A microphone is a transducer which converts acoustic energy into electrical energy. It is the exact opposite of a loudspeaker, and this is why you can use headphones as a microphone... although since headphones are optimized for a different form of energy conversion, your results may leave a little to be desired.

There are three main types of microphones, based on the way that they convert energy. These are the dynamic moving coil, the dynamic ribbon, and the condenser microphones. Without going into great detail (mainly because I don't know too many specifics), here are basic descriptions of the methods of operation.

A dynamic moving coil microphone consists of a diaphragm coupled with a coil of thin wire, the latter of which is placed in the air gap of a magnet, able to move up and down along with the diaphragm. When sound pressure differentials hit the diaphragm, its surface vibrates along with the sound waves, and in turn causes the coil to vibrate in exactly the same fashion. Because of the way solenoids work, when the coil moves up and down within the magnetic field, analog current which matches the original sound wave is induced in the wire, whose ends lead out to the output of the microphone.

A dynamic ribbon microphone is similar to the moving coil mic; it contains a very thin, corrugated metal ribbon stretched inside the air gap of a magnet, which is tied down at the ends and free to move in the middle. When sound strikes the ribbon, from either side, the ribbon vibrates and interferes with the magnetic field in the gap, inducing a voltage differential in the ends of the ribbon which has a waveform similar to the waveform of the sound wave striking the ribbon. Wires are attached to the ends of the ribbon, and lead to the microphone's output.

A condenser microphone consists of a gold-plated plastic diaphragm, which functions as one plate of a capacitor. The other plate is also gold-plated, and the two are seperated by air. A certain voltage, usually 48V, is applied to the diaphragm, so that when sound strikes the diaphragm and causes it to move, the distance between the plates changes, altering the capacitance of the condenser, and ultimately inducing a varying voltage in the second plate which is an analog of the original sound waves. The signal carried from this plate goes has to go through an amplifier inside the mic before it can be sent out. Thankfully, the amplifiers used for this purpose can be made out of transistors now. Back in the day, tube amplifiers were the only kind available, so condensor microphones had to be rather big.

"Ok, that's how microphones work, but what's the difference?" you ask.

You can't say something's categorically true of any specific kind of microphone, because there's just so much variety out there, and so many manufacturers out there doing new and innovative things. However, there are useful generalizations.

• Dynamic microphones are most of what you see, day to day. Compared to condenser microphones, they're "less crisp", which means less accurate and that they have worse high frequency response. This is due to the mass of the diaphragm, and that the sound wave must do work on the coil to cause the current. Much like cone speakers, the frequency response varies with size. A mic designed for a kick drum will have a diaphragm of an inch and a half, or more, whereas one designed for guitar micing or vocals will have one of an inch or less.

  Dynamics are also the most resilient mics. That old adage, "You can hammer a nail with a '57" is actually true. I wouldn't recommend it, but you've got to try hard to kill one. Also, most dynamics can handle pretty high SPL, so you won't destroy one by screaming into it, or placing it next to a snare drum.

• Condenser Microphones have a much wider frequency response curve than dynamics, due to their lighter and more sensitive transducer. Their sound is also described as being more detailed. Their really high sensitivity is why they're useful for micing things from far away; if you've ever seen mics hanging down above a concert hall or theater, you can bet that they're condenser mics (or electret, maybe, but that's beside the point). There are varying diameters of condenser mics, small diaphragm condensers are used for micing instruments, drum overheads, etc, while large diaphragm condensers are used for vocals primarily. Small diaphragms are more detailed, while large diaphragms smooth things out a little, and have somewhat better low-midrange response (good for vox).

  Condensers, though, are fragile. You can break one by saying "POP!" into it loudly. This is why you see singers in studios singing into a mic with a big circular thing between their mouth and the mic. That thing's (appropriately enough) called a Pop Filter, and it blocks the plosives by only allowing the sound to pass through, while blocking the blast of air. As Littlerubberfeet points out, though, what's more dangerous is moisture. Once some condensation gets in there, the mic is done.

  I just want to air one of my pet peeves, here: most all condenser microphones made for vocals (large diaphragm) are side-address; you're supposed to talk into the side of the mic, not the end (sidetalking!). However, in innumerable things I'm exposed to on TV, or in movies, people talk into the ends of such mics! It's wrong, and it looks wrong. Usually, sound is one of the few technical things they don't get wrong in film, because there's an expert on set, but he (note the stereotype! Unfortunately, it's usually true...) must've been drunk or sleeping, or something... Anyway, back to our regularly scheduled programming.

• Exotic microphones. With the last two types of microphone (or even just the first!), one could make a hit album. Most things you hear on a CD have passed through one of the last two kinds of microphone, but there are a few more types, so I'll give them some mention here.

  • Ribbon Mics are a very old design, and are seen as an anachronism, most of the time. They have detailed response like a condenser, but are dynamic. However, they are very fragile, and cannot handle loud sounds. Many have a figure-8 pickup pattern, leading to doing some interesting things with them, like setting up guitar cabinets on either side of the mic. There are some VERY good ribbon mics out there, and because they're obscure, they're generally used by people who've sought them out, and know what they're doing.

  • Electret Mics are much maligned, since old designs really did sound horrible. Modern electrets are getting much better, though, starting to compete with condenser mics. They still have a stigma, however. Give one a listen if you're in a position to, they're not bad (anymore)!

Anyway, so that's a whirlwind tour of the wild world of microphones. This is only a rough outline, and full of half-truths for the sake of simplicity. If you really want to know, look it up! I recommend www.prosoundweb.com for articles and info. Feel free to /msg me if you have additions/suggestions/comments, etc.

All info in this node from my experience, and from my copy of Modern Recording Techniques, fourth edition, by David Miles Huber, and Robert E. Runstein.

Mi"cro*phone (?), n. [Micro- + Gr. sound, voice: cf. F. microphone.] Physics

An instrument for intensifying and making audible very feeble sounds. It produces its effects by the changes of intensity in an electric current, occasioned by the variations in the contact resistance of conducting bodies, especially of imperfect conductors, under the action of acoustic vibrations.

 

© Webster 1913.