A ribbon microphone is a unique type of dynamic microphone that is based around a thin, corrugated strip of metal (often aluminium) or film suspended between two magnetic poles. Unlike traditional moving-coil dynamic mics, the ribbon element responds to variations in the velocity of air particles, rather than the pressure. As the ribbon vibrates within its magnetic field, it generates a tiny voltage that corresponds to these changes in velocity.
In classic ribbon designs, this level is very low compared to typical dynamic mics, and a step-up transformer boosts both the output voltage and impedance. Preamp choice is very important when using ribbon mics.
Because a ribbon mic has an extremely thin, delicate element, it is capable of capturing fast transients. Ribbons mics have a wide dynamic range and are capable of handling high SPLs at high frequencies. (Give them a try on brass or percussion.)
These mics are bidirectional by design, because the ribbon element responds to sound arriving from the front or back of the mic, and does not pick up sound arriving on its sides. This natural figure-8 pattern makes them ideal for stereo recording applications and is useful in applications where you want to eliminate unwanted noise between two sources (i.e. in broadcast).
Ribbon mics are very sensitive, but they are often quite fragile; delicate older models can be broken by strong gusts of air, voltage spikes or even by being stored on their side.