The microphone's directivity describes the microphone's sensitivity to sound from different angles. It can be divided into omnidirectional, unidirectional and bi-directional. Omnidirectional has the same sensitivity for sounds from different angles. It is common in recording projects that require the recording of the entire ambient sound; or when the sound source is moving, hoping to maintain a good radio condition; the speaker also has this type of lavalier microphone that is used in the speech. The disadvantage of Omni-directional is that it is easy to receive ambient noise, and it is relatively cheap in terms of price. The common unidirectional style is Cardioid or Hypercardioid, which has the best effect for sound from the front of the microphone. Bi-directional accepts sound from the front and rear of the microphone. There are few applications for direct use, and most of them are used for special purposes such as stereo recording (such as MS and Blumlein recording methods). Its internal structure is basically similar to omnidirectional. The main difference is in the circuit board (PCB).