Noise reduction for a concrete box

December 26, 2012 7:40 AM

Hometheaterfilter: How much soundproofing do I need to do in a concrete box?

I'm looking to convert a room in my basement to a home theater. The room is basically a concrete box. Poured cement floor, 4 hollow CMU walls, 2 windows in the CMU walls, and a poured (not hollow-slab) ceiling for the garage overhead. The floor is a single pour for the entire basement (whole house).

I like film, so there will be plenty of that, but I'm really worried about noise control when I have a bunch of rowdy folks over for prime time sports games while kids are sleeping upstairs. I want to eventually carpet the room. I want a livable, enjoyable space rather than an austere listening room. Also, I already have the message that a solid core door is key.

I'm thinking that, at minimum, a floating slab using something like Owens Corning quietzone underlayment will be necessary for managing bass. I'm just trying to get a feel for where the cost/value is sitting for things like studs that decouple the drywall, cotton fiber batting for the walls, and the variety of ceiling solutions.

I'm a numbers guy, so if there's an acoustics engineering 201 you can point me at, I'm more than happy to play with formulas. I just need some real-world testimonial reference points for what numbers constitute an acceptable range for my case. An X dB reduction is something I understand abstractly, but pretty much means jack to me from a "will this annoy my sleeping child" perspective.

posted by bfranklin to home & garden (8 answers total)

I had a heck of a time finding numbers for this when I built my workshop. I would love to have found dB numbers for various wall assemblies, but couldn't come up with any specifics, even from the people with the multi-layer sheetrock and the split stud assemblies.

I eventually ended up with 6" thick walls and 2x4 studs, with staggered studs so that I reduce the amount of solid sound transmission structures all the way through the wall. This was a compromise vs the split studs and z-channel hung drywall because I wanted to be able to put heavy shelves on the wall.

If you're in the north SF Bay Area you're welcome to come by my workshop and we can close you in and make noise outside (or vice-versa) to show you what that wall structure does. It is noticeably sound deadening, but it's not as dramatically "run the router table at midnight and have the neighbors completely oblivious" as I was hoping for.
posted by straw at 8:30 AM on December 26, 2012

You're attempting to deaden the sound of a rowdy room, which is laudable. But keep in mind that you're not simply concerned with the needs of other, less rowdy people in the house--you're building a home theater, which has its own acoustical needs. Screw these up, and you'll have the equivalent of an acoustical tomb.

I got a lot of bang for my buck from sheetrocking my basement theater, but it was still home to harsh sounds and unpleasant echoes. Adding wall-to-wall carpet solved that. No, I didn't use an underlayment, just a plain carpet pad. But my bass seems to be fine.
posted by Gordion Knott at 8:39 AM on December 26, 2012

Assuming you understand the difference between soundproofing and acoustic treatment (one meant to keep sound inside, one meant to make it actually sound good), some good resources for this would be home recording studio construction. There are a few books on the subject, and a lot of good relevant information.

The three biggest keys for sound reduction are mass, no air gaps, and having 2 layers with a space inbetween.

It sounds like you've already got mass with the concrete. That will go a long way.

As for no air gaps, you want to make sure with whatever you do, you don't have any holes where air can move through. Even something such as a 1/16 gap below the door can significantly reduce the effectiveness of soundproofing.

To get 2 layers with a gap inbetween is a great soundproofing technique, but hard to do right and potentially expensive. If you've got enough room down there, you could do this by framing out the whole room with 2x4's, making sure none of the 2x4's touch the concrete anywhere but the floor, and then drywall that, which would give you one layer (drywall) with an air gap, and then another layer (concrete). Doorways and air ducts are tricky here. With doors you have to either have 2 separate doors, or one very very heavy door.

As for the flooring, a floating floor will go a long way to reducing bass transmission, but it's expensive, and has to be done right or it's not effective. Rubber pucks are often used, but if you don't have enough compression, or too much, they don't work well.

So, getting to reality, it comes down to how big is your budget, and how much isolation do you really need? Having concrete all around is great, since that puts you at a much better starting point than most houses.

If you don't want to shell out a ton of money, than the main things I'd suggest are:

1 - Make sure you have heavy doors (solid core designed for outer doors are great for this) that seal well. If you can see light shining around the doorframe at all, it's not sealed correctly. For my studio I put in a door designed for the outside of a house, and it came with weather sealing all the way around.

