Introduction: Sub-Woofer Ottoman
I recently restored an antique rocking chair with the hopes of using it in my living room. It was a greatly satisfying project that I am very happy with. Soon after finishing the chair and testing it out, I decided that to really enjoy sitting in this chair I would need an ottoman. Its just necessary, in my opinion, to have my feet up while I rock. Hmmmm.... Rock.... The idea light went on in my head.
As you might be aware of, if you've seen my last couple of instructables, I've been pulling things out of my basement to repair and restore them. Having been in the basement so often recently, I remembered that I have this massive old sub-woofer down there from my years as a DJ, a 15" sub-woofer. It occurred to me that I could kill two birds with one stone. If I incorporate this woofer into my ottoman, I'd be getting rid of the cheap laminated box in my basement and upgrading my sound system for a better music and movie experience.
Step 1: Tools and Supplies
- table saw or circular saw
- jig saw
- drill and assorted bits
- tape measure
- large clamps
- hobby stapler and staples
- oil brush
- 7 feet of oak, 1"x8"
- 24"x48" pre-made pine plank (found at Home Depot)
- wood glue
- 1 1/2" wood screws
- stain color of your choice
- pre-made wooden couch feet, large dowel, or 2"x2"x48" deck railing support (just something to make feet for it)
- cloth 30"x30", what ever patern you like
- pillow stuffing
- 12" or 15" sub-woofer
- low pass crossover (Radio Shack)
- speaker wire connection plate (or whatever they're called)
- speaker wire
- finish nails
- color wood putty
Step 2: Building the New Box
Building a box is not terribly complex, after all, it's a box. For a speaker, it is important to provide the proper amount of air space it needs to sound right. This woofers original box was about 2 1/2 cubic feet of air space. My new box will be a little less than that, but I'm on a budget. You can calculate air volume by multiplying the inside length x width x depth in inches. One cubic foot is 1,728 cubic inches. Here is a volume calculator.
So mine, for example, is 18" x 22 1/2" x 8" which equals 3,240 cubic inches. Once you have your volume in cubic inches, you divide that by the cubic inches in one cubic foot, 1728. So for mine, I take 3,240 and divide it by 1,728, which equals about 1.9. So my box has about 1.9 cubic feet, about a half a cubic foot less than the original box. In building yours, you can adjust your measurements to fit the specific requirements of your speaker. Oak is kind of expensive which is why my box will be about 1/2 a cubic foot less than the original.
I picked out some oak that would match my rocking chair, seven linear feet of 3/4"x8" oak. This will be the side walls of the box. Cut into four boards, 2- 24" and 2- 18" lengths, I started by gluing them together into a rectangle, using a T-square to make sure the corners were square. A good layer of glue is required in each joint to ensure that the box will beas air tight as possible. Remember that it's important to clamp wood pieces together when joining them with glue. It forces the glue into the pores of both pieces of wood. When the glue was dry, I further reinforced the corners with 6" strips of 1"x1" that I glued and screwed into place.
Now that the side walls are built, I used them as a guide to draw the top and bottom of the box on the two pieces of pine. The pine will be connected to the top and bottom edges of the oak box, instead of fitting it inside the edges of the oak box. This will give the sub-woofer a little more air volume.
After I cut the top and bottom pieces out, I went ahead and glued on the top of the box. When that was dry, I again glued and screwed in some 1"x1" strips to reinforce the connection. A speaker box needs to be very ridged.
Step 3: Cutting the Speaker Hole
The bottom side of the box will hold the woofer, as it was a down facing speaker originally. First a hole needs to be cut for it. Using a compass, I drew a 14 1/8" circle on my piece of pine. The hole only needs to be a little more than 14" because the actual cone is about 14". The circle is off center, closer to one of the short sides of the board, about 1 1/2" from one end. Next I put a 1/2 inch hole inside this circle as a starter hole for the jigsaw, which I use to cut out the hole.
The reason the speaker hole is off center was to leave room for the wire connector plate that I took out of the original sub-woofer box. If you are building one of these, a connector plate and speaker can both be bought at Radio Shack or an audio store. I mounted this plate on the bottom next to the woofer. It required a small 3"x5" rectangular hole.
Step 4: Sand and Stain
At this point I sanded all the outside pieces with 150 sand paper. Because I don't have a miter saw, the corners are butt joints. I had to pay special attention to these while sanding, so that they were sanded down to a smooth transition from board to board. Once smooth, I stained the oak with the same Sikkens stain I used on my rocking chair. The top and bottom pieces, both pine, I stained with Minwax Cherry stain.
