A CONDENSER or ELECTRET microphone is one that is based on an electrically charged diaphragm (or back plate) assembly which forms a sound-sensitive capacitor.
On a condenser or electret microphone a very thin metal or metal coated-plastic diaphragm is mounted just in front of a ridged metal or metal-coated-ceramic back plate. This assembly is known as a capacitor (condenser) which has the ability to store a charge or voltage. When the element is charged an electrical field is created between the diaphragm and the back plate proportional to the spacing between them. Sound waves vibrate the diaphragm and it is the vibration of this spacing due to the motion of the diaphragm relative to the back plate that produces the electrical signal corresponding to the sound picked up by a condenser microphone.
The construction of a condenser microphone must include some provision for maintaining the electrical charge or polarizing voltage.
NOTE: An Electret condenser microphone has a permanent charge maintained by a special material (Electret) deposited on the back plate or on the diaphragm.