Polycom SoundStructure C16 Design Manual page 737

Once the "zones" are planned, the microphone locations are selected within those zones. The concept here
is to locate microphones and speakers such that each zone is completely independent in level and mix. This
way, microphones from a given zone are never played into the loudspeakers associated with that same
zone (mix-minus) and are sent at increased levels to zones further away (the inverse square law calculated
results drives the required level settings in the reinforcement system - zones twice as far away will typically
have 6 dB more level). To support zoning, a multi-channel amplifier must be used so that each loudspeaker
zone can receive separate loudspeaker signals.
There are two general concepts that are often used in voice lift. Needed Acoustic Gain (NAG) or "how loud
does it need to be" and Potential Acoustic Gain (PAG) or "how loud can it be without feedback" are the
calculations that can be done with a few different online tools, and will quickly help determine the sound
levels that can be tolerated within a room. For the room to be acoustically stable, the NAG must be less than
the PAG, and in fact should be less than PAG by some safety margin just to be safe.
Occasionally the PAG can be slightly improved with equalization, feedback eliminators (mostly these are
just fast reacting narrow filters that reduce the gain at the ringing signal at the onset of feedback), and
microphone/loudspeaker directionality improvements, but those are usually limited to less than 6 dB total
improvement. Placing microphones as close as possible to the local talkers, and minimizing the number of
active microphones will help with NAG/PAG.
Ceiling mounted microphones present particularly difficult sound reinforcement challenges due to their close
proximity to loudspeakers (decreasing PAG) and their long distance from the local talkers (increasing NAG).
The use of ceiling microphones and sound reinforcement must be designed extremely carefully with
conservative levels of reinforcement, large separation of zones, and limited volume control range to ensure
that the resulting room is stable. In situations where ceiling microphones and loudspeakers must be used
with the added requirement of in-room reinforcement, it is recommended that the design and installation be
performed by a professional who specializes in these applications.
In a room that has sound-reinforcement with inappropriately high gain settings, there is no longer any such
thing as a "side conversation". Everyone in the room will likely be able to hear all conversations, making it
impossible to have side comments that are private.
With reinforcement applications, remember that the key to success is setting the appropriate performance
expectations with the end user and it is the responsibility of the conference room designer to set that
expectation.
Polycom, Inc. 737