Automatic Microphone Mixers; Noise Cancellation - Polycom SoundStructure C16 Design Manual

is that the resulting signal picked up by the microphone includes significant amount of additional noise
sources that are closer to the microphone than the talker such as HVAC. In addition, the local talker's mouth
will be farther from the microphone, reducing the signal level that is picked up by the microphone. Any
additional gain added to pick up the local talker's signal will also amplify the background noise, exacerbating
the problem.
To set proper expectations, demonstrate the audio quality of a ceiling microphone in the target room as early
in the design process as possible. This can be done simply by temporarily hanging a microphone from a
ceiling with tape or some other temporary adhesive, and running the microphone cable to a location
acoustically isolated from where the microphone is temporarily installed. With the use of a microphone
amplifier and powered loudspeaker, it is a simple matter to have the customer listen to the audio quality and
agree that it is acceptable before additional work is performed. This will save costly re-installation work.
More often than not, once the conference room users hear the resulting sound quality of ceiling
microphones, they will allow tabletop boundary microphones to be used in their rooms, or at the very least
agree that the microphones must be placed closer to the local participants.
When using ceiling microphones, it is important to install the microphones away from noise sources in the
ceiling including HVAC, light fixtures, projectors, loudspeakers, and any other noise source. If ceiling
microphones are the only option, it is better to hang the microphone from the ceiling as close to the talkers
rather than to have them flush-mounted if possible. While aesthetically less appealing than a flush-mounted
ceiling microphone, the hanging microphone will move the microphone away from any ceiling noise sources
including the loudspeakers that will play the remote audio into the local room and get the microphones
closer to the talkers. If the ceiling microphone is close to the ceiling (less than a foot from the ceiling), but
not flush mounted, the same boundary affect that improves the sensitivity of flush-mounted microphones
will cause multiple delayed signal paths to be picked up by the microphone, unnecessarily degrading the
overall audio quality of the system. Ceiling microphones (hanging or flush mounted) should have no more
than a half-spherical pick up pattern. In typical conference rooms with 9 foot high ceilings, it is common to
hang the ceiling microphones one or more feet from the ceiling over the conference table. Again, the closer
to the talker's mouths the better the system can sound.
When planning for how many ceiling microphones are required, keep in mind that a ceiling microphone will
cover approximately 100 sq. feet of room space. This is dependent on the critical distance of the
microphones. For instance, rooms that have very high ceilings (defined as > 10 feet) should only use ceiling
microphones if it is possible to hang them several feet from the ceiling.

Automatic Microphone Mixers

As described elsewhere in this guide, the role of the automatic microphone mixer is to limit the number of
microphones that are open (or contributing audio) at any given time to only the microphones associated with
active talkers. By reducing the number of microphones that are active, the local speech that will be sent to
the remote site will be less reverberant and less noisy.
There is a significant reduction in audio quality if all the microphones are active all the time versus being
processed by the automatic microphone mixer. This difference is particularly obvious when ceiling
microphones are used due to the high noise and reverberation levels associated with microphones that are
farther away from the local talkers and approaching greater than 50% of the microphone's critical distance.

Noise Cancellation

The ambient noise in the room caused by HVAC, projectors, computers, and even noise external to the room
that is picked up by the microphones will reduce the signal to noise ratio at the microphones. This noise will
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