Microphones are how we convert sound into audio, or more specifically electrical impulses that represent sound waves using an analogues wave form where the various peaks and troughs of a sound wave are represented in both frequency and amplitude by an electrical signal. This process, as well as any process in which one form of energy is converted into another form of energy, is called transduction. In fact this is the exact opposite of how loud speakers work.
Different Kinds of Microphones
The key component of a microphone is the transducer which is commonly referred to as the capsule. In music production there are two basic microphone designs. These types are based on the type of capsule they use.
Dynamic or moving coil microphones such as the Shure SM57 (pictured above) use electromagnetic induction to recreate the electrical signal interpreted as audio. Sound Waves from your voice or guitar amp come contact with a mylar diaphragm which is attached to a coil suspended over, on, or around a magnet. The intended frequency response and polar pattern is the direct result of the size and type of magnet being used being ferrite or neodymium and the orientation of the coil.
Condenser Microphones condenser mics such as the Audio Technica AT2020 (pictured below) operate a little differently. Sound waves still contact a diaphragm in the same fashion but instead of being directly attached to a moving coil the diaphragm serves as one of two metallic plates suspended with an electrical charge between them. The diaphragm plate is free to move in direct correlation with the sound waves that contact it while the fixed back plate remains stationary in relation to the diaphragm. The electrical charge between the two plates exhibits a variable level of capacitance. As the plates move closer and further apart with the sound waves, a bias signal directly modulated by the capacitance is interpreted as audio. Because the transducer as well as the bias signal requires an electrical charge to operate, an external power supply is needed and is most commonly found in the form of Phantom Power. Phantom Power is +48V of DC power typically supplied by a microphone preamp, although sometimes condenser microphones are powered by internal batteries as well as AC power supplies.
Less physical energy is needed to move the diaphragm of a condenser microphone therefore they can be much more sensitive. For this very reason dynamic microphones are used more frequently in live sound reinforcement as open air monitors tend to cause condenser microphones to feedback. Dynamic microphones are favored on drums as their simpler circuitry results in a faster more articulated response as well as high SPL tolerances.