I must say these are very strange signals:
1. Half waves are very short for a mesh type pad. They are typically 2-4ms.
2. The signals are very irregular. They really should look like more or less as a decaying sine signal.
Are you sure you measured the signal correctly, i.e. on the input? If yes, then there is definitely something wrong. Try to get a scope reading on unconnected pad. Be careful not to hit too hard since unloaded (unconnected) piezo can produce very high voltage and damaged the sound card input.