piezo contact microphones are cheap, easy to diy and usefull tools for sound recording. while most microphones pick up sounds as vibrations that move through the air (more acuratelly – they transduce acoustic energy to electrical energy), contact mics are different in that they will pick up sounds as vibrations in solid materials.
why build and use contact microphones?
piezo contact mics, if properly used, can deliver great sounding results and can be used in a wide variety of situations. they can be used to amplify acoustic instruments and everyday objects, to record extremely quiet sounds without the interference of the surrounding soundscape, as pickups in sound and multimedia installations, as triggers in interactive situations, as mics for field recording, as hydrophones, as body pickups (heart beat, bone movements…) and anything you can think of!
most contact microphones are made from piezoelectric materials (often refered to as piezos). piezoelectricity is a property of certain materials (i.e. some crystals) to produce an electrical charge in response to mechanical stress or pressure. also, if an electrical charge is sent into a piezoelectric material it will vibrate. so, piezos can act both as mics or speakers.
commercialy, piezo elements are often used as cheap buzzers, small speakers in clocks, computers, toys, musical postcards or as tweeters (treble speakers). in the music industry they can be used as acoustic instrument pickups, drum triggers etc.
you can either recycle piezo elements from toys, clocks etc. or buy them new in electronics stores.
here is a picture of different shaped and sized piezos (in comparison to a two kuna coin).
HOW TO MAKE A DIY CONTACT MIC?
what you will need:
> a soldering iron, some solder, a wire cutter and optionally a set of helping hands
> a piezo element
> mono microphone cable (or some different shielded cable)
> a mono 6.35 mm (1/4”) audio jack
> hot glue gun, rubber seal or plasti dip
short – prepare the cable for soldering. put the solder on the shield (a) and the core (b) of the cable on both sides.
long – first, prepare the cable for soldering. get a microphone cable (or some other shielded cable) of your desired length. use the wire cutter to remove 2 cm of the outer layer of cable insulation (on both sides of the cable). when you remove the outer insulation, inside the cable you will see one or more insulated smaller wires and one loose set of copper wires (the ”hairy” part). the insulated wires are the core (a) and the hairy part is the shield (b). you need one core, so if you have more cores cut the ones you won’t use. now remove the insulation from the core (about 0.5 cm in length). twist the shield, so it isn’t ”hairy” any more, but braided. with a soldering iron put solder on the core and the shield. the result should look like this:
short – solder the cable to the jack. connect the core of the cable to the tip of the jack (a), and the shield of the cable to the sleeve of the jack (b).
long – now take your 6.35 mm mono audio jack, unscrew it and pull the cable through the hollow part. the jack consists of a tip and a sleeve. the tip caries the signal, and the sleeve grounds it. so, the core of the cable (a) should be soldered to the tip (a) of the jack, and the shield (a) of the cable should be soldered to the sleeve of the jack (b). see the following pictures:
short – solder the cable to the piezo. core of the cable should be soldered to the center of the piezo (a), shield of the cable should be soldered to the brassy edge (b) of the piezo.
long – the other side of the cable should be soldered to the piezo element. the piezo element consists of a thin layer of piezoelectric crystals (a – the part in the middle) and a brass ring (b). the core should be soldered to the the middle and the stranded wire to the brass part of the piezo (see pictures). the center part of the piezo is very sensitive to heat, so it is easy to damage it while soldering. it is best to get a piezo element with wires already attached to it, or to put a small amount of solder (tin) to the center and edge of the piezo before soldering the cable to the piezo.
test your contact mic.
at this point you should have a working contact microphone. plug it into your mixing board/sound card/ amp and see if it is working. it should not have a (strong) hum.
if you have made a piezo contact mic as described above it will have a series of limitations.
first, because of impedance mismatch your diy piezo will have a lack of low frequenciy response (to fix that you will need some kind of a buffer, usually a preamp). also, piezos don’t have a flat frequency response – they tend to have a specific resonance peak, which will result in a thin and metallic sound. you could try dampening your piezo. one option is to put hot glue over the piezo or to cover it with rubber seal. this will also protect it from moisture (if you want to use it in the field) and buzzing resulting from touching the (exposed) core of the cable.
at the bottom of this page in the links section you can find more diy contact mic guides and different tips for building them.
