What is "Directivity Index"?

Directivity Index

The ratio, expressed in dB, of a microphone’s output produced by a plane sound wave arriving in the direction of the reference axis, to the microphone’s output produced by a diffuse sound field having the same frequency or frequency band and sound pressure.

The typical directivity index for standard microphones is between 0 dB (omnidirectional) and 6 dB (hyper-cardioid). More directional microphones, like shotguns, can exhibit higher directivity, but this may vary with frequency.