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Discussion Starter #1 (Edited)
Hi, my theater will be about 13.5' wide by 20' deep. I plan on two rows of seating with a platform in the rear (stadium). The front seats will be about 11-12’ from the screen (106”) What, in your opinion, would be the optimum height and depth of the platform? I’m thinking 10” high by 65” deep. I’m not sure I need that much depth, but would be necessary if I want to be able to walk in front of the rear seats on the platform. Also, how should I insulate it? The platform will be framed using standard floor framing method (joists 16" OC w/ 3/4" plywood glued and screwed), should I fill my platform with standard fiberglass insulation? Sand? Foam? Any advice is appreciated.
 

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This calculator will give you the height you require for the riser..
http://www.theater-calc.com/

You only need to fill the riser with any standard fibreglass insulation to keep resonances under control..
Also a double thickness of the timber on top will help to deaden it as well..
 

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The width all depends on what you want to put on it. Most couches will be over 65" in width. Even two recliners can take up a lot of room. Also if you're looking to go 10" high or more then you might want an added stair on the edge to climb up. To really give you accurate advice we'd need a diagram of your room.

Without knowing what you're room is like then I would consider making the platform the entire width of the back wall. Depending on ceiling height I would go 12" with a 6" step. For a true stadium feel you'd have to go even higher but unless you have 10 or 12 foot ceilings then it would feel weird and would most likely not meet code (minimum ceiling height is 7' or 6'-4" under bulkheads in my area). As soon as you get over a certain height building codes dictate that railings must be in place also (over 18" I think).

You should also consider putting down two layers of ply or OSB for the deck and cutting it so that it hangs over an inch or so around the edges. That way when the carpet is tacked down it will give it more of a refined look (like most stairs are carpeted). One sheet of ¾ ply will make the edge fairly thin.

You will want to stuff it with insulation. Sand would be overkill I think and is hard to work with. Most sand you buy is damp and drying it out can be a pain. You would need a lot of bags to fill a 65"x48"x10" platform (18 ft³ worth). It would also be a nightmare if you (or a future owner) ever decided to re-purpose the room and demolish the platform. Stuffing it with normal insulation will make it feel really solid and stop any resonant frequency build up. If it were hollow it would sound like a drum. Stuffed it will fee like it's completely solid. I just stuffed mine with pink fiberglass and it worked really well (See this shack link).

If you used REW in your room and found and strong resonant low frequencies you could make the platform into a Helmholz resonator. Otherwise you could stuff it and bore a lot of holes through it make it into a huge bass trap.

Another thing to consider is if you want deck or rope lighting. If so then you'll need to wire it accordingly before you stuff it and screw the deck down. If you are planning on going all out then you might consider buttkickers (bass shakers) installed under the platform.
 

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My theater is 12.5' X 20'. My front row of seating is at 13' (eye position when seats are upright) and my rear platform is 6' deep and 9.5" high (2X8 with 2 sheets of 3/4 ply on top). Basically the rear of the front seats are pretty much right up against the riser. I think it works out perfect.

If anything, I would opt for an additional 6" of depth on the riser so it would be easier to walk when the rear seats are reclined.
 
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