An omnidirectional microphone, also called a pressure microphone or pressure operative microphone, is a microphone that is equally sensitive to sounds coming from all directions. Only one side of the diaphragm inside an omnidirectional microphone is exposed to the impinging sound, so that the diaphragm responds to the pressure variations uniformly. Omnidirectional mics are good for ambient sound and group vocals.

 

Omnidirectional polar pattern