Microphone Polar Patterns

When setting up a home studio, it is important to choose the right type of microphone or microphones. You see, each microphone has its own polar pattern. A polar pattern is a graphic representation of a microphone's sensitivity to sound relative to the direction or angle from which the sound arrives. There are four basic types of polar patterns for microphones: Cardiod, Super Cardiod, omnidirectional and bidirectional; each serves a different purpose.

A Cardiod microphone is a microphone that is most sensitive to sound from the front and least sensitive to sound from the back. Their unidirectional pickup makes for affective isolation of unwanted ambient sound and high resistance to feedback when compared to omnidirectional alternatives. A cardiod microphone are probably the best option for a live performance.

Supercardioid microphones offer a narrower pickup than cardioids and a greater rejection of ambient sound. However, they also pick up a small amount of sound from directly behind. For this reason, it is particularly important to place monitor speakers to the side facing the 'dead spots'. Supercardioids are highly suited to very loud stage environments as they are very directional with high gain before feedback. 

Omnidirectional microphones are equally sensitive to sound arriving from all angles. Therefore, the microphone does not need to be aimed in any particular direction. This can be particularly useful when using a lapel mic to capture a speaker’s voice, as the individual can move their head without affecting the sound. The disadvantage is that an omni mic cannot be aimed away from undesired sources, such as PA speakers, which may cause feedback.

A bidirectional microphone (also called a figure-8 microphone) picks up sound from the front and rear of the microphone, but not from the sides. Microphones with this patterns are usually ribbon or large diaphragm condenser microphones. These microphones are very sensitive and therefore should be kept away from monitors.

Sources:
http://www.shure.co.uk/support_download/educational_content/microphones-basics/microphone_polar_patterns