Choosing a Contact Mic For Field Recording

(This page is part of a series. See the rest of my contact microphone pages here. I also have a similar page about Choosing a Hydrophone for Field Recording.)

I’ve been experimenting with contact microphones for years, slowly refining my designs until they meet most of my needs, and trying out commercial offerings too. This page is my attempt to gather advice about selecting and using a contact mic for field recording. Scroll down for a table of DIY and commercial options.

What Are Contact Mics?

They are microphones that only respond to the vibrations of the objects they touch (rather than the vibrations of the air),  like sonic microscopes that reveal hidden vibrations too quiet to reach our ears through the air. On acoustic instruments, they are usually called “piezo pickups” to differentiate them from the electromagnetic pickups used on electric guitars. Specialized versions called accelerometers are used for scientific research. All of these contain piezoelectric ceramics (usually PZT) that transform vibration into electricity.

Only For Field Recording?

This guide is based on my own interests and my experiences with students, artists, and sound-designers who want to uncover hidden sounds from their environment. I hope it’s useful to a wider audience, but there’s a lot I don’t know: I’m clueless about which brands of acoustic guitar pickups sound the best, or how to choose an accelerometer for vibration research.

Resonant or Flat?

I classify contact mics on a spectrum from resonant to flat. (All transducers resonate at a certain frequency, but in this case I mean that they resonate within the audio band.)

  • Resonant contact mics are typically made with cheap piezo benders. They resonate sharply in the mid-range, producing a noticeable ringing or honking coloration to the sound. Frequencies above resonance will be severely attenuated. But the resonance also makes them very sensitive, so they typically require less gain.
  • Flat contact mics usually resonate in the ultrasound range, so their frequency response is quite flat throughout the audio range. They’re usually less sensitive, so the results tend to be noisier if recording very quiet sources.

Two Approaches

I think of contact microphones like musical instruments rather than instruments of science: I’m not interested in capturing an “objective” recording (if such a thing is even possible), but rather highlighting the certain details that are relevant to my interests.  I think it’s useful to consider two broad categories of contact mic technique:

  • Animation: You are putting something into motion: hitting, sawing, bowing, or otherwise “playing”an object like an instrument. For this technique, choose a mic and preamp that perform well with loud signals, and use low gain to avoid overloading your recorder. I usually prefer a flatter mic for this since I’m more concerned about balanced frequency response than noise.
  • Auscultation: You are listening to another world that is already in motion: a wire fence in the wind or the activity of an ant colony. These sounds are often tiny, so choose a mic and preamp that provide lots of gain and minimal noise. (The mid-range “honk” of a resonant mic may be unrealistic, but if it overlaps with a frequency of interest then I would consider the resonance as “free gain” to lift quiet sounds above the noise floor.) Prepare to mitigate challenges like rustling cables and footsteps accidentally transmitted through the ground.

Mounting

Since contact microphones are directly attached to the objects they record, physical details will have a big effect on the sound. Your contact mic has mass, so it might accidentally de-tune your object, or stop it from vibrating entirely. It seems counterintuitive, but it’s usually best to put the mic at a nodal point where the object isn’t vibrating much: the top of a bell instead of the edges, the corners of a metal plate instead of the middle. It takes a lot of experimenting.

Also consider where to support your object so it can vibrate freely: A vibraphone key has great sustain because it sits on soft felt pads positioned exactly at the key’s nodal points.

Attachment Methods:

  • Removable poster adhesive (Blu-Tak, etc) is a great way to quickly attach a contact mic. It’s flexible and doesn’t seem to attenuate the sound much. It won’t work on wet or dusty surfaces, though.
  • Beeswax seems to sound even better than poster adhesive, but it’s harder to find and can leave surface marks. Here’s a great recipe with sound examples.
  • A-Clamps work great. I have several sizes of inexpensive plastic ones. They tend to increase sensitivity and bass. (Not too heavy or you’ll stop the object from vibrating.)
  • Magnets are like invisible clamps. If your mic is thin, put a strong magnet behind it and it will stick to anything ferrous.
  • Convenient Rocks can be used in a pinch if your object is horizontal.
  • Tape is my last choice. It generally won’t provide a good bond unless you can wrap it completely around the mic + object sandwich, so it acts more like an elastic band. Thin double-stick tape is OK for permanent mounting but it’s hard to un-mount so you might damage your objects or mics.

You Need a Preamp

Like hydrophones, contact mics have extremely high electrical impedance, which is not well-matched to the mic inputs on recorders and mixers. You’ll get OK signal levels but very little bass, and an exaggerated “screechy” treble. The solution is to add a high impedance buffer preamplifier between the mic and the recorder. The difference will be dramatic!

