When discussing microphones, I like to use two analogies. First, I will suggest that mics are a lot like saws. There are many different kinds for many different purposes. Different microphone designs and technologies will make one microphone much better suited for one application than another, but for a different application, a different design or technology will be required. Like saws, where you *could* use a scroll saw to cut a board, you *could* use a small diaphragm omni condenser on a lead vocal... but in neither case is it typically going to be the best choice.
Generally speaking, the technologies involved in microphone design involve either the size of the diaphragm, the way the diaphragm converts sound energy to electrical energy, and the capsule design, which affects how the microphone responds to sounds coming in different directions.
Microphones have a very thin, flat moving object that is made to vibrate back and forth as it converts sound energy to electrical energy. This is called the diaphragm, and they generally come in two sizes - small and large. Small diaphragms tend to be more accurate, but large diaphragms tend to sound more robust, or even “larger than life” sounding, in much the same way that a larger speaker tends to sound more robust.
Dynamic vs Condenser:
Most microphones are either dynamic or condenser (sometimes called electret). The technology used does make a difference, but not as much as you might think. Many people are too hasty to make the over-generalilzed conclusion that a condenser microphone is “better” than a dynamic one. The SM7 is a dynamic mic with a large diaphragm. It was used to record the lead vocals on Michael Jackson’s Thriller album. It's a wonderful sounding vocal mic. However, a condenser mic is a more "standard" choice for vocals. They tend to more airy and open sounding, and are more sensitive to transients and dynamics. Dynamic mics introduce a little bit of natural compression to what you're recording, (a function of the physics that determine how they operate) which may or may not be desirable. Dynamic mics are less likely to feed back than condensers, which make them generally better-suited for live sound.
Both dynamic mics and condenser mics come in both small and large diaphragm varieties.
There are other microphone technologies, such as ribbon mics and PZM (pressure zone microphones, also called “boundary” microphones), but by the time you’re likely to buy one of those, your knowledge will likely surpass the scope of this article.
So... we have small-diaphragm dynamic (or more like medium diaphragm) mics such as the ubiquitous SM57/58. These are generally rugged microphones best suited for miking instruments and live vocals. We have large-diaphragm dynamic mics, which are good for lower, resonant sources, like the “big radio announcer” voice, or a kick drum. There are small-diaphragm condensers, which are great for instrument spot mics, and capturing accurate sounds in a room, like for miking an orchestra, or for drum overheads. Finally, there are large-diaphragm condensers, which are primarily suited for lead vocals, but can be applied to many other things that you might want to sound detailed, yet larger than life - an acoustic guitar, or even drum overheads.
Polar Patterns (directionality)
So, within all of these combinations above, any of them may be tailored to pick up sound better in some directions better than others. The two most extreme directionalities are hypercardiod and omnidirectional.
A hypercardiod mic will pick up sounds mostly directly in front of it. This is useful when you want to isolate one instrument from the sounds around it as much as possible, like miking an acoustic guitar while rejecting a singer in the same room. Another significant advantage with a hypercardiod mic is that, because they are so directionally specific, they are less prone to feedback in live situations.
At the other end of the spectrum is the omnidirectional mic. Omnis pick up sound more or less equally in all directions. These can make for very useful room mics, where you want to pick up many instruments at one time. Omnis tend to not make very good stereo overheads, though, as both mics generally wind up picking up the same thing, meaning there is little separation between right and left channels.
Cardiod mics are the most common as they are reasonably directional, but not too picky. Consider a live application where you have a singer/guitarist who plays while standing in front of a microphone. A hypercardiod mic would have the singer fading in and out as he/she made small movements away from the focal point of the mic. A cardiod mic would be more forgiving.
Finally, we have figure-8 polar patterns. These are found in microphones where you visually have a “front” and “back” of the mic. Visually consider what the number eight looks like - a top and bottom bulb, connected by an extremely narrow “waist.” The top bulb would represent what the front of the mic picks up, and the bottom bulb would represent what the back of the mic picks up. The mic does, indeed, pick up sound from both sides. This could be used for recording a pair of backup singers, for instance. What just as useful, though, if not more so, is the “waist” of the figure-eight. That “waist” represents the side of the microphone. A figure-eight microphone rejects all sounds coming from beside it. This area of rejection is called the “null.” A typical application of figure-8 microphones is recording a person who sings and plays guitar. One figure-8 mic will be used on the voice, but placed sideways so that the null of the microphone (the “waist” of the 8) points to the guitar, thus rejecting as much of the guitar as possible. Another figure-8 microphone will be used on the guitar, also placed sideways, so that the voice of the singer is in the null of the second microphone, thus rejecting the voice. This means a minimal amount of bleed between the two microphones, allowing for a good deal of control of both the voice and the guitar when mixing.
Choosing a microphone:
My second analogy is that microphones are a lot like wines. You know how some people or restaurants will have a collection of wines, because the trick is matching the right wines with the right meals? Mic selection works the same way. Some singers - not many, mind you - will sound fantastic with an SM58 thrown in front of them. I've only experienced that once, though. You could liken that to the bottle of Baby Duck that was just perfect with the potato salad at the family picnic last summer. Most singers - not all, mind you - will sound fantastic (or as fantastic as they're going to sound) when you throw a U87 in front of them. That would be the equivalent of saying that you can serve Dom Perignon with almost anything. That's one of the reasons that, like that bottle of Dom, the U87 is so expensive. (not even close to a Sony c800G, but the last time I spent that kind of money on anything, I drove it home...) To go back to the Thriller example I used earlier, the producer surely had access to a U87. There is no doubt about that. But in that particular case, it was not the bottle of Dom that got served. The producer found that the SM7 wine matched the Michael Jackson entree better.
Finally, I want to kill any notion you might have of using frequency specs to determine the quality or suitability of a mic I thought that flat and transparent would be the holy grail of microphone ideals at one time too. If you look at a frequency response curve for some of the most expensive mics, and they are NOT flat. That "not flat"-ness is the character they impart, and it is that character that people are willing to spend big bucks on. You don't pay big money for a Les Paul because it has a good balance of tone across the frequency range. You buy it because it sounds like a f**king Les Paul! Same thing with mics. (and preamps!) By comparison, the most accurate mics in my collection are a pair of Behringer ECM8000's. Their frequency response curve is more accurate than any other mic on the market until you start hitting the $3000 range. They are small-diaphragm condensers (no surprise). They were about $40/ea. I rarely use them. They are so accurate that they are entirely uninteresting, and entirely unflattering. I am reluctant to part with them though. They may well, because of their accuracy, be just the right wine for the right meal at some point.
One last thing....This may be the greatest resource on microphones on the planet:
It is a condensed version of a 97-page thread from another site. Harvey Gerst, the author, has 50+ years of experience as a gold record songwriter, recording engineer, producer, and product design specialist. He's worked with Bob Dylan, Albert King, Jimi Hendrix, The Doors, and Jefferson Airplane. He designed prducts for JBL, Trident, Morley, etc. In short, there are few people who know more about this stuff than him, and he has chosen to share it.