What is "Bidirectional Microphone"?
This type of microphone is based on the pressure gradient principle. The sound can reach both sides of the membrane. The microphone provides a signal whenever the sound is different on the front and the backside of the diaphragm. If the admittance for the acoustical sound is the same at both sides of the diaphragm, the polar pattern of the bidirectional microphone forms a figure 8 and the microphone is equally sensitive in the two opposite directions. It should be noted that the two lobes are oppositely phased.
The bidirectional characteristic can also be obtained by using two diaphragms, one on each side of a common backplate (The Neumann principle), or the directivity can be obtained by using two oppositely phased pressure (omni) capsules.