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.

Most people believe that a microphone is a simple enough instrument that they don't even think about what can go wrong with it. You talk into it, it records you. However, in a similar manner to amateur photography, a little bit of ignorance goes a long way and can produce avoidably shoddy results. There are several quick and easy tips that can help people avoid the most common mistakes with a microphone without going into full-blown sound engineering.

One of the most world-changing things about the internet is that it gives normal people a global audience for blogs and podcasts. But just as poor spelling and grammar can drive people away from a blog, poor recording quality can ruin a podcast. Fortunately, you don't need to hire a professional editor to write a passable blog, and you don't need a recording studio to get decent audio for your podcast. The plastic mic that came with your computer certainly won't let you achieve truly professional results, but as long as it isn't actively bad most people won't care.

Holding the mic

It all starts with this. If possible, you'll want to use a microphone stand. A mic lying on a desk, especially the desk your computer is sitting on, will pick up vibrations such as the computer fan that will be audible in the recording. A mic stand also brings the mic closer to your mouth. I once resorted to making my own mic stand out of a bent coat hanger. Hold the mic if you must, but avoid holding or covering any part of the sensitive area. If you can wear a headset, you can listen to your recording as you make it to help avoid the other problems.

Breathing

One of the most irritating disruptions on any recording is the speaker's breathing, because when it does come up, it tends to be persistent. If at all possible, avoid letting the audience know that you need to breathe. It's very easy to accidentally exhale onto the mic, and if you don't control your breathing effectively you can even be audible inhaling through your mouth. Move farther away from the mic if these issues pop up, and try to inhale through your nose instead of your mouth. Even the Chocolate Rain guy knew to move away from the mic to inhale, although most people will not find it necessary to take it to the excessive degree which he does.

Avoiding plosives

The second most distracting thing I hear on amateur recordings is a puff of air that hits the mic whenever the speaker pronounces a plosive consonant, especially p and b. There are several ways to avoid this. You can put a cover over the microphone, sit back a bit farther from the mic, put the mic a little to the side instead of directly in front of your mouth, or hold a piece of a paper between your mouth and the mic. Avoiding this one simple mistake can set you a head and shoulders above most other amateur recordings.

Volume

Volume is important, but usually not critical. Your audience is usually willing to turn up their speakers if necessary. To some degree, you can turn the volume up on your recording with even the simplest audio editor, but if you need to turn it up too high you will amplify ambient background noise that lowers the overall quality of the recording. Most audio recorders can display a volume bar as you make your recording. You will get the best results by getting this bar as high as you can while staying below 100%. This would cause clipping, which means the audio wave will be cut off at the extreme ends. This results in critical nuances of the sound being lost as the input is saturated beyond its ability to record them. This is hardest to achieve if sound effects or other loud noises will be part of the recording. Remember that if you need to move back from your mic to prevent the other problems, you may need to speak louder to achieve the volume you want. If you're having trouble reaching the volume you need, check your computer's volume settings, you may have a microphone input setting you can adjust.

Radio voice

Most people record in their normal speaking voice. This is usually fine, but you may want to make some sample recordings in different voices to see what sounds good. Remember that your voice sounds differently bouncing around in your own skull than it does recorded and played back. Pay attention to enunciation, and try speaking from your diaphragm. Try not to slur, drop the g's from the end of your words, or skip over consonants that require tongue movement such as t and k. These bad habits people use in ordinary speech (some regional) can affect how understandable you are.

Preventing feedback

Feedback is a rare issue in podcasts, but for completeness I'll mention it. Feedback is caused by the microphone picking up signals from the speaker, amplifying them in a feedback loop. Usually recordings are not done with the speakers on, but this can be useful if recording with a headset (which will prevent feedback because the mic cannot pick up signals from the headphones). If you must leave your speakers on while recording, for example to hear your results in real-time, turn them away from the mic.

Editing

Slightly advanced, but not too difficult with modern point-and-click audio editing tools such as Audacity. Few people can talk or read for long periods of time without stumbling occasionally. If you do this, you don't need to start over if you can edit your recording. What I generally do is stop at the point where I stumbled and start the sentence over. You can mark this position by blowing into the mic or making another loud sound that will be identifiable when looking at the visual representation of the recording in the editing software. You can then select and erase the bad version of the sentence, leaving only the good version that followed it. Be careful to leave a short period of silence following the previous sentence when you do this, though, so the transition has a natural-sounding pause.

Now go forth, and amaze your global audience with your newfound recording skills. Or at least don't disappoint them by sounding like you don't know what you're doing.

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.

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