 7/8 plaster over 15/32 osb on 2x6's with r19 and rc2 with 2 layers of 5/8" drywall. I am hoping not to use green glue because that would cost a fortune on a building this size and I feel this wall should give a reasonable stc or oitc rating. Does anyone know of a source for sound transmission ratings for this wall assembly?

    I would like to be able to record drums bass and guitars live with a scratch vocal without being too loud during the day. Neighbors are about 60' away or so. I have never had a problem telling people we could only record drums for a few hours in the afternoons. Drummers have a short attention span anyways. I'm not looking to be a commercial studio but I can get pretty busy if I have the right space to record.

    Also, what is considered a reasonable stc for between control room, main room etc? I have recorded a lot of drums in a living room with 1/2" drywall on the wall and a bedroom for my control room and wished it was quieter but always got it done. Also, I recorded at a world class studio with 3 layers of 5/8 on each side with green glue. While it was nice I don't feel I need quite that much isolation.

    I don't have a studio plan yet. I'm really having a hard time sectioning off the space because I like the idea of being able to get a big drum sound. I might just have to put a sliding glass door going out of the main room into the garage so I can open it up to get room mics. I think in my old space the bathroom was like 30' away from the drums and I liked to put a mic in there for some space. Sorry for the rambling. Thanks- Griff
  2. Griff

    Griff Active Member

    Forgot to mention... For the control room I don't want to decouple the floor. But would be willing to spend some money on a door and window ( or just a sliding glass door) with decent stc rating. I was thinking if I can get a 55 stc door just shoot for that on the walls so I'm not wasting money. Does this sound good?
  3. Space

    Space Well-Known Member

    "For the exterior wall my plan is to put 7/8 plaster over 15/32 osb on 2x6's with r19 and rc2 with 2 layers of 5/8" drywall."

    Why have you decided on this type of construction?

    The STC and OITC (Outdoor/Indoor Transmission Class) are two different things. Stc, sound transmission class, has to do with what the human ear can hear and mostly has to do with interior boundaries and the other has to do with how well the exterior of a structure is doing.

    The TL (transmission loss) of the assembly is the more exact. While the information on your specific assembly is not immediately available, it can be located with more research.

    For me, I would not set my mind on RC until I have defined my building and the room(s) it may contain. It is a known issue that RC introduces a smaller air cavity which will alter the low frequency containment ability of the assembly, and this is the place you want to be the highest in respect to an audio environment.
  4. Griff

    Griff Active Member

    I decided on that because my plan shows 2x6's and shear panel, and I want plaster. I do steel framing, drywall and plaster for a living so ideally I would have built it out of steel but I couldn't get plans for a steel building the way I wanted to do it with engineering for a good price so I settled on wood.
    As far as the rc2 and 2layers of drywall that's what i am thinking but I'm open to suggestion. I am not familiar with the small air cavity you speak of unless you are thinking I'm planning on doing a triple leaf system?
    I get the difference of sic and oitc but never tried a search with just transmission loss. I'll try that next.

    Thanks for the reply-Griff
  5. Space

    Space Well-Known Member

    Well plaster is fine, shear is going to be there since it will be a requirement for the building to handle seismic and environmental loads.

    What we are discussing here is a decoupled mass/air/mass assembly, it is the standard for high degrees of transmission loss, what the webheads call sound proofing. RC is used in an attempt to establish a decoupled assembly. Problem is that it is not a true decoupled method and has data that shows you will loss transmission in the low frequency area from the small air cavity and the RC bars themselves.

    So while you are developing your plan you might consider, budget unseen, making the environment that you select to use as your musical area, to be a true room in a room fully decoupled assembly.

    Just my thinking.
  6. Griff

    Griff Active Member

    Ok, now I follow you. Yes rfc is not a true decoupling, I will keep looking for a rating on my wall assembly to see if it might get me where I need to be.

    You might be right, for the tracking room maybe the only way to get there would be a full decouple. There won't be any doors or windows going outside, but the sound may travel through the concrete but I'm not sure how much.

    As far as budget, I don't have one. I'm not rich, and I have to figure out how to add onto the house too since we had kids. I am mixing an album for a guy that does grading right now so he will grade it and dig my footings. I have friends that do concrete. An electrician friend wants to trade a truck I have for electrical. And the rest I can do. Still probably cost at least $50 or $60 k but I don't plan on counting just try to take it step by step. Treating it will be another big expense I guess.

    For the control room and rest of the garage- I have a shed right now I mix music in. I stay out there all night mixing and no neighbors can hear me. It is hardi siding, 2x4, r 13, rfc, 2x 5/8" drywall. So my wall I suggest should work for that.

    Thanks for the input you have me reconsidering my plan which is why I posted in the first place.
  7. Griff

    Griff Active Member

    Also, my footings will be 12" x 30" deep. Maybe that's a good chunk of mass to keep sound from flanking.

    I think I can get concrete done for about $10000 hopefully and the trusses are $2900. All plan and permit costs are paid. And I have material lists to send out for the lumber yards to bid as soon as I start the concrete. Spent about $5000 so far on soils report, plans , engineering, etc.
  8. Space

    Space Well-Known Member

    What do the reports say about the soil? Is it sand or rock? Here is the thing. With the kind of area you purpose and the still as yet undefined isolation requirements, You could have an isolated slab poured underneath the footprint of the actual recording environment.

    This would eliminate the heavy footer you are suggesting and get the cost down to a regular approved footer to support the exterior of the structure and then have the interior of the assembly sit on another slab.

    You have to think about this in 3D. Every possible area that can be decoupled should be decoupled.
  9. Griff

    Griff Active Member

    The top layer of soil has a lot of clay in it but further down it gets more solid, so that's why the footing has to be so deep. I guess you have a point. It probably wouldn't cost much more to isolate a section of concrete.

    I guess a loud band is 120db and my neighborhood is probably 60db so 60db of transmission loss would be nice if possible. I have a couple of neighbors who play loud music, not enough to interfere with recording but enough to mask some sound.
  10. Griff

    Griff Active Member

    I understand that a triple leaf wall assembly is not effective. But does that same rule apply if I build a room inside my garage since the airspace will be so much greater than a wall assembly?

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