Explain this schematic
The diaphragm is a variable capacitor.   The resistor feedback  (three sections of NOR, U3 output to U1 input) acts to bias the gates as a linear amplifier cascade.   I think you are looking at a capacitive barometer sensor, possibly with oscillation frequency dependent on pressure.  It might also be a nonoscillating
amplifier, which would be useful as a stethoscope (blood pressure is usually measured with a pressure gage and a stethoscope).