Press your ear to a radiator, a fence post or the hull of a wooden boat and you hear something the air never carries to you. Solid things are busy with vibration: footsteps travelling through floorboards, wind singing in a steel cable, water knocking inside a pipe.
A contact microphone listens to that hidden layer. Instead of catching pressure waves in the air, it picks up vibration straight from the surface it touches, which is why it has become one of the most useful and most misunderstood tools in sound art.
This guide explains how a contact microphone works, why so many of them sound thin, how artists have used them since 1960, and which ones are worth buying or building when you start.
What a contact microphone actually does
Most contact mics are built around a piezoelectric element, usually a thin brass disc with a layer of ceramic bonded to it. When the disc bends, even microscopically, the ceramic produces a small voltage that follows the vibration.
Taped, clamped or pressed against a surface, the disc moves with that surface. It hears the object from the inside and ignores most of the sound in the room around it.
That isolation is the point. A contact mic on a guitar top or a piano frame barely registers the drummer beside it, and it can sit on a loud stage without the feedback problems of an ordinary microphone.
It also hears things no conventional microphone can: the creak of a tree trunk in wind, the hum inside a lamp post, the ringing of a long steel bridge as a bicycle rolls across it.
Why your first piezo contact mic sounds tinny
Almost everyone who plugs a bare piezo contact microphone into a recorder is disappointed. The sound is thin and scratchy, with no low end, and it is easy to blame a cheap mic.
The real problem is electrical. A piezo element behaves like a small capacitor, so its impedance rises as the frequency falls, and it wants to feed an input of around one megohm to keep its bass.
A typical line input sits near 50 kilohms, which turns the pairing into a high-pass filter that starts cutting below roughly 200 Hz. The plug-in-power mic inputs on many portable recorders are lower still, and the loss can begin around 1 kHz.
The fix is a buffer, also called a high-impedance or impedance-matching preamp. It does not need to add any gain; simply giving the piezo the high load it wants brings the low frequencies back.
Trying to rescue the sound with EQ afterwards mostly boosts hum and noise along with the missing bass, so it is worth solving at the input.
How artists have used contact mics
The contact microphone entered experimental music early. John Cage's Cartridge Music, completed in 1960, had performers push feathers, wires, matches and pipe cleaners into phonograph cartridges, while contact microphones were fixed to chairs, tables and other furniture.
The piece was first performed in Cologne in October 1960 by a group that included Cage, David Tudor and Nam June Paik. Cage's Music for Amplified Toy Pianos, from the same year, also relied on contact mics.
Since then the tool has spread into every corner of sound practice. Composers clip them to instruments, installation artists fix them to architecture, and field recordists press them against fences, ice, branches and machinery.
If you are drawn to objects that are built to be heard, our piece on sounding sculpture and how artists turn objects into instruments looks at the sculptural side of the same tradition.
Choosing the best contact microphone for your work
There is no single best contact microphone, because the right one depends on what you plug it into and how rough the conditions will be. These three are widely used and sit at very different levels.
The Korg CM-300 is a clip-on piezo pickup made to feed instrument tuners, and it costs very little. It has a 1.5 metre cable, a 6.3 mm mono jack and a clip that opens to 30 mm.
Treat the Korg as a cheap way in. Its clip suits the bell of a wind instrument better than a wall or a branch, and it sounds far fuller through a high-impedance preamp than straight into a recorder.
The JrF C-series mics are hand-built in Yorkshire by the sound artist Jez riley French, and he says thousands of artists, recordists and ecoacoustic researchers use them. They come with a 2 metre cable and a quarter-inch jack.
French recommends pairing them with his XLR impedance-matching adaptors when you record into XLR inputs, which deals with the bass loss described above.
The Cortado MkIII from Zeppelin Design Labs in Chicago puts its piezo sensor in a steel puck, linked by a 1.8 metre cable to a Class-A preamp with a balanced XLR output. It runs on 24 to 48 volts of phantom power.
It has a switchable 10 dB pad for loud sources and a bass boost switch that moves its high-pass filter from about 150 Hz down to 23 Hz. Because the preamp is built in, it suits anyone working with standard XLR recorders and mixers.
Whatever you choose, check your input first. A passive piezo with a jack plug needs a high-impedance instrument input or a buffer, while a mic with its own preamp only needs phantom power.
Building a contact microphone yourself
A DIY contact microphone is one of the classic first projects in sound art, and it fits into an afternoon. The simplest version is a piezo disc, a length of shielded cable and a jack plug, soldered together.
Protect the solder joints with a rubber coating or a little epoxy so the leads don't snap and the disc can't short against a metal surface. Shielded cable matters too, because a bare disc acts like an antenna and picks up mains hum.
Then add a high-impedance buffer. Published circuits built around a FET-input op-amp are a common route, and several makers sell small kits and boxes for exactly this job.
Building your own teaches you more about vibration than buying one. You quickly hear how the size of the disc, the way it is mounted and the coating change the tone, and you can make several cheaply to spread across a large object.
Getting a good recording
Contact mics are fussy about attachment. The tighter the coupling between element and surface, the better the sound, so use putty, clamps, gaffer tape or a magnet rather than holding the mic in your hand.
Placement changes everything. Sliding the element a few centimetres across a metal sheet or a tabletop can shift the tone from hollow and boomy to bright and ringing, so listen on closed headphones while you move it.
Leave plenty of headroom, since a hard knock can produce a surprisingly strong signal. Keep the cable still as well, because any handling noise travels straight into the recording.
Many recordists pair a contact mic with an ordinary pair of microphones in the air, catching both the inside and the outside of the same sound. Our guide to field recording and how artists capture the sound of a place covers that airborne side, including recorders and wind protection.
Contact mics, hydrophones and coils
A contact microphone is a close relative of the hydrophone, which often uses a piezo element too but is sealed and built for water. They are not interchangeable: each is designed and coated for a different medium.
If you want to move from solid surfaces into rivers, harbours or the sea, our article on hydrophones for whales, ice and rivers explains how underwater recording works.
A third family, electromagnetic coil pickups, listens to the fields around electrical devices rather than to vibration. Together these tools open up the parts of the world that ordinary microphones miss.
Start small and listen
You don't need expensive kit to begin. A cheap piezo, a buffer and a pair of closed headphones are enough for an afternoon spent listening to radiators, railings, trees and kitchen tools.
What you find there, the way a fence sings in wind or a bridge rings under a passing bike, often becomes the first material of a new piece.
About Dr. Mira Solheim
Dr. Mira Solheim is an art historian and writer focused on artistic research, Nordic visual culture and the intersection of art with technology and film. She writes for Artistic-Research.no on methodology, institutions and practice.