Understanding Microphone Pickup Patterns
Microphones are made with certain applications in mind. For example, stage use, studio use or field recording use. The way that a microphone picks up sound from various directions is known as its pickup pattern. There are a few standard pickup patterns: Omnidirectional, Unidirectional, Bidirectional and Cardioid. Pickup patterns are usually depicted as polar diagrams, a circular graph of sensitivity of a microphone from various directions.
Omnidirectional
Literally, from all directions. Omnidirectional microphones pick up sound well from all directions, and are frequently used for recording ambient and background sound. Omnidirectional microphones are also used for vocals, because of their lack of proximity effect. They could be used for recording of a group of vocalists, although the preferred method would to split the group into individual singers and each one having their own microphone.
Unidirectional
Literally, from one direction. Although cardioid microphones could arguably be unidirectional, the term 'unidirectional' is now usually reserved for 'gun' microphones. These microphones are long and rod shaped. Grooves on the side of the microphone allow sound coming from the sides to either pass through without reaching the pickup or cancel each other out. As a result, only a thin, cone-shaped area in front of the microphone is picked up satisfactorily. Gun microphones are good for recording individual voices in noisy locations, such as interviews, as well as picking up sound from a long distance.
Cardioid
Cardioid microphones have a heart-shaped pickup pattern. Probably the most common microphones in use today, They reject sound coming from the back of a microphone and are progressively more sensitive to sounds as the direction approaches the front of the microphone. They are favored for stage use as they do not pick up the sound from on stage speakers or monitors so readily, thus preventing feedback. There are versions of the Cardioid pattern called Supercardioid and Hypercardioid, which represent increasingly limited ranges of pickup. As the pattern narrows, feedback rejection improves even more, but due to limitations of construction, a narrowing of the pattern does add a little sensitivity directly behind the microphone. The narrowing does cause the sound recorded to be more pinched and less flattering, but in a stage situation with many speakers placed in unsuitable places, a hypercardioid microphone can be a highly effective feedback prevention measure.