BUILDING YOUR OWN KICK DRUM MICROPHONE USING A COMMON 6 1/2" WOOFER SPEAKER
By Captain Ogre

It's old school. Way old technology that still works. It's been used to gain bottom-end emphasis out of early media, such as vinyl records and good old analog recording tape, where the technology of the day just wasn't quite to where we are in the brave, new, digital world.

The theories are all listed in any basic electronics textbook found anywhere. and it's incredibly simple to build your own using any myriad of household items. Maybe even using up some old, forgotten utility items that you have laying around in your garage somewhere. In fact, the basis of the design allows for your imagination to run wild!



How does it work? Well, simply put, a speaker, when wired in reverse- intentionally mis-wired where the positive lead from the source is wired to the negative terminal of the speaker, and vice-versa, changes the movement of the speaker coil to where it runs in reverse. The coil is actually pulling IN instead of pushing OUT. The speaker is acting like a microphone! (Remember this. This is why it is important to wire your car stereo speakers properly.)

Now, since the speaker has become a microphone, it should be noted as to what effect this will have on the overall sound of whatever you stick in front of it. Mics, in general, can be thought of as a small-diaphragm reverse-wired speaker. However, you should remember that they are NOT intended to be used as a speaker (sound goes IN, not OUT). And, most mics are designed to reach the mid- to high-frequency range- in other words, your vocal range. Ahh, but what about kick-drum mics? Well, mics that are designed for kick drums are built to reach the low end of the frequency spectrum. But, consider the size of their diaphragms- they're still just not made large enough to reach the absolute bottom of the spectrum. Plus, the really good ones are reeeeeeeeeeeeaaaaalllllyy expensive...

Now consider any given woofer-range speaker. Most woofers, especially those that are designed today, are designed to reach frequencies below 100 Hz- right about where your kick-drum sound is. So, having one rewired into a microphone just makes sense. If your PA system can deliver the full range of sound that you want to deliver, why wouldn't you want to get the full bottom end from your drums delivered out front?

A SPEAKER COMPARED TO A MICROPHONE
Most microphones have an impedance of around 150 ohms. This resistance is necessary to "attenuate" the input signal- in other words, turn the volume down just a bit so you won't blow an input channel. Speakers have an impedance rating that is much lower- 4 ohm, 8 ohm, 16 ohm, etc. This is quite low for the input of a channel. In fact, when I tested mine this way, I had the channel fader and gain control completely OFF (knobs all the way down), and I was still pegging the peak meter on that channel. Therefore, the speaker-mic needs to be turned down a bit so it's not too hot in the channel. That's where an attenuator comes into play. It will be wise to make, buy, or otherwise acquire, a -20dB attenuator. For those that aren't too electronically inclined to build their own, you can buy an attenuator that will connect inline with an XLR cable. Just remember to connect this on the speaker-mic end, and not the channel end. (You're simulating a mic, remember?) For those of us that like a cheap challenge, the following shows how to build your own.

HOW TO MAKE A -20dB ATTENUATOR
Below is a schematic that demostrates a typical H-pad attenuator, mathematically set for -20dB:



The top and bottom resistors that are connected to the speaker simulate the required input impedance of the channel, at 150 ohms. The top and bottom resistors connected to the XLR plug simulate the actual attenuation. On an XLR plug, pin 1 is ground, which is connected to the speaker frame, pin 2 is the plus (or HOT) wire (usually RED), and pin 3 is the minus (or COLD) wire (usually BLUE. Get it? Red for Hot and Blue for Cold, just like your faucet).

