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DIY Sub-Mic

Somewhere in your collection of gear, you might have an old guitar amp, or a blown pair of speakers lying around. If you can salvage an intact (unbroken/blown-out) speaker cone, you can make your very own sub-mic for next to nothing.

Speakers and dynamic microphones are made with some of the same kinds of components: the sizes are different, and signal flows in opposite directions. One converts sound into signal, while the other converts signal into sound. Wire them backwards, and you can turn one into the other. Wiring a speaker backwards will turn it into an extremely large-diaphragm microphone, ideal for capturing the extreme lows on kick drums and other bass instruments. Speaker-mics typically have poor high-end response, but when the signal is blended with another microphone, the combination can produce massive full-spectrum sounds.

SpeakerMic18The wiring for this is fairly simple: connect the positive terminal on a speaker to pin 2 on an XLR cable (or to the tip on a 1/4″ TRS plug), and connect the negative terminal to pin 3 (negative on the TRS). Ground can be wired to pin 1/ground, if present. At this point, the speaker-mic is functional: you can plug it in and record. However, there are a few problems to deal with. The speaker-mic has a figure-8 polarity pattern: it will capture sound sources from in front of and behind the cone. The design is also awkward, so we will need to figure out a way to stabilize and mount it in front of a kick drum. One of the easiest ways to do this? Stick it inside another drum!

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The Yamaha Subkick ($400) is a 6″ speaker mounted inside a 10″ shell. Mine is twice the size at a fraction of the price.

What you will need

SpeakerMic003One functional speaker cone  (not blown out), at least 6″ diameter (bigger is usually better for our purposes: mine used a 12″ speaker salvaged from an extra guitar amp). If you have a pair of monitors with a blown speaker, use the good one: a lot of recording studios used Yamaha NS10 cones when one of theirs blew out. Amps & subwoofers work too.

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One drum shell, preferably wood: snare drums work well. remember, it needs to be larger in diameter/depth than your speaker. I used a 14″x5.5″ Pearl Export snare, but a 6.5″ depth may be easier to work with, depending on the speaker.

IMPORTANT: notice how the hardware attaches to the side of the drum on that shell. Two bolts/holes for each piece of hardware: one bolt by each rim of the shell. This design will make mounting the speaker easier in most cases. Also a drum with 8 lugs (or even 4 for that matter) will be more convenient when building the shock-mount.

1 standard XLR microphone cable (I used an old Switchcraft male XLR jack and some scrap Belden 8451 cable – balanced audio cable). Some people use an unbalanced 1/4″ plug (guitar cable) and plug straight into an external direct box – whatever is more practical for you.

2 mesh drum heads, or two regular drum heads & some speaker grill cloth

A few feet of rope (small enough to thread through the screw holes on the speaker)

Half a dozen bolts, nuts, & washers (to mount the XLR jack, if you’re using one)

If you want to get creative, paint/stain & varnish: one color to base-coat or stain, another to write on your name. For the fine-detail work use some thicker paint (I used acrylics) that won’t run. Spray paint if you’re lazy.

1 postcard

tape (scotch or electrical)

zip ties

some foam (destroy some old cushions or something)

Preparing the Shell

Since I wanted mine to stand out (i.e. I don’t want anybody ‘accidentally’ walking away with my new toy) I decided to paint my name onto it. Since I also decided to mount an XLR jack onto mine, I had to drill a few holes first: better to do this before you paint in order to make it look nicer. NOTE: if you actually want to keep the shell intact, you can just as easily slide the mic cable through the riveted vent in the side of the shell: just remember to have the male end sticking outside of the shell when you’re ready to solder.

Skip ahead if you don’t want to bother painting/stenciling the shell.

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Take off all of the hardware, sand off any glue residue (leftover from the shell wrap) and drill your holes. As you can see, the 3/4″ hole is a bit lopsided. This is because I had the bright idea of using the existing vent as a pilot hole. It is usually too wide to keep a large (flat) wood drill bit steady, so you’re better off drilling a fresh hole. Attach the jack now (loosely) to see if you need to make any adjustments. If all is well, take off all of the hardware so you’re left with the bare wooden shell.

SpeakerMic07PAINT IT BLACK (or neon pink). I used a “Water based (oil modified)” stain for mine. Lay down some newspaper and follow the directions on the can. As soon as I set the can on the table, my dog knocked it over, staining the floor of my garage. A little stain goes a long way, so I actually had enough left over to finish two drum shells with the dregs – two coats.

