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One of the major drum companies offers a "sub-kick" product that is designed to capture the extreme low end thump of a kick drum. It's packaged in a fancy drum shell and has a fancy stand and a fancy price tag too (over $250). Luckily, you can make one really cheap. All you need is a woofer-- 6-8" works great--some wire, an XLR jack and something to prop it up.
First, I bought an 8" foam-surround woofer on eBay for about $10. Next, I wired an XLR(M) jack to the speaker. I wired (+) to pin 2 and (-) to pin 1. I used some old scraps to make a bracket, and used some old rubber plumbing washers as gaskets (to isolate it a bit from the mount). I used a PVC T fitting to connect the bracket to the mic stand. Then I cut down a cheap MF mic stand and mounted it. It is actually plenty stable, and fits right up next to my kick drum.
The cost of mine was probably under $40 although I didn't actually keep track and many of the parts were just laying around anyway.
Update1: It sounds really great but the signal was REALLY hot, so I built an attenuator pad into the XLR jack. I created a simple "L pad" by placing a 10K ohm resistor in series with pin2 and a 1K ohm resistor (shunt) across pin's 1 & 2 (I didn't get very scientific with the resistor sizing, so this probably is not the optimal impedance matching - but hey, it works!). This drops the signal around ~20dB and helps so it doesn't overdrive my preamp.
I usually use it with an Audix D6 inside the kick.
Update2: I got an old 10" drum shell ($10), stripped the hardware off and mounted the speaker (mic) in it. This can prevent the sound from reaching the back of the speaker and causing phase issues (or cancelling some frequencies). I stuffed some foam in the back to reduce the potential of any "ringing" inside the shell.
Update3: I received some requests for some sound samples of the subkick. These are excerpts from some recordings I have done recently. If you have good speakers or headphones it should be obvious that the "A" sample is with the subkick.
Update4 (3-6-2010): I get emails about the attenuator (see Update 1), and even though I tried to explain that my resistor choices were non-scientific, everyone still seems interested in it. The fact is, you may not even need it depending on what kind of mic pre you are using. At the time I was using one of the built in pres on a Digi 002r and it did not have a pad and the signal was too hot. I was actually wiring it as unbalanced, so that may have had something to do with it.
Anyway, I did some experimenting with the H-Pad Calculator found here, and came up with a more scientific choice to try to match the impedance as best as I can. A mic would usually present a load of around 150-200 ohms and a speaker is typically around 8 ohms. So, by plugging in the following values:
we get an R1/2 = 0, and R2 = 10 and R2/3 = 100. I chose these values because then you can build this with off-the-shelf parts easily attainable, even at Radio Shack and it's pretty close to what might be ideal. In effect, you place a 10R across pins 2 & 3 and then the 100R goes from pin 2 or 3 to the cable you attach (that go to the speaker). And now I am running it as a balanced signal as well. Although I used some shrink tubing on mine, here's a picture of what it looked like before I put some tape on it and closed it up again.
It doesn't sound much different than it did before - but at least now we can all sleep better knowing our preamps are happy with their input loading.