Since a number of people have asked about building contact microphones, particularly from my youtube videos, I’m posting a very brief tutorial here regarding my own experience. There are other web pages here and here that elaborate on the building process. You can listen to some recordings I have made using my self-built contact mics.
Contact microphones are actually piezoelectric transducers and are mostly used for producing sound as buzzers, beepers and small speakers in electronic devices. If the signal is reversed using a piezo transducer as a mic, you’re actually picking up vibrations and having it produce voltage. Connecting this to a recording device or mixer allows you to hear the vibrations and resonances as sound. Piezo transducers come in all shapes and sizes, the most common ones ranging from 25-50 mm in diameter. They can be ordered from electronics components distributors such as Maplin in the UK or DigiKey in the US. As for what kind of disc to buy this is a question of great debate, but I generally stick to mid sized discs of 30-40mm with a resonant frequency of 1khz. Best of all they are cheap, usually 50 cents or less, so order a few different kinds to test and see which work better.
As for building contact mics, the process is fairly simple. A soldering iron and raw solder is needed. Some flux paste is helpful for getting the wires to stick more firmly to the contacts. As you can see some piezos come with leads already attached. All you need to do is connect then to a wire and the appropriate plug. The center circle of the disc is the ceramic piezo element (what produces the voltage). The ‘hot’ positive lead needs to be connected to this and the negative “ground” wire needs to be connected to the outer metal part.
What plug you use depends on what kind of device you use for recording the signal. Most likely you will connect your contact mic to a mixer or directly to a recording device such as a flash recorder (Sony PCM-D50, Edirol R-09, Zoom H4n, Tascam DR- etc.). I often work in the field and connect my contact mics as a stereo pair to a flash recorder with a 1/8″ mini-jack but I have XLR versions as well.
To make a stereo pair, simply solder up two single discs and attach the ‘hot’ +positive leads to the two center signal leads on the mini-jack plug. Then connect both -negative leads to the outer longer lead on the plug. Be sure to insulate any contacts with electrical tape to avoid shorting the signal. Getting good soldered contacts takes practice so always get extra plugs in case you make mistakes.
Finalizing your contact mics is an important step. Piezo transducers tend to be fragile. If the ceramic crystal is damaged in any way you’ll immediately know it. At the very minimum its good to put some kind of glue over the back of the disc to protect the crystal like ‘hot glue’. I sometimes put the felt furniture pads that are used on chair leg bottoms over the crystal to protect it then I cover the whole disc with electrical tape. I also glue a thin piece of wood to the front side with epoxy. This is to make sure the disc does not bend and crack. The wood also “softens” the sound as piezos tend to have a sharp metallic sound (since they are actually made of metal). Some people also coat their mics with something like plastidip which makes a nice clean protective finish.
Another reason to protect the piezos is because they tend to work better when a bit of pressure is applied. The sensitivity increases and you tend to get much better bass response. With the felt furniture pad backing and wood front I’m not afraid of bending the disc or crushing the ceramic part when using strong clamps or clips. It is also handy to use something like blue-tack to make a firm connection to the surface of what you are trying to record. After all its the Contact that makes a contact mic.
It is good to carry around a number of different clips and clamps for the materials and situations you might encounter. Most of these are cheap and lightweight and can be found from building supply stores.
Lastly, this is a device I often use when recording with my contact mics. The FEL Microphone Amplifier is small, runs from a 9V battery and can boost a stereo signal up to 50dB. This really helps when the source is more quiet or you want to pull some bass out. Most of the time I need to boost the signal about +15-30dB, but occasionally you need more. As a general rule, the better the preamp, the better the signal.
For examples of my recordings using contact mics check the posts with the tag *contact mics*
Hopefully this helps anyone who is interested in working more with contact mics. Most importantly, remember there are no fixed rules about the best ways to use contact mics and it a matter of continuous experimentation. I will update this page if more questions or info comes in.
I made a simple set of contact mics for XLR plugs. I searched around for some basic designs and didn’t find much. Here is what I came up with for a setup. I used 2 piezos instead of 1. The center ‘positive’ leads of each piezo are connected to the 2 and 3 leads of the XLR cable, while the outer ring (ground) of both piezos are connected to the 1 (ground) lead of the XLR cable. The diagram can be seen in the first photo. *NOTE* Some have noted that this wiring should ‘phase cancel’ yet for some reason it works. If you encounter problems, simply use a single piezo and do not connect anything to the negative #3 lead.
The top faces of the piezos are turned inward toward each other. On each face is a self-adhesive felt circle (for furniture legs). Once the two felt circles are in place they can be glued together with a semi-flexible glue like ‘moment glue’. In this case the felt circles were smaller than the piezos so I wrapped the edges with nylon string and filled it with epoxy for strength. When this was done and all glues were dried I wrapped everything with electrical tape. The result is high sensitivity, low noise, contact mics. All this (good cable, neutrik plugs, piezos, felt) still cost lest than 20 euros for two.
Recording a suspension bridge in Calgary Canada (above).
Recording a dead tree in the wind on the Lithuanian coast (below).
Below is a video from MAKE Magazine about building a contact microphone that includes the schematic plan for a 9V powered amplifier. The amplifier can be handy if you don’t have a suitable amplifier on your recorder.
Top image: Recording wire fences in Avebury UK.