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Raised platform - need construction help!

4531 Views 10 Replies 5 Participants Last post by  ALMFamily
Hey guys. This is my first post however, have been reading thru the forums for days. The amount of info and education has been incredible...glad to be here!

I am having difficulty taking a decision for the raised platform in my theatre room. Room is 11x17 and in a basement thus, my floor is a concrete slab. Platform will be 12" high (I have 10' ceilings) and 6 1/2' deep. Platform will be built entire width of room with two 6" steps up on both sides.

What I am struggling with is this idea of floating the platform on rubber (u boats seem to be the common theme). Some say they are needed, others say no. Is this necessary? Why not just pressure treated lumber on the slab? Is it correct that I should not not attach this to the rear and side walls?

Another topic is about porting 2" holes on the front between each joist (carpet over). Is this recommended? I do plan to use 2 layers of plywood on the top and fill (top to bottom) with insulation as I understand both of these are consistently recommended.

I am not so much worried about cost rather building this and then carpeting and having this awful hollow sound when my sub fires. Thanks in advance for the clarity!
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Welcome aboard.

Someone who has done this will chime in with an answer.

I think the idea is to decouple the riser from the room. I also think this is more important in an upstairs room where sound is more readily transferred through the walls. You will get some improvement in tactile responce decoupled. Make sure you cut holes in the rear of the riser on the plywood floor to allow sound to escape. What kind of sub(s) are you using?
Thanks Tonto. This is also my understanding however, I am looking for a sound view on whether it's really need on a basement floor (slab)?

Regarding the holes, are you saying to put the holes in the front AND back? For reference, the back of the platform will be against the rear wall. As for sub, my budget is eaten up on he room so I have tomuse my old equipment. I have an old school Velodyne 12" sub which still sounds great!
If it's in a basement, the rubber isolators are optional though still can be useful. As as been said, the idea is to decouple the riser from the room - which means building it after the room is drywalled and keeping it 1/4" or so away from the walls.

To use it for broadband control, if you want to cut holes in the joist cavities is fine, though I would want something larger than 2" diameter.

You can also cut some slots in the top deck in the rear of the riser by the wall where nobody will be walking and will be behind the 2nd row of seating. This gets some additional absorption in the 'corner' formed by the wall and the riser.

Thanks Bryan. Ok so definitely you answered my question about securing to back and side walls - answer is no, leave a 1/4" gap on each side which the base trim will cover.

Regarding the holes, are you saying if I do the front then I have to do the back? I assume the idea is to give the sound wave an escape point? If I don't do these, will it really make a difference? I am thinking so because the room is small. Regarding hole size, what do you recommend? I will be using 2x12s. Thx
No holes in the rear of the platform itself. Holes at the rear but in the top floor behind the seating. Not so much an exit point as an entry point by the 2 boundaries.

If 2x12's, 4-5" holes would easily still fit and leave enough strength.

Yes, you need to do the back. When I did my riser, I cut rectangular holes in the plywood flooring between each floor joist close to the back wall. In the front, instead of cutting holes, I ripped a 2x6, notched the joists & placed 1 on the bottom & 1 on the top, leaving a rectangular hole in the front for sound to enter (10" joist).
Thanks Tonto. Clear on the back however, how wide and long? I am using 16" spacing.

Not quite following you on the front - are you saying you only did 1 longer rectangle? As for layout, my platform will be 11' wide. On the ends, I will have a 28" stairway and just a row of 2 chairs in the middle. Also, if I go with the u-boats (1/2" spacing off floor) will i still need front ports?

At this point, I think I may have confused myself. The platform will be 11' wide, 6.5' deep and implan to fill with insulation. If I am not interested in it being a bass trap, can I just frame it, enclose it and be on my way or will I have sound issues? Remember, it is on a poured concrete floor.

One last question. Regarding the insulation in the platform, assume this is the full depth from floor to bottom of plywood (10.5" in my case so R30)?
One last question. Regarding the insulation in the platform, assume this is the full depth from floor to bottom of plywood (10.5" in my case so R30)?
Yes - fill it without compressing it.
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