Polycom C16 - Manual (Page 465)

Designing Audio Conferencing Systems
B - 3
Microphone Selection And Placement
The type of microphones used and their location will have the largest impact
on the audio conferencing quality. Microphones translate the acoustic signals
from the local talkers into electrical signals that can be processed and sent to
the remote participants.
Microphone Fundamentals
Most microphones used in conferencing systems are electret microphones, a
version of condenser style microphones where an acoustic signal on a thin film
dynamically varies the capacitance of an electrical circuit which in turn creates
an electrical voltage that represents the microphone signal. Condenser
microphones require a bias voltage, called phantom power, to operate
properly. Electret microphones are a variant of condenser microphone that
replaces the thin film with a dielectric material that is permanently charged
and suspended above a metal plate. While electret microphones don't require
a bias voltage to operate due to being permanently charged, they do typically
contain an integrated preamplifier that is powered using the phantom power
from the device the microphone is connected to. Due to the design of electret
microphones, these microphones come in a large variety of sizes and shapes
and can provide excellent audio quality.
Phantom Power
Electret (and condenser) microphones require a power supply, called
phantom power, to power the electronics of the microphone. This power
supply may come from a battery or from the electronics that the microphone
is connected to. Microphones typically operate with phantom power voltages
ranging from 9 to 54 V DC (with 48 V specified in the standard IEC 61938)
although there are some microphones that only operate with a more limited
range and will not operate with 48 V. Electret microphones typically require
approximately 2 mA of current of phantom power although they can require
as much as 10 mA.
The phantom power is supplied across the positive and negative balanced
audio signals with respect to the ground/shield of the microphone.
Directional vs Omnidirectional Microphones
Electret microphones are either omni-directional and directional in their
pick-up pattern depending on how the electret microphone element is
physically mounted within the microphone enclosure.
Omni-directional microphones will pick up sounds from all directions around
a microphone (a 360 degree pick up pattern) while directional microphones
have been designed to pick up signals better in the pick up zone of the
microphone and to reject signals outside of the pick up zone.