There are many mic brands and models out there, too numerous to review here (and there are plenty of reviews already). The only one we can personally recommend from experience is a Shure SM57 -- the audio industry's workhorse -- the mic of choice for the White House press room, touring musicians and countless radio station field reporters.
If you only need one mic, it is well worth the $99 -- it will last for years and years. It is unidirectional, that is, it only picks up sound from a narrow range directly in front, and a bit off to each side, of the mic element -- good for limiting background sound and noise. Picture the TV reporter speaking into a mic, then turning it towards the interviewee, and you'll have a good idea of what you will need to do during interviews using the SM57. Alternatively, sit side-by-side with the mic positioned in the center about 12-18” away. An optional windscreen is available, and worth the 15 or so bucks if you’ll be recording outdoors or holding the mic closer than 6” to your mouth.
Probably the most important consideration when purchasing a microphone is its impedance. Without getting too technical, the impedance is what determines a mic's compatibility with an audio device. Computer inputs and portable digital recorders typically have a high impedance input, professional recording equipment (including the mixers mentioned below) typically have a low impedance input. The SM57 is only available in low impedance, but XLR Jack-to-1/4" adapters are available for under $20.
For virtually all computers and digital recorders you will need an plug adapter; a ¼" female to ⅛" male adapter is a few bucks at Radio Shack. The hi-low adapters are bulky, not a good match for a petite recorder or laptop, and so you may want to consider having a cable interface to relieve strain on the recorder input. Make sure to get mono, not stereo, adapters and cables.
If you do want to use more than one mic, you will need a mic mixer. There are many good ones available, 2-8 channels, for around $50-$150 (and lots more, of course!). Just a few inexpensive, yet high quality brands to consider: Behringer and Peavey, to name two. It is critical that you do your homework; many mixers have USB outputs and may not be 100 percent compatible with recording software such as the free, open source Audacity (you can always use the always-present analog audio stereo output into your computers stereo line-in. Virtually all mixers have inputs for both high and low impedance mics (though some very basic ones only have high impedance). One very important thing to consider: most mixers use an AC power adapter, so you will need access to a power outlet. There are a few that can operate on battery power -- the ART Three Channel Microphone Mono Mixer is one.
Many of the new digital recorders have more than one built-in mic making it easy to set for different situations, and you can send each to a separate track, making editing individual voices fairly easy, e.g., boost a quiet talker. More on digital recorders here.
Feel free to ask us for more detailed advice, although, we are by no means audio-engineering experts. Many community colleges have basic recording courses, and you also might check with your local music store or recording studios; many offer lessons on recording basics.
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