Using a piezo element as a contact mic
If you want to record or amplify the sound vibrations from a surface, you can use a piezo element as a contact mic.
However, do not hook them up directly to other audio equipment! There can be harmful high voltage spikes.
A simple and o.k. preamp to use is this one from collins lab
If both sound quality and equipment safety is not your greatest concern, a very simple solution is this one
If you want to hook it up directly to a speaker:
(The 220 resistor can be varied, for more or less gain. The 3.3M can be replaced with a 1M resistor which is easier to find...)