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 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 so.

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. 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 voltage). The ‘hot’ lead needs to be connected to this and the ground needs to be connected to the outer metal part.

What plug you use depends on what kind of device you use for 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-9, Zoom H4 etc.) or Minidisc. 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.

To make a stereo pair, simply solder up two single discs and attach the ‘hot’ leads to the two center signal leads on the mini-jack plug. Be sure to insulate any contact to avoid shorting the signal.

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. 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 clamps or clips. Its also handy to use something like blue-tack to make a firm connection to the surface of what you are trying to record.

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 most 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.

For examples of my recordings using contact mics check the posts with the tag *contact mics*

The two digital recording devices used are the Edirol R-09 and a Sony PCM-D50. For some some microphone tests and personal commentary on these check here and here.

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.

*UPDATE*

I made a simple set of contact mics for XLR plugs. I search 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. 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 tops 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 sensitive, low noise, contact mics. All this (good cable, neutrik plugs, piezos, felt) still cost lest than 20 euros.

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Recording a suspension bridge (above).

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.

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Recording fences in Avebury UK (below).