Building the computer subwoofer
I got sick and tired of the thumping noise coming from the gray box under the computer desk that was supposed to be a subwoofer. Hmmmm, what to do about the problem? I've always been a Do It Yourself kind of guy, so of course I would MAKE something better. I'm also "King of Cheap", which means that whatever I made not only had to sound better, but be inexpensive to boot. These pages describe how I put together something that exceeded my expectations regarding sound quality and didn't cost much either.

Let me say right here that I'm not a speaker designer. I found all I could on the topic of speaker construction on the Internet (lots of stuff). Then, I did nothing but read about it all for nearly a month straight before I made any purchase at all. That first purchase was actually two books on speaker construction authored by David Weems. In my opinion, this project was a very good way for me to introduce myself to speaker construction. There is no crossover network to design or construct, and no diffraction or imaging problems associated with hi-frequency drivers.
OK. Here are the design goals I had in mind:

1. Size: No more than 12 inches high. Width and depth were flexible because there is quite a bit of room under the desk in the corner where the box sits. I use the box for a footstool while at the computer. Plus it's in the den, so I wanted it to blend into the surroundings as much as possible.

2. Low frequency sound quality: I'm not a game player, but like to listen to music while sitting here staring at this screen. As mentioned, the old subwoofer was a one-note thump box that didn't have much musical quality. It sounded good making helicopter sound effects though.

3. Price: The goal was to keep the price under twenty dollars....a goal I nearly met using materials at hand. The only real expenditure was for the woofers.

4. SAF (Spouse Approval Factor) Definition: The sometimes elusive qualities of an object that define its acceptance to the wife. In my particular case, the order of priorities that would gauge the success of this project would be: #3, #1, and #2 above. I'm proud to say that this project is a 10 on the SAF scale. :)
I began the project by searching for woofers. I had to keep in mind that the "amp" that would be driving the sub was of questionable quality and power. Whatever speakers were used had to be fairly high efficiency for decent sound levels. I found drivers listed on E-bay that were supposed to be for Boston Acoustics subwoofers. They were small (4.5") 4 ohm, and cheap. They came in the mail two days that E-Bay. The only problem with them was they didn't have any listed specifications (T/S Parameters). Once they arrived, I tried for a while to get these measurements via computer programs using the soundcard in my PC with no success. So, I just dove into building a box. The first incarnation was a 1 cubic foot box. It was too big for the woofers, allowing them to suffer overexcursion problems because of underdampening. So I put a divider down the middle of the box, effectively cutting it in half. This did the trick. Sound improved immediately. The over-volume problem was quickly taken care of on the table saw....the box was cut in half. Here's what it looks like....
Click on photos to enlarge
The woofer cut-outs have been made in the plywood. I used a flycutter to do this. Clamping the material to the drillpress table is a must when using this tool. Cabinet was constructed of simple butt joints because I would be covering it later. If I were to do it over again, I would make the mounting holes for the woofers closer to the corners.
I added one brace to the inside of the cabinet to reinforce the area. I had read that the subwoofer cabinet should be as strong as possible. I didn't see a need to brace the sides because they weren't all that wide. The box measures 12" x 12" and is 7.5" deep (internal measurements). The plywood is surplus stuff that I get for 2 dollars a sheet. It's mostly void free and pretty sturdy.
I knew that I would be experimenting with the box for a while, adding fill, changing port lengths, etc.. This was the first speaker I've constructed, so I wanted to play around with it. With this in mind, I made the top of the box easily removable. It has an inset panel and is sealed with foam weatherstrip. I did everything I could to ensure an airtight box.
The woofers mounted. I ended up filling the box loosely with poly stuffing from Walmart, leaving the area of the port open. I also tried poly batting on the walls, but preferred the fill instead. Maybe the fill isn't technically correct for a ported box, but beauty is in the eyes of the :)
I experimented with quite a few lengths of ports. I figured what the heck, I had to buy this whole piece of pipe just for one piece, so I might as well see what different lengths do. The one that sounds best to me is 3.25 inch in length. With an internal box volume of .63 C.F., that vent tunes the box to around 36 hz.
The subwoofer is being given the first sound tests. Everybody had their go at it. I played classic rock, top 40 stuff, and Boston. It even had some Madonna through it. I don't have test instruments, but  ran some frequency sweep tones. Useable bass is present starting at 35hz. I estimate the F3 point is 40 to 42 hz. The amplifier may have a bass compensation circuit. Note the dinky old subwoofer in the foreground.
Page two shows how the sub was finished
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