So if i understand this correctly, if the same piezo disc is used as a sensor, the 1 Mohm resistor must be add in parallel while if it is used as a speaker, a 100 ohm resistor must be added in series.
What happens though if it is used as a speaker (with the 100 ohm resistor) but someone knocks on the sensor? Isn't there the possibility that too much voltage will reach the Arduino pin (even if the pin is set to output)? Also if i drive the piezo with a higher voltage (for example with 9V through a transistor) shouldn't the sound be louder? Is there a datasheet that is common for all piezos, or each has different characteristics?