The first condense microphones were developed at Bell Labs in early 1916. The condenser microphones is build up by air gap capacitors with a back-plate and a flexible diaphragm. When the diaphragm moves in response to sound, the capacitance changes and the resulting voltage is amplified.
Modern capacitive MEMS microphones operate on the same principle and are basically miniaturized condenser microphones manufactured in a silicon wafer process. Capacitive microphones use two or three plates, a diaphragm and one or two back-plates, that form an air gap capacitor that has a high bias charge.