Part III
Now for the fun and sawdust in the house. Time to cut the hole in the floor after careful measurements and some pilot holes to find the I-beams. The goal is not to cut any part of the I-beams.
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Mini Sawsled works perfect for this cut:
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Now it is done. No Turning Back Now!
Here is what we are looking at for existing framing: You can see these are composite I-beams on 16" centers with 14" between the flanges. I need to finish these to match the THT when viewed from above.
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I will need to build a support "floor" for the speaker to sit on, and box in the I-beams with plywood so the THT throat will match the finished opening in the floor. Luckily, the standard plans THT throat opening exactly matches the space between the I-beam flanges, but I will have to frame out the web to match the I-beam flange.
I started with framing up a floor for the speaker to sit on with 2" by 4" in the crawlspace, that way I could stand on it while working on boxing the beams. I just used 2" by 4" for a "floor" tied to the existing walls by the corner and used one 4" by 4" post on a concrete pier block. It is important to leave more than 36" between the floor and the bottom of the joist to fit the 36" tall THT, or you will never get that box in there. I left the front of the support down 1.5" (37.5" clearance) and the back down 0.5" (36.5" clearance) so the box will sit on the platform with about 1" clearance in front and 1/2" clearance in back for positioning and adding caulk. I will shim the box into final height.
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With the support floor built, I can stand on it and wrap the inside of the I-beams with 3/4" plywood. A 3/4" plywood fits on the side of the I-beam and matches the I-beam flange perfect. Here it is test fit before I nailed it flush.
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Now filling in the ends between the I-beams: the ends are each three pieces and nail to the side pieces like a box. The three pieces are:, the top piece between the flange, the middle between the webs, and the bottom piece will be a 2" by 4" since the speaker top opening will mate with it, I want the bottom mating surface wide for a good seal. I glued and nailed everything. PL not needed, since I will cover all this surface with formica countertop material.
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Here is the bottom flange created with the 2" by 4" material: In hindsight I should have tacked a ledger around two sides of the bottom flange to line up the speaker perfect to the opening, since it is impossible to see what is going on below when positioning it. I ended up with many trips up and down to check for alignment.
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Working around fiberglass insulation always makes it more fun

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Finished framing waiting for the THT.
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At this point I wrapped the inside of the plywood opening with some formica I had. It doesn't match, but I can sand it with 240 grit and paint it to match if needed. I used formica so I didn't have to finish the plywood, sand it, bondo it, sand it again, prime it, paint it. I am done with sawdust inside the house.
Now I don't have any photos of the giant THT being dragged, strained, cursed, and wrestled into place 3' up onto the support floor, since I did it myself and couldn't hold the camera, but use your imagination what it looked like. It took many trips up and down to get the throat of the THT to line up with the opening before setting it. I had it perfect, then shimmed the THT box up tight to the floor after applying some clear latex caulk for sealing. Sometime during the final shim placement, I was too eager and hammered in one shim in too much and it moved the box 1/8" out of alignment. I wasn't able to move it back without pulling out shims, but it is not real visible from above so I left it.
Here are the finished in place pictures from above.
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With amp on wall hooked up and up and running. I may move the amp over by the stereo, it was already placed here when I had my THTLP in this location.
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I am toying with the idea about adding a light down in there, firing up for affect. I still need to weld a floor grate to keep pets from falling into that chasm. My welding skills are horrible and those photos may not be posted.