Unlike normal microphones that measure tiny changes in air pressure, contact microphones are much more tactile and are used to measure physical vibrations in objects. This makes them great in recording and amplification applications with acoustic instruments, they sometimes can make a good replacement to a standard microphone as they can be (not always!!) less prone to feedback. I have been using contact microphones in my own work to amplify everyday objects, and turn them into expressive instruments, pretty much any material can be discovered to have interesting and unexpected acoustic properties.
Contact microphones come in a range of shapes and sizes – from chunky expensive piano microphones to flat copper discs that cost pennies. You can also find flexible ribbon contact microphones that have some interesting applications. The sound quality that can be achieved from these different types is an argument for elsewhere as I find that you can achieve great results from the cheapest contact microphones available.
Whilst these piezo disk types could be wired straight to a jack and plugged into a guitar amp or interface, the sound quality won’t be great. This is because contact microphones are not well matched to typical inputs on these devices, you will probably find that anything you connect the mic to will just sound tinny and lacking bass response – as if listening with a broken speaker. This is because a contact microphone is basically a capacitor, and wiring it this way is creating a crude hi-pass filtering effect. Piezo disks seem to have an ill reputation for lacking bass, and I say this reputation is WRONG!
The circuits formerly presented on this page are currently being updated (August 2014) so please stay tuned for a better piezo preamp!