Boundary effect microphones
The original boundary effect microphone was the Crown PZM (Pressure Zone Microphone) so the boundary effect microphone is often referred to generically as the PZM.
In this mic, the capsule is mounted close to a flat metal plate, or inset into a wooden or metal plate. Instead of mounting it on a stand, it is taped to a flat surface.
One of the main problems in the use of microphones is reflections from nearby flat surfaces entering the mic.
By mounting the capsule within around 7 mm from the surface, these reflections add to the signal in phase rather than interfering with it.
The characteristic sound of the boundary effect microphone is therefore very clear (as long as there are no other nearby reflecting surfaces).
It can be used for many types of recording, and can also be seen in police interview rooms where obviously a clear sound has to be captured for the interview recording.
The polar response is hemispherical.