eseb wrote:The aim is to measure the propagation speed of waves in different media for geology. One causes a shock that one piezo sensor registers and another one retrieves further in a material: the propagation time of the wave in the material is measured.
OK, sounds interesting.
So I presume that you are able to solder up your own leads to connect your "sensors" to the sound card?
Piezo devices produce extremely low current, but relatively high voltage. The impedance ("resistance") of the input that the transducer is connected to is therefore very important. If you connect a piezo to a low impedance (Low Z) input, the signal level will be extremely small. Ideally the input impedance should be 1 MOhm or more.
I don't know what level of electronic experience you have, but a good way to interface piezo transducers with the "Line" input of a sound card is to use a simple JFet Op Amp buffer. As a starting point, choose components to give an input impedance of 10 MOhms. A suitable (and cheap) Op Amp would be "TL051" or similar (TL052 for two channels in one chip). The basic circuit is very simple:
R1 would be a high value to provide high input impedance.
R2 sets the gain - higher values give higher gain.
For a split rail supply, vref would simply be grounded. For single rail supply, use a voltage divider between vcc and vee.
To connect the outputs to a standard computer "line in", the output of one op amp would connect to the tip of a stereo mini-jack, and the output of the other op amp would connect to the ring. The sleeve of the mini-jack would connect to common ground. For safety (so that you don't blow up the sound card, you should place DC blocking capacitors between the op amp output and the jack plug, and start with a low value of R2.
If that all sounds too complex, then you could try just connecting the piezo transducers to the stereo mini-jack plug via 500 Ohm resistors. Again, one should connect to the tip, and the other to the ring, with the sleeve as common ground. If the signal level is very low, it may not be visible in Audacity until you amplify / normalize it ("Amplify" or "Normalize" effects).