How long are the wires between piezo sensors and Arduino? With impedance levels of 1M or so it doesn't take much stray capacitance for strong crosstalk, the signals should be individually screened if any significant cable runs.
How long are the wires between piezo sensors and Arduino? With impedance levels of 1M or so it doesn't take much stray capacitance for strong crosstalk, the signals should be individually screened if any significant cable runs.