2 - Isolate your subwoofer. If you can get your subwoofer so that it doesn't have a solid contact with the ground, you won't transmit nearly as much bass through the house. You won't get as solid a bass sound in the home theater, but the rest of the house won't be booming all the time. If you have a light subwoofer, this could be as easy as putting a 2 inch piece of foam underneath it, so that the vibrations are reduced before making their way to the floor.

3 - Address your air vents. If you just have standard heating vents down there, sound can move right through. The way to address this is similar to a car muffler. Build a box outside the air vent opening. Line the inside with sound reducing foam, and put an opening on the box which is not in line with the air duct. This way, any sound will have to go into the box, bounce around in the foam, and then enter the air duct. Keep in mind, this will reduce the amount of air flow somewhat, so you may have to supplement the heating of this area with a space heater, although since it's a basement space, you might be fine.

Doing those three things should make a noticeable difference on a relatively small budget. To get more results than that will require a significant amount of money, or a compromise of some sort (such as turning things down when the kids are asleep, or turning off the subwoofer at night, etc.).
posted by markblasco at 9:19 AM on December 26, 2012 [2 favorites]

Oh, and one last thing to keep in mind, the more soundproofing you do, the more you are going to need to acoustically treat the room for sound quality afterwards. If you make a budget for this, don't spend all of it on the soundproofing, otherwise you'll have a room which is very soundproof, but sounds terrible on the inside.

Acoustic treatment doesn't have to be super expensive, but you'll definitely want something more than just carpet if you want a room that sounds good.
posted by markblasco at 9:21 AM on December 26, 2012

My small house has a basement room we use for this kind of purpose. Construction is almost identical (cement floor, cement block walls, and, a cement slab above the ceiling (it holds the radiant floor heating for the floor above). Dropped about 6 inches below the cement slab above is a cork panel ceiling (about 1/2 inch thick cork).

There is very little sound that comes upstairs (other than what comes up the stairs if the bottom room door is open...
posted by HuronBob at 9:24 AM on December 26, 2012

markblasco: Any specific books you'd recommend on the subject? I wish I could be more specific on budget. Contractors want ~40k to finish my entire basement without any fancy soundproofing and that room is probably 1/4 of my square footage. So my answer is less than the 10k to have someone else deal with my headache, but enough to consider the job done right.

I've been flirting with the idea of using commercial-style steel framing in the basement, which would make gapping the wall pretty easy -- yay, powdered actuated fasteners. I think I read that I'd need > 2" gap to avoid resonance. Losing 6" in width isn't a huge deal.

Additionally, are you referring to anything beyond addressing first order reflections with addressing sound quality? I've seen a few vendors that will do acoustic wall treatments with your own photography, so I was thinking about going that route.
posted by bfranklin at 10:38 AM on December 26, 2012

I am planning on doing something similar, and keep running into reviews of GreenGlue. It might be useful for you.
posted by blue_beetle at 11:26 AM on December 26, 2012

If you are already going to pay to have someone to this, than you can probably get it done the right way without it being much more expensive. The goal here would be to have the contractors finish the room with walls and a ceiling that do not connect to the existing walls or ceiling in any way if possible.

This is going to be a great place for advice on design of the room for both soundproofing and acoustics:

recording studio construction forum

If you are already in the house, the first thing I would do though is to put a sound system down there and see just how much sound leakage you get. Honestly, you may need very little if any expensive soundproofing. It may be that getting a good solid door is all you need to enjoy films at reasonable volume at night. Without being there to see and hear the space, I can't say what will be needed.

If you are really concerned about the noise, and this is a priority, than hire contractors who have experience with this type of thing. They will know what works and what doesn't.

GreenGlue is great for adding mass in existing walls (I believe you put it inbetween drywall layers), but I don't think it would do much for an existing concrete structure. I could be wrong, but that was my impression when I was researching this stuff a long time ago.

I ended up doing very little soundproofing to my space because to get it done right was going to be very expensive, but if you're allready going to be putting money into getting this room finished, than it might not be that much more to get it done in a way that is good for sound. Other than the link above, I don't know if I would have any suggestions as to how to find a good contractor for that.

One last thing which may be worth looking into if you don't want to spend a ton of money on this: You can try to find a tuner for your theater which has some built in compression, which in essence turns the volume down every time things get loud. By reducing the dynamic range, you can have your softer parts turned up to a decent level without the room exploding every time it gets loud. My tuner has that feature (it's called night mode), and it allows us to watch movies at night with our kid right above us without having to constantly turn the TV up and down.
posted by markblasco at 1:59 PM on December 26, 2012