Since you’re going to want to put some sort of identifying name/logo/symbol/etc on the shell, make a stencil so you can reuse it. I made my design with Adobe Illustrator along with a stencil font modeled after the the band Crass – fitting for a low-budged DIY project.
http://www.dafont.com/crass.font
Remember, you’re making a stencil, so closed letters & symbols like A, B, D, O, P, Q, R, and so on need to have the breaks & openings.

SpeakerMic09I made this to fit in between the lugs, centered over the XLR hole, so the dimensions will vary depending on the size of your shell.

Print it out, and clip it on to a postcard (something sturdier and more moisture-resistant than paper – just as long as it’s still flexible)

When you’re ready, attack the paper with a scalpel until the postcard underneath looks like a functional stencil. Try to be neat, you don’t want to ruin the stencil: otherwise, you’ll have to do it all over again. This part can be very time-consuming, especially when the stencil for the other mic reads “THIS SPEAKER-MICROPHONE IS PROPERTY OF THE LOS ANGELES VALLEY COLLEGE RECORDING STUDIO”

SpeakerMic12Tape it to the shell (mask off the rest of the shell with newspaper if you’re going to use spray paint), and get your paint ready: remember, you want to use thicker paint for this, so it doesn’t form a runny mess underneath the stencil.

Warhammer nerds, put those painting skills to use. I’m going to paint this red, but I’m painting on a coat of white(ish) in order to make the red more vibrant, since acrylic paint will actually show some of the color beneath it (alternatively, if you’re trying to make the letters darker, use a darker color underneath). It’s actually better to paint the lighter undercoat in the center of the stencil (avoid the edges of the letters so you don’t have a white outline around your red letters).

Since it’s a black shell, you can fix any mistakes with a sharpie. When you’re done, throw on a coat or two of varnish to protect everything. Let the paint/varnish dry.

Mounting the Speaker

Some people have routed out the center of a circular piece of plywood, cut/sanded to fit inside the shell, but our rope method will function as a shock-mount, without damaging the shell. Depending on the size/weight of the speaker, you may be able to get by with bungee cords more effectively, but ours is too large & heavy for that.

Attach all of the hardware (minus the snare strainer/bracket, etc. Set the rims aside for now) – just the lugs, tightened on halfway so the bolts/washers stick out inside the shell.

Lay the shell down on its top, and prop up the speaker so the front lines up with the first set of bolts sticking inside the shell: books, magazines, etc, just as long as the pressure is on the outside frame of the speaker, not the cone itself. If your drum has 8 lugs, line them up with the 4 screw holes in the speaker. Try to line up the terminals on the speaker with the XLR jack or vent hole (if you didn’t mount the XLR jack). Once the speaker is centered at the right height, grab your rope.

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Tie a knot in one end, and thread it through the screw hole in the speaker.

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Tie a knot in the other end, wrap it around both lugs, and adjust it so the speaker is suspended in the center – not touching the shell. When it looks right, use a zip tie to lock the rope in place, and tighten the lugs down on the rope. Repeat on all of the other three lugs, opposite side first.

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Use the other 4 lugs to support the back of the cone, making sure that the upright speaker is completely parallel to the rim of the shell.

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Solder the Wires

If you aren’t using an XLR jack, now is the time to chop your microphone cable in half – use the male (pointy) end and feed the wire through the vent (which should be more or less lined up with the terminals on the speaker), letting the plug hang outside the shell. Try to secure the cable on the inside of the shell so you don’t rip the wires out by accident.

SpeakerMic20My speaker came with a ground wire attached (screwed on) to the frame, so solder it to the ground (pin 1) if you have one.

Before you attach the heads, hook the speaker-mic up to a mixer and tap/talk into the cone: it should be working.

Take a piece of foam, and trim it to fit behind the cone: the foam should be able to hide flush inside the shell.

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If you have mesh heads, go ahead an attach them. If you need to make you own, cut the center out of some old drum heads (leave the portion that makes contact with the rim) trim/lay down the speaker grill cloth, and tighten the heads on over them, front and back.

You now have your very own speaker-microphone capable of capturing bowel-moving low end frequencies. Throw it in front of a kick drum (usually a few inches to a foot away from the resonant head) along with another microphone on the beater to pick up the full range of the drum.

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