What are Omnidirectional Microphones?
Omnidirectional microphones are microphones that pick up sound with equal gain from all sides or directions of the microphone. This means that whether a user speaks into the microphone from the front, back, left or right side, the microphone will record the signals all with equal gain. This is in contrast to unidirectional microphones, which pick up sound with high sensitivity only from a specific side.
Omnidirectional microphones have a polar plot which is circular, because it records sound from all directions, 0º to 360º, with equal gain. The polar plot image shown is a typical polar plot response of an omnidirectional microphone. You can see the circular pattern. So whether a user speaks into the microphone from any direction or angle, the same gain will be achieved.
Omnidirectional microphones are advantageous over undirectional microphones in that they allow greater flexibility in the directionality of sound pick-up. Stated again, a user can speak into any side of the microphone and it will still pick up good gain from all sides. This can be very useful in applications where sound needs to be recorded from multiple directions. An example of this may be a musical performance where there is a band of musical instruments in front and a choir singing behind. A undirectional microphone would only be able to pick up one of the two with good gain while recording the other very lowly, if at all.
However just because omnidirectional microphones pick up sounds from all directions while undirectional microphones only pick up sounds from one direction doesn't mean they're always superior to unidirectional microphones. In many applications, picking up sound from all directions may be largely undesired. An example of this is recording a professor's lecture in a classroom. In a scenario where only the lecture of the professor needs to be recorded without any noise that may be coming from the students behind, a unidirectional microphone is best to be used. A common microphone that is used in a situation such as this is a shotgun microphone, which uses a unidirectional microphone. Shotgun microphones focus on the sound coming from directly in front of them while ignoring sounds from the sides and rear. This allows the shotgun microphone to pick up the desired sound directly in front of it with great gain while ignoring all noises that may be present on the sides and rear. Thus, a cleaner recording can be achieved when unidirectional microphones are used, as long as the sound source to be recorded stays stationary in front.
So both omnidirectional and unidirectional microphones have their advantages and disadvantages. The important thing is to know the application of the microphone in use so you can decide which would be better for the given situation at hand.