by Austin
My first exposure to contact microphones was when I first wanted to amplify my autoharp. I found a contact mic from a local guitar store and used it for a while. I use a magnetic autoharp pick-up now, because I found the contact microphone amplified the noise of the bar chords being pressed and the loud clack of my belt buckle against the body of the autoharp. The contact microphone did work to amplify the vibrations within my autoharp, however, and I used that pick-up for many months.
What I now understand is that though contact microphones can loudly amplify some unwanted sounds and may not be ideal for some instruments, they can unlock and amplify the hidden sounds within everyday objects; they are a dousing rod for unearthing an entire world of new sounds and instruments. At Bent Fest 2007 I got to see Tim Kaiser perform for the first time. He proved to me and the rest of the audience the vast sound potential within a contact microphone, some hand-selected/-crafted objects, and lots of delay. He and Logan Erickson also hosted a workshop at Bent Fest 2007 on using contact microphones. The big lessons I took away were “listen with a stethoscope to find good placement” and “make good, solid physical contact with the device and the contact microphone.”
So, what goes into a contact microphone? A contact microphone can be made out of just about any piezoelectric disc, which is a brass disc with a smaller wafer of crystal soldered directly to one side. When the crystal vibrates, it releases an electric current. The crystal translates vibrations from objects it touches into electric currents, which travel through wires to your amplifier. It is similar to the way a regular microphone works; a paper cone vibrates a magnetic coil, which in turn creates a electric current. Piezo-electric discs, however, sense vibrations directly from the object itself, not the air the object is vibrating. This allows them to amplify minute vibrations from within tiny objects that we would otherwise not be able to hear well through the air. Piezo discs can also serve other functions as well, such as velocity sensitive drum triggers and drivers that will physically vibrate objects.
There is a wonderful, in-depth chapter on piezoelectric discs in Nicholas Collins’ book Hand Made Electronic Music. This book is a must-read. In it, he details how to make contact microphones. He recommends soldering a piezo-disc directly to a shielded audio cable, then dipping it in Plasti-Dip. My friend Mike Taylor (or Mic Tailor, if you will) and I made a couple of contact microphones this way, in 2006, using broken audio cables I had found at a thrift store and some piezo-discs from American Science and Surplus. Mike and Matt Dotson used several of them to put on a performance of John Cage’s “Cartridge Music” at a John Cage symposium here at NIU, using them in place of phonograph cartridges. Phonograph cartridges predate piezo-discs but essentially do the same job, amplifying tiny vibrations. The mics we made were decent, but not great. The discs were very thin and relatively small in diameter; even with a couple of coats of Plasti-Dip, I still ended up snapping the crystal wafers on some, thereby ruining both the mics AND several long, shielded cables. In a second attempt, I did better, but not best – I filled a bottle cap with hot-glue, placed the piezo-disc in the glue, and dipped the entire assembly. They ended up looking something like chocolate covered Oreos on the end of a guitar cable. I solved the durability issue, but the hot-glue filling the cap slightly dulled the piezo-disc’s sensitivity.
Now, back to Bent Fest 2007, the first time I met Nick Pelzwik. Nick had a display with a circuit-bent wobble television and some circuit-bent Barbie Karaoke delays. At Circuitastrophe 2008, I saw Nick again, and this time he was armed with a ton of delightful contact mics. Nick’s solution was simple, but ingenious: A Plasti-Dipped piezo disc soldered to a 1/4″ jack inside a protective cap via some short wire leads. They look and sound very good. Nick performed at Circuitastrophe using them on a trumpet, toy piano, his own throat, and other objects to produce a very rich soundscape. Nick makes and sells these on eBay, I might add.
Nick shared with me his source for good quality piezo-discs, so I ordered a bunch in the hopes of trying to improve my initial design. I found that these discs were wider in diameter, coincidentally the same diameter as beer bottle caps. The discs that I had used before were smaller in diameter than the bottle caps, causing me to pool the hot glue into the caps. Now the discs rest only atop a ring of hot glue on the crimped circumference of the bottle cap. This allows the discs to vibrate more freely than before while still remaining somewhat protected from breakage by the bottle caps. Instead of soldering directly to a shielded audio cable as I did before, I soldered to a short length of wire which connects to a 1/4″ jack housed inside two facing 2-liter bottle caps zip-tied and hot-glued together. So, the result is a solid, protective housing for the piezo disc and jack, made from mostly recycled materials, definitely following the re-use mentality of circuit-bending. The beer cap allows you to hold the piezo-disc firmly and directly to objects like a stethoscope without finger noise and to even use it like a guitar pick without the worry of snapping the crystal. I’m also happy with the two 2-liter cap solution for housing a 1/4″ jack; they are pretty solid once attached with zip ties and hot glue and very colorful.
I have made about 30 of these for starters and they are available at the GetLoFi store for $15 apiece or $65 for a six-pack, shipping included. And now, here they are in action:
[youtube]http://www.youtube.com/watch?v=ReuB3p7Wkig[/youtube](Note: In the video the magnetic autoharp pickup is not being used, a Bottle-Cap Contact Microphone is clamped to the autoharp.)
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