  • Passive contact mics are just the piezo element and cabling. They require a preamp. (I’m highly suspicious of any claims that a passive contact mic sounds good without one.) Passive mics are cheaper than active, and more flexible since you could choose several different sensors to plug into a single preamp.
  • Active contact mics include a preamp, sometimes hidden in the sensor head or the output connector. They will require some sort of power (“plug-in-power” for 3.5mm types, “phantom power” for XLR types, or internal batteries). They are easier than a passive mic + preamp combo, but less flexible and usually more expensive.

Contact Mics Compared

A table covering many contact mics that seem appropriate for field recording, sorted loosely from expensive to amazingly affordable. ALL PRICES ARE APPROXIMATE. Manufacturers can change their prices at any time.  Get in touch if I missed or mis-represented something! (I don’t do affiliate links, and all opinions are my own.)

Model Approx Price (USD) response active? Plug and Power Notes
Barcus Berry Planer Wave Transducer 450 flat active XLR
9v battery or phantom
Expensive but sounds way better than the simple piezo benders. You can buy it with a preamp (4000 system) or just the pickup (4000PI) to plug into your own preamp. After spending the money I don’t think it’s a huge improvement over a thick PZT humidifier disc for most sources, and the shape makes it somewhat difficult to mount.
Trance Audio Amulet series
340 and up flat active 1/4″ with
9v battery or XLR with phantom
Expensive but sounds way better than the simple piezo benders. Smaller transducer and preamp than the Barcus Berry. (I haven’t used these but they are very well respected.)
LOM Geofón 165 EUR bass only! n/a XLR The LOM Geofon is wildly popular (especially on social media) but I’m not sure what all the fuss is about. It’s a moving-coil seismograph sensor that only picks up bass. Like all LOM gear it’s made in small batches several times a year, so scarcity adds to the mystique. You can get the same results from any active contact mic and a low-pass filter!
Cortado Mk III Contact Microphone 160 flat active XLR
phantom
Inline preamp based on Alex Rice design. DIP switches for bass boost and pad. The sensor (piezo bender) is potted in a metal case for controlled resonance. It sounds pretty good. (You can also buy a cheaper kit version or just the passive sensor. Very nice.)
Metal Marshmallow Pro 150 flat active XLR phantom The sensor head contains a unique preamp with adjustable gain knob. Low noise and high quality. I highly recommend this contact mic!
Jez Riley French (“JrF”) 35 GBP and up resonant passive 1/4″ A respected UK field-recordist and sound artist who also makes contact mics and hydrophones. Similar to other DIY piezo bender designs. (Jez tries to keep mics in stock, but it’s a side-project, so there can be delays.)
Monkey Sound 55 and up resonant passive 1/4″ Alejandro Garcia, a  Spanish maker of hydrophones and contact mics. Similar to other DIY piezo bender designs.
Crank Sturgeon 35 and up resonant passive 1/4″ Lots of oddball transducers from a veteran of the Northeast USA noise scene. Similar to other DIY piezo bender designs, but some have unique features that make them awesome for performance.
Korg CM300 Clip-On
15 and up resonant passive 1/4″ It’s just a small piezo disc mounted in the jaws of a plastic A-clamp, intended for digital instrument tuners so the sensitivity and frequency response is not great. Very convenient, though!
Round Piezo Pickups for Guitars (or Drum Triggers)
5 and up resonant passive 1/4″ (available cheaply on Amazon and eBay) They are just piezo discs like other DIY piezo bender designs, soldered to short cables with a female 1/4″ jack. They are often fragile and not well-shielded, but they get the job done and are very cheap.
Under Saddle Guitar Pickups AKA “Rod Piezos” 10 and up flat passive 1/8″ or 1/4″ (available cheaply on Amazon and eBay) An easy non-resonant option for louder sound sources. Inside you’ll find one PZT wafer for each string, mounted in a shielded metal channel. (Guitar pickups are pretty long but ukulele or mandolin pickups are shorter.) More info from C.B. Gitty.
DIY with Piezo Bender Disc 5 and up resonant passive n/a The most common DIY contact mic is a piezo “bender” soldered to a guitar cable. Piezo benders are cheap and there are many sizes (and resonant frequencies) to choose from. They are not known for their durability or sound quality! Buy extras because some will be destroyed during soldering and daily use. Here’s my quick-and-dirty construction recipe. (LINK COMING SOON)
  DIY with Piezo Bender Disc and extra metal disc 5 and up flat(ish) passive n/a My “daily driver” contact mic is a piezo bender glued to a metal disc, with a rugged 3D-printed back shell. The metal disc lowers the sensitivity but raises the resonance to a much higher pitch where it contributes to “clarity” instead of mid-range “honk”. Here is my recipe for a durable stereo contact mic that pairs well with my stereo plug-in-powered preamp. (LINK COMING SOON)
DIY with Humidifier Ultrasonic Disc 5 and up flat passive n/a (available cheaply on Amazon and eBay) These are the PZT wafers at the heart of ultrasonic humidifiers (AKA misters or nebulizers). Not as sensitive as benders, but no resonance to speak of. They’re cheap too, and come with leads attached so you won’t hurt them during soldering. I highly recommend them.
DIY with PVDF Film Sensor 10 and up flat passive n/a PVDF film is a flexible, non-resonant plastic with piezo properties. But it has very low sensitivity. Avoid for all but the loudest sources! (It also melts when soldered, so be sure to get one with wires attached already.)

