A majority of MEMS microphones are inherently omnidirectional by virtue of having a single sound port (typically located on either the top or bottom of the package) which allows sound pressure to push and pull the diaphragm.
This type of microphone is also known as a pressure microphone because its electrical output is proportional to the pressure present at the sound port.
The ICS-40800, on the other hand, is a dual port MEMS microphone offered by TDK which inherently has a bi-directional polar plot. This type of microphone is known as a pressure gradient microphone because its electrical output is proportional to the difference in pressure between the front and back of the diaphragm.
The following application note describes the general theory of operation as it relates to beamformers and an example of how to employ the ICS-40800 to achieve a cardioid-shaped polar plot for directional microphone applications.