Originally Posted by cuzed2
I am certainly far from an expert; but you should study up on how to construct the riser in such a way so that you get some "Bass-trap value". This will likely involve having some of the internal bracing narrow, so that it does not contact the floor (in other words you don't want to make many smaller cavities from this larger volume). If your perimeter is built of 2x10's on edge, than the you would want to "hang" some interior 2x6" joists from your joist hangers. Also before carpeting; you will need some bass-wavelength friendly-holes" either on the 2 x 10 faces, or perhaps on the horizontal deck back by the walls?
I had actually planned on doing this just out of cost savings without thinking about any other benefit as it would be cheaper to use smaller lumber for the interior of the riser and not having the insulation touch the carpet. I had planned on basically building a wall only on it's side say out of 2 x 8's with 2 x 4's 16" on center. Is this not the way to do it then? Do you possibly have a link or a diagram I could reference? The bass trap idea is great, too bad it was not my idea.
Originally Posted by HopefulFred
Having only just started thinking about designing a riser, here're my thoughts.
1. Start by knowing your distances to the screen. You'll have to commit. (says me)
2. Through experimentation, figure out how high the riser needs to be. Stacks of lumber, someone to help lift the chair, and that same person to sit in the front row and put their head in the way will all be required.
3. The easy part is managing the steps and lateral dimensions (though these are crucial practical considerations)
4. Do like your cousin said, and figure out some way to get some bass energy into it so that it can be absorbed (holes in surfaces - either along the wall or in the riser wall itself). Ideally, this would happen in a way that focuses the absorption at problematic frequencies (room modes), but really (so I've read) if you loosely fill the cavities with pink fluffy insulation, you broaden the frequency range that gets absorbed, while also diminishing the maximum effectiveness at any frequency. This is the most fool-proof approach, I believe. Though I could be wrong - it has happened before.
Great minds think alike. Here is what I did yesterday when Mrs RTROSE was home to be my Guinea pig. I'm glad I did the trial and error as I would have build the riser way too short. I was going with a 6" riser but I actually need a 10" riser.
My "test" riser.
Mrs. RTROSE was very concerned about my construction technique regarding the temp riser. To prove she would be safe I scaled the chair and riser combo, it creaked and groaned under the immense weight, bowed slightly but held its own. After being convinced she would not perish in a fall from the great height of 9 1/2" she became a willing participant to the riser height experiment.
Originally Posted by TMcG
RTROSE - here is the formula for calculating the resonant frequency of a Helmholtz Resonator
where f is the resonant frequency (Hz), c is the speed of sound (1120ft/s), p =3.14, S is the area of the port (ft 2 ), L is the length of the port (ft), and V is the cavity volume (ft 3 ).
I would build the riser 11" high using 2x10 on its side and then two layers of 3/4" plywood glued and screwed. HopefulFred is right about building a resonator by spanning the riser depth with 2x8 lumber on joist hangers and only using fiberglass batts for the 2x8 depth to dramatically increase the surface area within the volume and leave a 2 inch continuous gap at the bottom of the riser base which and vented through a port of a specific, calculated size...HOWEVER....
I doubt all of this effort would really be necessary as your room doesn't have any acoustic treatments whatsoever and has a large opening without a door to the right side of the theater. IMHO your time would be better spent trying to make up some 1" or 2" acoustic panels to help tame some of the primary reflections / room modes. At least you have the information and calculator to attempt the resonator if you so-choose.
One other suggestion - I would actually use some 1/2" or 3/4" plywood on the BOTTOM of your riser base so that you are distributing the weight load across the top of the carpet. The next owner could then easily remove the riser base and repurpose the room or have a couch, etc. Just a thought.
Oh great more math. I hate math.
I have to be very careful to how high I go with the riser as my ceiling is only 7' 9" Even if 11" was ideal I don't know if I could go that high. 10" is going to be my max I believe. Plus do to excellent planning on my part the projector will be hanging down in the "walk way" of the riser. I could possibly move the projector back, but don't want to do that unless it is absolutely necessary.
Putting the plywood on the bottom to spread the load is a great idea, I had thought about putting plastic or landscape fabric on the underside to help protect the carpet but doing the plywood would serve two purposes. I will just have to steal that idea for sure.
I had already planned on doing some type of acoustical treatments to the room, but as you point out with a rather large opening on the right side of the theater I'm fighting an uphill battle, and that opening is one of the main reasons why I did not go full bore on trying to do any sound isolation.
Thanks guys for the input. I've got some reading and research to do. Any other ideas or suggestions are welcomed and encouraged.
Oh and other links or info you could point me to would be great as well.