One of the most popular DIY microphone projects is the "subkick" and it's been around as long as rock and roll and spare NS10 woofers sitting around studios. It's an easy and satisfying build. There are probably hundreds of DIY subkicks on Google if you want to gather ideas.
First is my new Yamaha-style subkick. I used a cheap MCM Electronics 8" woofer and mounted it in a cheap 8x5" Ludwig tom tom. I cut away the drum head leaving about half an inch of material connected to the ring of the head. I was able to barely slip the head over the edge of the woofer, and the little bit of plastic I left was enough to firmly hold the speaker in place, while leaving room for the speaker surround, not interfering with it. It was a simple bit of construction and worked out pretty well. I'll probably add a $6 Pearl mesh head to cover the rear of the shell to keep things clean.
The tom I bought came with an arm (there were several in different colors on eBay) and the base I just used a junky old snare stand I had sitting around, that lucky enough has the same diameter bass as the tom arm. Buying the tom was the hardest part of the project for some reason. I wanted to keep it cheap. I paid $40 for the tom and arm, and about $10 for the speaker, which makes this a nice $50 project.
I drilled for an XLR jack on the side of the drum. The soldering is pretty straightforward. A speaker is backwards from a microphone diaphragm, so the (-) speaker terminal is actually the one that will be in phase. So I wired the (-) speaker tab to pin 2 of the XLR (hot). The (+) speaker terminal I wired to pin 1 (ground) on the XLR. I also wired pin 1 to a wire I ran to one of the lugs to ground some of the metal in the drum, why not I guess. Probably not strictly necessary. Pin 3 is unconnected. This is an unbalanced microphone but I ran into no noise issues in my testing. You might want an inline pad if you're getting really loud, something like a Naiant or Shure XLR pad, or you could build one into the subkick. I did not find the level to be an issue with my Mackie Onyx preamp, but I did have the gain at a conservative setting. With an API preamp you'd probably have to engage the pad.
Here's some photos and I'll follow with the construction and technical details:
Here's the first subkick I made back in 2010, it got used on an album by Green. Sounds very different from the new one. This one I just attached a mic mount directly to a 6" guitar speaker, and an XLR plug, and put some burlap fabric (acoustically porous) over the front to protect the speaker.
Here are two sound clips of the subkicks on the same drum, a 22" Pearl Export, which was not tuned (!) between takes.
I think you will hear that they sound drastically different, even though the drum tuning and setup is identical.
The answer is in this article from Sound on Sound:
Here's the gist of it in a quote,
"Returning to the Subkick idea, basically what is on offer is a relatively high-mass diaphragm (cone) with a huge copper coil glued on the back, adding more mass and inertia. The whole assembly is horrendously under-damped and almost completely uncontrolled. Not surprisingly, when employed as a microphone it has what under normal circumstances would be an appalling frequency response, completely lacking in high-end. What is important, though, is the very poor damping, because this means that the diaphragm (cone) will tend to vibrate at its own natural resonant frequency when stimulated by a passing gust of wind — such as you get from a kick drum. So really, the Subkick isn’t capturing the kick drum’s mystical subsonic LF at all — it’s basically generating its own sound. In other words, what we actually have is an air-actuated sound synthesizer, not an accurate microphone!"
So what you're hearing is the tone of the speaker at its resonant frequency, more than the actual sound of the drum. This is not an accurate microphone in any way, more of a bass resonator. I think listening to my two files will show you how much of the speaker you're hearing and how little of the actual drum. Pro Tip: Hold the woofer up next to your ear and gently tap the cone. You will hear a low note ring out, and this is what you're roughly going to end up with when you build your subkick. You could use this "tapping" method to select a driver that sounds kind of how you want before attempting a build.
Acoustically speaking, the smaller guitar speaker resonates around 80-100 Hz, as you can see in the first Pro Q2 screenshot below. Subjectively speaking, it's got a little less sustain, and a little more high frequency information, such as the click of the beater around 1K. If you load these files into your DAW and use something like Pro Q2 with a band-listen function, it's really informative to sweep around these sound clips and hear whats going on at different frequencies in isolation.
The big tom tom subkick puts across a much bigger "sub" sound, resonating at 60-ish Hz, with tons of information below that in the subs, and very, very little coming through in the midrange or top end. I hear a little more sustain to this one, which I attribute to the softer rubber surround on the speaker, as opposed to the stiffer paper surround of the guitar speaker.
So you've got two very different subkicks which I both think have usable sound. Every subkick is going to sound different and that's fine. Recording Hacks has a good shootout that will show this to be true too http://recordinghacks.com/2012/02/05/moon-mic-review/ Although I have to say, right now I'm very pleased with how the big blue one came out. It just looks impressive in person, people say "wow!" and I can't argue with the big low end. I think I'd rather set up a subkick than use an LF synthesizer like Max Bass, but those are fine too and something you can always pull up no matter what mics you tracked.