Erinys writes “Here’s how to build a contact microphone. A step by step I put together years ago, but it is surprisingly still of use. Contact microphones may be used to record acoustic waves that propagate through media such as wood, sheet metal, or a block of ice. Doesn’t sound “pretty” in a classical sense, but if you’re into noise and field recordings, you’ll love what you can coax out of an old tin can with one of these attached.” Link.
HOW TO – Furious contact microphone assembly