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Riser Help

post #1 of 5
11/30/12 at 8:26am
Thread Starter 

Hello all.

I followed something similar to the way BIGmouthinDC showed in his riser construction (thank you sir!) but now I've read too far into other threads and being a noob I've gone and confused myself.

These are 2x12s, do I need a gap under the joists? I was going to use 2x10s or something like the other thread showed but I thought that was just for saving a few bucks and since the price was basically the same I kept it simple and used only 2x12s but now I worry if I should have space under the joists for air to flow? Maybe this only matters if I plan to use it as a bass trap?

That's where I really got over my head, the bass trap. The sub(s) will not go directly on the riser, I don't have the sub yet so I don't really know what the bass sounds like in there just yet. I have everything else but my sub isn't that great so want to get a new one. Is it a good idea to go ahead and drill the holes on the wall facing boards on the riser or is this something that must be tuned to how the bass works out in the room? Carpet will be going over this and I'm not doing the carpet but I'd assume with this pushed up 1/4 inch from the walls in the corner that they won't carpet those sides facing the wall so those holes would then be uncovered. Part of me wants to just insulate this riser and put the top on but I don't want to regret skipping something.

Another question, I already had carpet padding down from the previous carpet. Is this good to go between the floor and the riser or should I use 30# roofing felt. I have that on hand but wasn't sure if one would be better than the other here. Seems like both serve it's purpose for moisture but didn't want to leave the pad under there if it's going to have a negative affect with sound that I'm clueless on.

Finally, didn't realize there were so many types of insulation until starting this room. Would some pink R-19 serve it's purpose to fill the riser or do I need to go with this mineral fiber stuff or something more for acoustics?

Otherwise, I plan on following what I was reading and putting 2 to 3 layers on top, carpet and will have some lights. Feel like maybe I'm worried too much about it but really don't want to have the carpet go down and then have to pull it back to fix something I could have done better with some more effort.

Thanks in advance
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post #2 of 5
11/30/12 at 2:02pm
If you're just building a riser, and not going to concern yourself with the potential of using it for bass trapping, go ahead with your plans. 2x12 all the way around presents no disadvantage. Any fluffy insulation is fine. I don't think carpet padding beneath the riser under-performs roofing felt or anything else.

If you really want to get into the bass trap possibilities, it's a whole different beast. First, strictly speaking, anyone who is not Dennis Erskine or one or maybe two other people is mostly guessing about the effectiveness of their designs. Dennis posted once that he and only one other designer he was aware of have documented repeatable success with this - so caveat emptor. The design Dennis uses relies on openings at the wall - so the riser will have to be positioned along a wall, not in the middle of a room, like BIG's. It's been my observation that most DIYers who try this resort to just opening as much area as possible in the surface of the riser in the hopes of allowing it to breathe enough to function as a broadband absorber - as though the structure wasn't there at all, and there was a big pile of insulation on the floor. This design is not a pressure based absorber, and can't be tuned to address specific modal problems like Dennis's can.

Others may disagree with this analysis, but I think trying to tune a riser as a pressure-based absorber is a fool's errand for a DIYer.

Since you've already got one sub and are getting another, you're probably in better shape if you work carefully with positioning and allow the two subs to work together than if you relied on one sub and a riser trap.

Fred
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post #3 of 5
11/30/12 at 5:38pm
Subscribed!

Agree with the comments prior to mine from Fred. If you want to use your riser as a bass trap. You need to design it from the beginning. I tweaked mine a bit to attempt to be a bass trap, not sure of it's effectiveness, but I had the extra materials so, HEY.

Agree with the openings at the wall. Most of the time the riser is positioned in a corner/against a wall where the vents along the wall make sense. In my case, my riser is sitting in the middle of the room and I installed a cleat to stop the couch from sliding off the back if you plopped down on it! eek.gif So I put vents on the back sides and front and none on the top. Again, not sure if I wasted my time, but is sounds more solid and does not resonate.

About what to stuff it with...regardless of it's bass-trapping potential, stuffing it with unfaced "pink stuff" will keep it from resonating during you movie enjoyment AND while you are walking on it. Before I stuffed mine it sounded like you were beating an empty cardboard box with a stick. 2 x 12's go with the fluffy stuff. My own personal OPINION, the mineral wool works better in tight spaces (hence me using it in the majority of my build), where there is not a lot of room and you need max absorption. Having 12" worth of space, the fiberglass stuff will be perfect and most likely cheaper.

One suggestion is to put a pad under the carpet that you use on the riser. I skipped that part, in many ways wish I would not have.

The pad under the riser is just fine, and in some ways probably better than just roofing paper (vibration, leveling, etc)

Good luck
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post #4 of 5
12/1/12 at 9:15pm
Just moved from SoCal to WA and got a nice big house with a big bonus room. Thinking of building a riser in the future so we have two rows of seats. (family of 6, and friends) I'll be watching and learning! Thanks for posting this OP!
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post #5 of 5
12/3/12 at 7:00am
Thread Starter 
Appreciate the replies, thank you! I'm going to tinker with placing the sub (my old one still works ok actually) a little bit and see if I can understand this bass trap idea a little better. I'm wanting to finish closing the soffit and top off this riser but can't really do any of that until I place that sub if I want the wires out of the way. I see some good posts of constructing a bass trap riser so with mine built already, wondering if I'll just go ahead and do the holes (wall facing), fill it with mineral fiber and hopefully just don't construct something that has an adverse affect. Can't put my finger on it yet but there is something in this basement room that is not playing nice with acoustics. It's pre-finished, three concrete walls since it's on the end of the house but the walls are insulated and drywall over them but maybe just poorly insulated? Dunno, but think I need to look into this. Again, appreciate your thoughts on the riser.

Thanks!
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