In-Room Reinforcement - Polycom SoundStructure C16 Design Manual

audio conferencing device (typically 0 to 10 dB). If the recommended ERL is exceeded, it will be necessary
to review the gain structure (lowering the loudspeaker amplifier settings and increasing the remote audio
levels), ensuring the loudspeakers are pointed directly into the pick up pattern of the microphones, or
lowering the gain on the microphones.

In-Room Reinforcement

In a given room, sound reinforcement may be desirable if it is not possible to have listeners easily hear other
talkers in the room. The objective of this type of sound reinforcement (sometimes referred to as "voice lift"
in the generic sense) is to augment the local talker's voice so that local listeners still have the perception
that the audio is originating from the local talker but now the level of the speech has been increased subtly
via the installed loudspeaker system.
One of the keys to making a system of this type stable and work reliably is properly setting user expectations
and perception. It is critical that the users do not expect audio levels similar to a "paging system" or "public
address" (PA) system. Paging and PA systems are designed to broadcast a single voice loudly, overcoming
background noise, side conversations, and inattention in order to deliver important messages. The sound
levels generated by these systems are much louder than local participants in a room would ever need to
subtly reinforce their voice and maintain the perceptual directionality so the local listeners' attention remains
focused on the talker and not on the loudspeaker.
Voice reinforcement or enhancement is supposed to be just that; reinforcing the level of the talker to improve
intelligibility and not re-create the sound level of an emergency page. In a properly configured and calibrated
voice lift system, the best way to find out if the system is functioning properly is to disable it and see if the
local participants notice the resulting loss of intelligibility. If they do, then the system is set correctly. The
reinforced levels should never exceed conversational speech levels (approximately 70 dBA SPL typical at
the listener's ear) or the result may become unstable, creating residual echoes to the remote listeners due
to low ERL and worse may generate very loud acoustic feedback in the room with loudspeaker audio
coupling into the local microphones.
Voice lift is typically needed when the room is large enough that the local talkers' audio drops below 60 dBA
SPL when heard by the farthest listener in the room. Based on normal speech conversation levels of 70 -
77 dBA SPL this roughly corresponds to listeners that are approximately 20 to 25 feet away from the talker.
Another way to view this is as a critical distance issue. If the local listeners are farther away than the critical
distance form the local talker, some form of voice lift may be required for adequate intelligibility.
For rooms smaller than approximately 20 feet x 20 feet, the voice lift application is really not necessary and
is just starting to be of some benefit in rooms 30-feet (9 meters) square. Just as adding gain to a microphone
to try to compensate for a critical distance issue does not work, adding sound reinforcement to compensate
for a noisy room doesn't work well either. The microphones that pick up the noise will reinforce that noise
into the room, adding to the noise rather than making it easier to be heard. The correct approach would be
to get rid of the noise and make the room useful in all applications.
So, how does one realistically go about making this system work? One must be careful planning microphone
and speaker locations, so that the system stays acoustically stable (that is, no squealing and howling of
feedback as different microphones come active) while providing the necessary pickup of local talkers and
reducing the "effective acoustic distance" (how far away do they sound) of the listeners. First, consider the
number of "zones" or areas of independent loudspeaker playback. A "zone" is most often sized close to the
largest room size that does not need reinforcement, or about 20-feet square. A room 20-feet by 40-feet
would be 2 zones, one 40-foot square would be 4 zones and so on. Long, narrow boardrooms will often
require this type of voice reinforcement application.
Polycom, Inc. 736