Microphones' Manufacture Methods
Microphones are transducers, which means they take information and change it from one form to an entirely different form. Sound, one type of information, is an air pressure pattern. What microphones due is change this air pressure pattern into electric current patterns. The accuracy of the transformation of these currents is called fidelity. Sound engineers are very interested in the fidelity that various microphones offer.
There are several different techniques that are used to build microphones. The two that are most common in microphones, especially those used in studio recording sessions, are magneto-dynamic microphones and variable condenser microphones.
Magnetic dynamic microphones are more commonly referred to just as dynamic microphones. With this design, the sound waves create a movement in the diaphragm of the microphone as well as in the coil of wire attached to the mic. The diaphragm is thin and metallic. A magnet then creates a magnetic field surrounding the coil. The motion of this coil in this field then makes the current flow. These are the same principles that are applied to the production of electricity, only on a much smaller scale. What's important is that the current is created by the diaphragm's motion and that the speed of the motion is what determines the amount of the current. These dynamic microphones are called velocity sensitive.
With condenser microphones, however, the diaphragm is right next to a backplate. This backplate is very rigid and might or might not have any holes. The diaphragm must not touch the backplate. A battery connects both of these metal microphone parts. This battery creates a charge between the two parts. The battery voltage determines how much of a charge is set off. Other determinants are the size of the backplate and diaphragm and the amount of distance that separates the two metal parts. The distance does change, however, as sound makes the diaphragm move. Current flows through the wire in direct proportion to the diaphragm's displacement. This current is so miniscule, though, that it still has to be amplified by electricity before leaving these condenser microphones.
The condenser microphones can vary in design. Many of them use an electret made of some sort of plastic. This permanently charges the diaphragm. Plastic is great for diaphragms because it is so easy to measure exactly. The most disadvantageous feature of electrets in microphones, however, is that they only stay charged for a few years and then you have to replace them - and probably the microphones themselves.