Polycom SoundStructure C16 Design Manual page 718

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 Omni-directional 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.
Directional microphones are most often used in conferencing systems due to the rejection of the
background noise, reduction of the reverberation, and the rejection of the audio from the loudspeakers.
Directional microphones also increase the gain-before-feedback in sound reinforcement applications due to
the increased rejection of the loudspeaker signal when it is directed towards the rear of the directional
microphone.
As seen in the following figure, directional microphones typically have three degrees of directionality -
cardioid, super cardioid, and hypercardioid - each with increasing directionality (that is, decreasing areas of
pick-up). A microphone with a cardioid pick up pattern will pick up signals within about 130 degrees of the
front of the microphone, a super cardioid about 115 degrees, and a hypercardioid about 105 degrees. From
the following figure it is clear that the supercardioid and hypercardioid microphones have a lobe in the rear
of the microphone that will pick up background noise more than the cardioid microphone.
Shotgun microphones are even more directional that hypercardioid microphones and attain this
directionality by having the microphone element in a tube with slots cut along the side. The signals from a
narrow arrival angle will reinforce constructively while those from off angle that travel farther will cancel each
other out, providing an extremely narrow pick up pattern. Shotgun microphones are typically not used in
conferencing due to the extremely narrow pick-up pattern - should the local talker move out of the narrow
pick up pattern, their voice will be significantly attenuated. To the remote listeners, it will sound like the local
talker has faded out.
Polycom, Inc. 718