What is a dynamic microphone?
The term “dynamic” can have a lot of meanings, especially when it comes to sound and music. In this case, however, it has little to do with “dynamic range” or a “dynamic performance". In this context, it refers the kind of electromagnetism that happens for instance inside your bicycle’s dynamo: When an electrical conductor moves in a magnetic field, an electric current is induced.
Dynamic microphones, thus, are microphones that convert sound into an electrical signal by means of electromagnetism. They fall into two categories, moving coil and ribbon microphones.
Moving Coil Microphones (aka “Dynamic Mics”)
Moving coil microphones are probably easiest to understand, because they are basically built like a loudspeaker: A coil is glued to the rear of a membrane, and there is a strong magnet surrounding this coil. When sound waves hit the microphone, the membrane moves to the rhythm of the sound waves, and the coil on its back moves along with it. The relative movement of the coil within its (stationary) magnetic gap induces a small signal voltage in this coil. There’s your microphone, a device that converts sound into an electrical signal.
Moving coil microphones are often preferred for use on stage, because they are quite sturdy and do not require external power. In the studio, engineers usually prefer condenser or in some cases ribbon microphones, which are less robust but offer superior sound reproduction.
Moving coil microphones are by far the commonest type of dynamic microphones. And since “moving coil microphone” is quite a long term, most sound engineers prefer to call them “dynamic mics” or just “dynamics”, thus perceiving ribbon mics as a different category. While this is technically incorrect, it makes a lot of sense from a practical standpoint, because ribbon mics are quite exotic beasts, which sound and behave different than moving coil dynamics.
Ribbon microphones work by the same basic principle of electromagnetic induction. However, instead of having a membrane and a coil, a ribbon transducer uses a narrow strip of extremely thin aluminum foil. In other words, the membrane itself is the electrical conductor that moves inside the magnetic gap. Such a thin piece of aluminum ribbon is much lighter than a membrane with a coil of copper wire attached to it. A ribbon transducer is therefore able to follow the movements of the sound waves more accurately than a moving coil capsule.
However, with just one conductor inside the magnetic gap instead of a whole coil of wire, it also produces much, much lower output. Ribbon microphones therefore contain a step-up transformer, which multiplies the transducer’s output voltage by a factor of about 30. Even so, ribbon microphones typically have lower sensitivity (i.e. output level at a given sound pressure level) than a moving coil microphone. A ribbon mic therefore requires a very low noise preamp with lots of gain.
By nature, ribbon mics are bidirectional, i.e. they are equally sensitive to sound coming from the front and sound coming from the rear. But sound waves coming from the sides do not set the ribbon in motion. This pickup pattern is called figure-8.
Ribbon mics are very fragile and must be treated with great care. Another drawback is that the treble response of most ribbon mics is quite limited. Today, ribbon mics are used for special applications, only, where extended top end is not required, e.g. for guitar cabinets, or not wanted, e.g. to tame overly bright brass instruments.
A relatively new development are so called active ribbon microphones, which contain an amplifier circuit for higher output. Active ribbon mics require phantom power, just like condenser microphones.