How a Microphone Works
While there are many ways to convert sound into electrical energy, we’ll concentrate on the two most popular methods: dynamic and condenser. These are the types of microphones most often found in recording studios, broadcast, motion picture video production, and on stages for live sound reinforcement.

Why Microphone Selection is Important
The microphone is, by its nature, at the very beginning of most sound systems and recording applications. If the mic can’t capture the sound clearly and accurately, and with low noise, even the best electronics and speakers following it won’t produce the optimum sound. So it’s important to invest in good microphones, to maximize sound system performance potential.

Dynamic Microphones
Comparing microphones to loudspeakers may help you to understand their operation. Dynamic microphones are similar to conventional loudspeakers in many respects. Both have a diaphragm (or cone) with a voice coil (a long coil of wire) attached near the apex. Both have a magnetic system with the coil in its gap. The difference is in how they are used.

With a speaker, current from the amplifier flows through the coil. The magnetic field created by current flowing through the voice coil interacts with the magnetic field of the speaker’s magnet, forcing the coil and attached cone to move back and forth, producing sound output.

A dynamic microphone operates like a speaker in reverse. The diaphragm is moved by changing sound pressure. This moves the coil, which causes current to flow as lines of flux from the magnet are cut. So, instead of putting electrical energy into the coil (as in a speaker) you get energy out of it. In fact, many intercom systems use small speakers with lightweight cones as both a speaker and a microphone, by simply switching the same transducer from one end of the amplifier to the other! A speaker doesn’t make a great microphone, but it’s good enough for that application.

Dynamic microphones are renowned for their ruggedness and reliability. They need no batteries or external power supplies. They are capable of smooth, extended response, or are available with “tailored” response for special applications. Output level is high enough to work directly into most microphone inputs with an excellent signal-to-noise ratio. They need little or no regular maintenance, and with reasonable care will maintain their performance for many years.

Condenser Microphones
Condenser (or capacitor) microphones use a lightweight membrane and a fixed plate that act as opposite sides of a capacitor. Sound pressure against this thin polymer film causes it to move. This movement changes the capacitance of the circuit, creating a changing electrical output. (In many respects a condenser microphone functions in the same manner as an electrostatic tweeter, although on a much smaller scale and “in reverse.”)

Condenser microphones are preferred for their very uniform frequency response, and ability to respond with clarity to transient sounds. The low mass of the diaphragm permits extended high-frequency response, while the nature of the design also ensures outstanding low-frequency pickup. The resulting sound is natural, clean and clear, with excellent transparency and detail.

Two basic types of condenser microphones are currently available. One uses an external power supply to provide the polarizing voltage needed for the capacitive circuit. These externally-polarized microphones are intended primarily for professional studio use or other extremely critical applications.

A more recent development is the electret condenser microphone. In these models, the polarizing voltage is impressed on either the diaphragm or the back plate during the manufacturing process, and this charge remains for the life of the microphone.

Other Types of Microphones
There a number of ways to translate sound into electrical energy. Carbon granules are used as elements in telephones and communications microphones. And some low-cost microphones use crystal or ceramic elements that are generally OK for speech, but are not seriously considered for music or critical sound reproduction.