One important characteristic of microphones is the polar pattern, also known as the pickup pattern or directivity.
There are two main types--omnidirectional and directional. For now, let's concentrate on the omni.
An omnidirectional mic, as the name implies, has a uniform response (up to some frequencies) to sound coming in from all directions. At higher frequencies, omnidirectional mics exhibit some directionality, with response stronger at the front of the mic than at sides or rear. Just what that frequency is depends on the microphone capsule and body design.
Omnis are very useful for interview mics, since you don't have to be concerned about pointing the mic at just the right angle to capture the latest "no comment." This is useful in situations where every news organization is jockeying for mic space at a press conference and also for simpler two-way interviews.
Use omnis as podium mics. When the presenter turns to look at the slides the voice level and frequency response won't drop off, assuming the presenter still remains at the same distance from the mic. But even if the presenter does back away from the mic, the timbre of the voice won't change, only the level, which is a lot easier to deal with at the mixing console.
Full fidelity musical instrument recording benefits from an omni mic with an even polar pattern over a wide frequency range and also an overall smooth frequency response. Omnis are great when two or more singers use a single mic.
But omnis can have some downsides because they do pick up sound, both wanted and unwanted, from all angles. If used in stage situations with sound reinforcement systems and monitor loudspeakers, they could, depending on the location and distance of the loudspeakers relative to the mics, pick up the sound emanating from the loudspeakers with enough level to cause feedback. Having said that, omnis have been used very successfully in staged productions, where their smooth frequency and polar response characteristics, as well as physical placement, have been carefully balanced to actually achieve better gain before feedback.
In studio situations where you want to isolate each musician and musical instrument for multi-track recording without physically isolating them, omnis may not be the best choice, as they will pick up nearby sound sources.
And omnis can more easily pick up ambient room noise and reverberation, the level of which can become quite noticeable when the mic is held at some distance from the desired sound source.
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