What is "Proximity Effect"?

Proximity Effect

An inherent characteristic of pressure gradient microphones, resulting in a boost in the low-frequency response when the microphone is brought closer to the sound source. Figure-of-eight microphones exhibit the highest degree of proximity. The more the characteristic approaches an omnidirectional characteristic, the less proximity.

In practice, this phenomenon exists within the length of an arm.

Pressure microphones (omnis) do not exhibit proximity effect.