Polycom SoundStructure C16 Design Manual page 689

Designing Audio Conferencing Systems
Tabletop Microphones
In many conferencing applications boundary tabletop microphones are used
and mounted at locations around the table as shown in the following figure
where one microphone is used for each two participants.
All participants that are seated at the table are well within the microphones
pick-up ranges as shown in the next figure.
C - 7