A microphone's polar pattern is represented by a 2 dimensional chart that visually shows the microphone's sensitivity at different angles and distances. Though the chart shows the microphone's polar pattern in 2 dimensions, in reality the microphone's actual polar pattern is 3 dimensional.
Though there are many different polar patterns today, early microphones were either omnidirectional or bidirectional.
OmnidirectionalOmnidirectional microphones are sensitive to sound from all directions pretty much equally. They work by measuring sound pressure at a certain point, and therefore are not sensitive to the direction the sound came from.
Omnidirectional microphones won't pick up wind noise or plosives like other microphones, and don't cause a proximity effect as the source gets closer to the microphone.
Unfortunately, since omnidirectional microphones pick up sound from everywhere, they do not provide good sound isolation. Also, this polar pattern is pretty much useless in live music situations.
Being able to pick up sound from all different directions certainly has its place in music recording. Omnidirectional microphones work great for recording ambience, as well as orchestras, and pianos. These microphones lead to a natural sound and work best in rooms with good acoustics.
Due to the nature of omnidirectional microphones, they also work great as lavalier microphones. Since they don't pick up plosives or have a proximity effect, they can pick up the human voice quite well. Also, the omnidirectional pattern prevents these microphones from being affected by head movement.
BidirectionalAnother common type of polar pattern is bidirectional (figure of eight). Bidirectional microphones pick up sound equally from the front and back, but pick up almost no sound from the sides.
All ribbon microphones are bidirectional since the ribbon is sensitive to sound from both sides. Even some large diaphragm condenser microphones are bidirectional.
Bidirectional microphones are useful in some types of stereo recording, and have even been used to record two singers at once when recording engineers had limited tracks to work with.
CardioidBy far the most popular polar pattern of microphones today is cardioid. Cardioid microphones have a heart shape pattern that is most sensitive to sound from the front, less sensitive from the sides, and not sensitive from the rear.
The cardioid polar pattern is formed by the interference between the omnidirectional and bidirectional polar patterns. At the front, the two polar patterns interfere constructively, boosting the signal and making it the place where most of the sound is picked up. No interference happens at the side, so this level remains lower than the front, and is only picked up by the omnidirectional pattern. The omnidirectional and bidirectional patterns interfere destructively at the back, leading to no sound being picked up there.
Cardioid microphones are used when isolation is needed, whether that be from other instruments, or trying to avoid picking up room ambience. They work great for recording guitar amps, vocals, and drums. Cardioid microphones also produce better results than omnidirectional or bidirectional microphones in rooms that are not acoustically pleasant.
By only picking up sound from one direction, they reduce ambient noise and are resistant to feedback in live environments. However, cardioid microphones do pick up wind noise, plosives, and are susceptible to the proximity effect.
An extension on the cardioid polar pattern is the supercardioid. These microphones have an even narrower pattern than cardioids so they pick up even less sound from the side. Supercardioid microphones are less likely to have feedback issues than cardioid microphones, but do pick up some sound from behind. Due to their tight polar pattern, supercardioid microphones work great in loud, live environments.
HypercardioidThe final type of microphone polar pattern is hypercardioid. This polar pattern is even narrower and tighter than supercardioid and picks up even more sound from behind. These microphones are also known as shotgun microphones, and are typically used to pick up a specific sound source from a distance, especially in environments with a high level of ambient noise.
In the end, choosing the right polar pattern for a situation is very important to gettting good results when recording. Whether recording room ambience or needing tight isolation for live recordings, there's a microphone polar pattern that's right for the job!