GETTING A BETTER SOUND OUT OF YOUR DIY CONTACT MIC
as mentioned above, passive piezo contact mics are faced with a series of limitations that will drastically reduce their sound quality.
the biggest challenges are impedance, resonance and interference.
a piezo disk soldered directly to a guitar jack and plugged directly into a mixer or amplifier will have a ”thin” metallic sound. the reason for this is impedance ”mismatch” between piezos and regular audio inputs. piezos need to work at a higher impedance (over 1MΩ) and most audio mixers have line inputs at about 50 kΩ. what that means practically is that a piezo plugged directly into an audio mixer will have all frequencies below 200Hz cut off. a more technical explanation of impedance can be found here, here and here.
solution: IMPEDANCE CONVERTERS
there are two types of impedance converters: passive and active. most common passive impedance converters are DI boxes and they are usually used to match the impedance of an instrument (i.e. electric guitar) to that of an audio mixer input. i have not tested any passive impedance converters with piezo contact mics, and generally speaking the second type of impedance converters are better to use in this case.
active impedance converters are the best solution for good sounding contact mics. they contain an actual audio amplifier circuit that will make possible a full frequency response of a piezo contact mic. they usually work by using battery power, or phantom power provided by the mixer or sound recorder. both commercial and diy options are available. during my research i have tried out various kinds of piezo preamps.
on the following soundcloud links you can hear different combinations of piezos and preamps used with an acoustic laptop built by hrvoje radnić. the acoustic laptop is made as a box with two parts, and for every recording there is one contact mic placed on the part of the box with springs and another contact mic placed on the part of the box with a string and music box. the audio files are made as stereo recordings, where the right channel is the part of the box with the guitar string and a music box, and the left channel is the part of the box with the springs. sounds from one part of the box bleed into the other as well. all the sounds were recorded using a Focusrite Scarlett 6i6 2nd Gen sound card
the musician is also playing his acoustic laptop freely:
because of the way they are built, piezo discs do not have a flat frequency response. they usually have a frequency peak somewhere between 2 and 6 kHz that gives them their characteristic metallic sound.
solution: reduce RESONANCE, increase the mass of the disc
one solution to getting a ”flatter” frequency response from your piezo contact mic is achieved by increasing it’s mass. the simplest way to do that is by covering the piezo disc with hot glue. some great tutorials on how to build a good sounding piezo can be found here and here. i have found the JrF C-series pro contact microphones to be piezos with a great sound as a result of the way they are built.
i have made a video comparison between 3 piezo build-types. first one is a diy piezo with no added weight (”naked” element), second one is a diy piezo covered with hot glue, and the third one is a JrF C-series piezo. All contact mics were plugged into Radial Engineering StageBug SB-4 Piezo preamps, and recorded with a Focusrite Scarlett 6i6 2n Generation sound card with the same gain on each channel. as you can hear the naked element gives off a sound with most treble, and the weighted ones give less treble and more mids and bass. i would say that each of the builds has its pros and cons, all depending on what you want to achieve with your recording.
most diy piezo contact mics are made with unbalanced audio cables. thus, when using a long cable electromagnetic interferences can ruin the sound quality. i remember using longer (arround 3m) cables with diy piezos with one of my first sound works back in 2013 (kitchen turntable) and it resulted in piezos picking up radio waves.
best way to reduce the interference on your diy piezo is making it on a shorter shielded cable and pluging it into an active DI box. the DI box will fix the impedance mismatch, aswell as making you signal balanced. then you can use a long balanced cable to connect your DI box with you mixer/recorder/sound card. one more suggestion is to cover the piezo disc with copper tape that is connected to the system ground.
ATTACHING A CONTACT MIC
there are different ways to attach your contact mic to an object you want to record. one suggestion is to use clamps. the other is using a two sided tape or the sticky mass for attaching posters to the wall (in croatia we call it patafix, other name i found online is blu tack). i prefer to use the two sided tape and it works well with clean flat surfaces. patafix is good for rough surfaces. electrica tape is also a good choice since it doesn’t leave any sticky residue on the piezo.
a link to my piezo sound sample archive can be found on this link:
feel free to use these samples any way you like, no need to credit me or anything, although i’d be happy to listen to your works containing these samples!
EXPERIMENTS WITH PIEZOELECTRICITY
as mentioned above, piezoelectricity as a phenomenon can be observed in various materials. i have grown rochelle salt crystals and used them as piezoelectric contact mics. on the following video the crystal is clamped between two peices of brass on a plastic clamp. a wire is attached to each peice of brass and connected to an audio jack plugged into a piezo preamp and then into my soundcard. by hitting the crystal (with a screwdriver or pen) a small electrical charge is created, resulting in an audible sound (no sound from ‘the air’ is recorded here, only the electrical charge – sound produced by the rochelle salt crystal).
since piezo contact microphones can pick up very low amplitude and inaudible sounds, i have recorded a sound of a guitar string cooling down. first the string is placed onto a piezo, then heated with a candle and it produces an interesting sound effect while cooling down.
i was interested in how certain materials sound when they go through a physical change. so i recorded a sound of melting polystyrene in contact with acetone. also i recorded sounds of piezos being materially altered with different acids (you can find lab recordings in my sound sample archive).
here are useful links for your further piezo research: