Omnidirectional
Picks up everything equally. No proximity effect, maximum room sound. An omnidirectional mic responds only to pressure, not pressure gradient, so the angle of the source does not matter.
Equal in Every Direction
An omnidirectional microphone responds equally to sound from any angle. Rotate it, walk around it. The level stays the same. This is because it responds only to pressure, not pressure gradient. A pressure wave compresses the air equally from all directions, so the diaphragm does not care where the sound came from.
No Proximity Effect
Because omni mics are pure pressure transducers, they do not exhibit the bass boost that directional mics show when close to the source. This low-frequency emphasis, called the proximity effect, comes from the pressure-gradient component in directional mics. With an omni, the tone stays consistent regardless of distance.
Maximum Room Sound
The downside of hearing everything equally: maximum bleed from other sources and maximum room ambience. An omni mic captures the full character of the space it is in. This is great for recording a room (orchestras, choirs, ambient recordings) but challenging when you need isolation from other instruments on stage.
Try it: drag the sound source around the mic and notice the level stays constant at every angle. Only distance changes the level.