Yes, you can get 62-Ohm resistors (www.partsexpress.com), but they're not always available at your local Radio Shack. You could try potentiometers, but this lends itself to constant calibration and recalibration (not to mention having to have a can of contact cleaner handy for it most of the time)- too much cost, too bulky, and more trouble than its worth. So, let's look at the "in-a-pinch" option- about $4 worth of parts and a lot less time:



OK, let's explain what's going on. If you chain two resistors together in series, the total resistance sums up (33 ohms + 33 ohms = 66 ohms). So, we have four series-wired 33 ohm resistors, each chain getting 66 ohms. Since common resistors have a tolerance rating of somewhere between plus or minus 5% and 10%, you now have an approximation to the mathematical 62 ohms in the previous example, above. The center resistor is also close enough to approximate 35 ohms mathematical at 33 ohms actual. That's close enough to your -20dB target, and allows for plenty of "headroom" in your input channel.

WHAT PARTS MATTER WHERE
Well, the speaker itself doesn't really have to be anything fancy- its wattage rating does not matter, since it just became a microphone. It does, however, have to be a woofer, since you want the bottom end, not the top end where a tweeter runs. You could K-Ball one from an old stereo speaker box. You might even try a small subwoofer. The speaker's impedance might have an effect on the attenuation, if its impedance rating is greater than 16 ohms. But, for the most part, all you would mainly be concerned about is a torn cone, sprung frame, busted wire, or any other physical damage to the speaker.

And, since your drum kit is going to be used mostly on the stage, looks may be pretty important. You may want to dress up a surround for your speaker mic that not only supports all your parts, but can handle the rigors of the road. Also, if you have some outstanding artwork on your front skin, you may not want your speaker so large that it covers the front skin up. I used a 6 1/2" (8" actual) diameter woofer. It's small enough to where it can fit near the side of the skin and be plenty effective, and not detract from the entire front-skin art. You also want to consider placement, in the form of distance from the front skin. Obviously, this thing's going to be hot enough to eat a channel quickly if it's too close, so don't plan on hanging it directly inside your kick drum- bad things will happen. The way I did it was I targetted my projection spot on the front skin, started the mic placement at about 12 inches away from the drum, and moved the mic forward until it peaked at 0 dB on the channel, then backed it off just a bit and tweaked the channel to achieve the best sound.

Also, I used a "free-air" mounting method- that is, it pretty much "hangs out" in the free air. To do this, I simply used nylon wire ties to mount it to the frame. They're adjustable, provides a decent open-air shock mount, and are dirt cheap to replace if need be.

For the attenuator, resistors that are 1/4-watt to 1/2-watt will work. They're small enough to get on a perfboard and not be too ugly. You won't need anything greater than 1/2-watters- you're not dealing with a whole lot of voltage here. Speaking of perfboards, Radio Shack does sell small, pre-drilled electronic PC perfboards that will work just fine for this. If you can engineer your own, I highly recommend it.

As far as the XLR cabling goes- well, you might just have to get inventive here. In my case, I had a busted one and cut it right at the broke spot. It's possible to get XLR cabling and connectors at a well-stocked Radio Shack, or you could just find a cheap one and make it a sacrificial lamb for the cause of rock and roll. (Snip...)

HOW IT SOUNDS, AND WHAT ELSE CAN IT BE USED ON
The sound achieved is PHENOMINAL, when punching it through almost 3000 watts of amplification and out of a pair of 18" Black Widows. The bottom-end thump that knocks you around a bit is one of the most exhilirating experiences you can get. And the drummer is in total lust with it, too...

There are very viable real-world applications you can use this on. It is possible to mic all the beaters in your drum kit besides the kick drum- try one on your floor tom and see what that sounds like. It's possible to chain two speakers together in series, feed it to the attenuator, and get two mics into one channel. (NOTE: I've not tried this yet for various reasons, one being a possible feedback issue.) You might also try one in front of your amp- if it's lacking some low-end punch when miking it through the board, try a speaker-mic with it. For recording, you may find it wise, when miking drums, to use the speaker-mic in conjunction with your regular drum mics. Although you'll use a few more channels, it should provide a really full-sounding drum set in the final mix!

Build one, and give it a shot. You'll be amazed at what old-school tricks can do for your sound.