The dynamic (moving-coil) microphone operates by electromagnetic induction to generate an output signal voltage. It is like a miniature loudspeaker working in reverse. The diaphragm is attached to a coil of fine wire. The coil is mounted in the air gap of the magnet such that it is free to move back and forth within the gap. When the sound wave strikes the diaphragm, the diaphragm vibrates in response. The coil attached to the diaphragm moves back and forth in the field of the magnet. As the coil moves through the lines of magnetic force in the gap, a small electrical current is induced in the wire. The magnitude and direction of that current is directly related to the motion of the coil, and the current then is an electrical representation of the sound wave.
One of the major drawbacks of the dynamic microphone relates to the mass of its moving coil. Due to this mass, the dynamic mic has a relatively poor transient response, and is less sensitive on the average than the condenser mic.
The other major microphone type is the condenser. The diaphragm of a condenser microphone is a very thin plastic film, coated on one side with gold or nickel, and mounted very close to a conductive stationary back plate. A polarizing voltage is applied to the diaphragm by an external power supply (battery or phantom power) or by the charge on an electret material in the diaphragm or on the backplate charging it with a fixed static voltage. All Crown mics are the electret condenser type.
The diaphragm and back plate, separated by a small volume of air, form an electrical component called a capacitor (or condenser). The capacitance between these two plates varies as the freely suspended diaphragm is displaced by the sound wave. When the diaphragm vibrates in response to a sound, it moves closer to and farther away from the back plate. As it does so, the electrical charge that it induces in the back plate changes proportionally. The fluctuating voltage on the back plate is therefore an electrical representation of the diaphragm motion.
Because the diaphragm of the condenser is not loaded down with the mass of a coil, it can respond very quickly to transients. Also, the condenser capsule can be made very small. Condensers generally have excellent sonic characteristics, and are widely used in high-quality professional microphones in sound reinforcement, measurement and recording.