 

Preamps Compared

There are too many options, but here is a selection of piezo preamps to pair with your contact mics, sorted loosely from expensive to amazingly affordable. ALL PRICES ARE APPROXIMATE. Manufacturers can change their prices at any time.  Get in touch if I missed or mis-represented something! (I don’t do affiliate links, and all opinions are my own.)

Model Approx Price (USD) In Plug Out Plug & Power Notes
Triton Audio BigAmp Piezo 85 1/4″ XLR Phantom This small phantom powered inline preamp simply transforms impedance and adds a few dB of gain. It works fine but it’s very expensive for what it is.
Aquarian Audio preamps 40 and up 1/4″ various A USA Hydrophone manufacturer with several excellent preamps (PA1, PA4, PA6) that work great with contact mics. These have approx 26dB gain (for boosting small signals) and there are options for battery power, “plug-in powered” 3.5mm outs and phantom-powered XLR outs. Highly recommended for a no-solder solution!
Metal Marshmallow DIY Kit 24 and up n/a n/a Michael Krzyzaniak sells the preamp board from his Metal Marshmallow contact mics as a separate item, so you can build it into your own design (soldering required). It’s a very tiny PCB, available in 2 versions. The low voltage version can be powered from 3.5 – 5v DC. The phantom-powered version runs on standard P48 phantom. Highly recommended for DIYers! Very low noise with adjustable gain.
Stompville preamps 24 GBP and up various various UK shop that sells an improved version of the Alex Rice XLR preamp, and an unbalanced variation too. This is a reliable, inexpensive option if you’re in the UK. (See the related blog posts for a discussion of how the circuit works.)
Impedance Transformer 25 and up 1/4″ XLR A passive solution if you really hate transistors? This 1/4″ – XLR adapter includes a matching transformer. (Hosa MIT 129 has 50k input impedance compared to > 1M input of a preamp so it will still lack in bass.) Sound samples here.
AMZ Guitar Pedal PCBs 17 and up n/a n/a Jack Orman sells compact 9v-powered JFET preamps designed for guitar. I haven’t tested the noise floor, but they sound good with contact mics and require minimal soldering. The JFET Buffer Module has unity gain. The Mini-Booster Module has more (adjustable) gain.
Internal Piezo Preamps for Acoustic Guitars 10 and up 1/8 or 1/4″ 1/4″ Acoustic guitars often have piezo pickups inside, with cheap 9v-powered preamps built-in to the guitar body. They take many forms from slim “endpin jack” styles to larger boxes with equalizers. One caveat: Guitars are loud, so these circuits have very low gain. They will be too noisy for quiet sources. More info: Easy DIY Pre-Amp: Makes Piezo-Equipped Cigar Box Guitars Sound Better
“Alex Rice” Preamplifier DIY n/a XLR phantom A DIY balanced 48v phantom powered circuit. (It requires that the piezo be wired in a balanced configuration with a foil tape shield, which may be hard for certain types of sensors.)
Richard Mudhar designs DIY n/a n/a In his article Using Piezo Contact Mics Right, richard Mudhar shares several designs with thorough explanations.
Tillman FET Preamp Cable DIY 1/4″ 1/4″ This design was intended for guitars (and the FET is out of production) but it’s a good intro to designing simple FET buffers.

Further Research

Tim Prebble’s The First Rule of Contact Mic Club

LOM Knowledge-base article about contact mics

Rod Elliot’s schematics and explanations for Piezo Pickup Preamplifiers

Michał Fojcik’s Contact microphones (& attachement methods) shootout

David Dunn’s Microphones, Hydrophones, Vibration Transducers: Rolling Your Own

Tim Chilina’s Building Contact Microphones From Piezo Discs

Bert Vertessen (Phase57)’s guide to building a Piezo Contact Mic

Tin Dožić collected a lot of first-hand info here: research on piezo: contact alchemies

Felix Blume’s guide to building a DIY Hydrophone