Why is the other end of the sensors connected to 3V3 instead of GND?

[apricot-azumi]

[LuiCat]

It is intended to bias the piezo sensor, so that input voltage doesn't get under 0v. It might not be the best way to set up the circuit, but connecting to 0v would absolutely clip the input voltage under 0.

[LuiCat]

@apricot-azumi Haven't heard from you in a while. Any feedback or further questions?