The reason I stained the wood before I assembled everything, is because I used two different colors. If the box was fully assembled, it would be nearly impossible to keep the oak stain from bleeding into the pine, or vise versa. If you build this and use only one color, you can wait until the box is fully assembled to stain it.
Step 5: Clear Coat
This ottoman is intended to be used with my rocking chair so, like the rocking chair, I used Danish Oil on the oak. As for the pine top and bottom pieces, I wanted a little more protection so i used Helmsman's Spar Varnish.
Step 6: Feet
I used the legs of an old chair as feet for my ottoman, 3" pieces. You could use anything for yours; large dowels, 2"x2" squares, or a set of pre-made feet that you can buy at Home Depot. I measured in 1 1/8" from each corner and drilled a pilot hole in the bottom board. Then I glued and screwed a leg to each corner of the bottom board.
Step 7: Connect the Woofer
Before attaching the bottom board to the box, you will want to connect the woofer to the connection plate with speaker wire. Its very simple, positive to positive, negative to negative. Mine is a dual voice coil speaker so in the picture you will see two sets of wires.
When buying a subwoofer, there are a few considerations to make, and a few requirements of a home audio system. The woofer itself needs to be an 8 ohm speaker, home audio components run at 8 ohms . You don't need to use a huge woofer like mine to achieve good low end sound. A 10" or 12" woofer will be very sufficient for most of your music and movie needs, and will require less air space than mine. Another thing to think about is power, how much power does your sound system offer to run your subwoofer? Each speaker you look at will have a specific power rating which you should try to match to power output of your home audio system.
Update: A recent comment by Aeszok, reminded me that I forgot to mention a very important part of building subwoofers. When you send music to them through speaker wire, you are sending the full range of musical frequencies. Subwoofers are designed to pump out the low frequencies that your smaller speakers cannot achieve. To maximize this potential it is important to incorporate a Crossover. Crossovers separate low frequencies from middle and high frequencies, and allow a certain range of frequencies to pass on to your speaker. The idea is to send low frequencies to subwoofers, medium to high frequencies to mid-sized speakers, and high frequencies to tweeters.
There are many kinds of crossovers, each designed for specific uses. Active crossovers are usually a whole component that you incorporate into your sound system and will use power from your house, they plug into the wall or draw power from your car electrical system. Some of these crossovers will allow you to manually set which frequencies go to which speaker, from low all the way to high frequencies.
Passive crossovers use only the power sent through the speaker wire and are designed to allow only a certain set of frequencies through to a speaker. They are usually inside the speaker box. To my knowledge, most pre-built subwoofers that you buy in a store already have a crossover mounted inside its box, which is why I forgot to mention it.
A crossover that allows low frequencies to pass through should be used when building your box and can be bought at Radio Shack or other audio stores when you buy your woofer. Ask for a low frequency 8 ohm Crossover, it should look like a large capacitor. Also ask how to wire it in. It's been so long since I've installed one but i believe they are wired into the positive wire leading to your woofer.
Having one wired in will give you that low rumbling that you're after.
Step 8: Attach the Top and Bottom Board
After mounting the speaker and wiring plate, the top and bottom were ready to be connected to the box. They were both glued and clamped on to complete the box.
Step 9: Trim Pieces
Now that the box is essentially built, I needed to add the trim pieces that will cover the pine end grain and hold the cushion. I cut 1/4"x 1" strips of pine from the left over pieces of the top and bottom of the box. They were stained with the same color as the top and bottom pieces, Minwax Cherry, and finished with the varnish.
When they were dry I glued them to the pine top and bottom boards, with a 1/4" lip above the top board and down from the bottom board. This 1/4" lip on the top holds the cushion in place, and the bottom lip eliminates the speaker from view.
To further support these strips to the box, I put a couple finish nails into them. With a nail set I set them below the surface of the wood and then filled the holes with a little colored wood putty.
Step 10: Top Cushion
The top cushion of my ottoman was made so that it can be removed. In this way, the ottoman can become an end table if I want. I had to make a board for this, so I glued 3 - 3/4"x6" boards together. When that was dry, I cut the whole thing down to the same dimentions as the top board.. This will make the cap that I will attach the padding and fabric to.
When the "cap" is all cut, the pad is the last thing to be made. I just put the pillow stuffing on the cap, stretched my material over it, and stapled that to the underside. Simple as that.
Step 11: Enjoy It
Now it's finished and I can enjoy it. I think tonight I'll watch Die Hard with the sub-woofer hooked up, and feel all the explosions rumble beneath my feet as I relax in my chair.
Tomorrow I get to throw out that huge ugly sub box that was taking up room in the basement. I hope you enjoy yours like I know I will enjoy mine.
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