Polycom C16 - Manual (Page 189)

Installing SoundStructure Devices
7 - 13
The following figure shows different room gain measurements that may be
found in a typical room. Room gain is considered good if it is negative, mean-
ing that the echo picked up by the microphone is less than the level that is
output to the amplifier. Acceptable room gain occurs when the room gain is
less than +10dB. Not acceptable room gain occurs when the room gain exceeds
+10dB.
Tabletop microphone applications will typically have room gains that are 0 or
less while ceiling microphone applications will typically have room gains that
will be positive due to the proximity of the loudspeakers and ceiling
microphones.
Negative room gain indicates that the AEC has a good level for the AEC refer-
ence and there is not excessive acoustic echo. Positive room gain indicates that
the relative levels of the AEC reference to the microphone input should be
reviewed and if the level of the reference is too low, the input gains of the
remote audio sources may need to be increased while at the same time the
in-room amplifier level reduced so that the overall level remains the same.
Reducing High Room Gain
A common issue is for the AEC reference signal level, the remote audio, to be
too low and the in-room amplifier turned up to compensate for the lower
signal level coming into the SoundStructure device. When this happens, the
room gain will be increased by the amount the amplifier gain is increased. The
convergence of the AEC can slow down when the room gain exceeds approx-
imately +10dB. In general, the higher the room gain the longer it may take for
the AEC to converge completely. This may have the effect of the remote site
hearing residual echoes while the AEC converges.
To fix this issue, check the input signal level meters for the remote audio that
is coming into the SoundStructure device to ensure that the appropriate signal
gain has been applied. When the level of the remote audio is increased the
in-room volume will also be increased and the amplifier should be turned
down to compensate for the higher signal level.
Another common issue is for the loudspeaker audio to be coupled directly into
a microphone. In this case, it may be necessary to relocate the microphone
away from the loudspeaker source, redirect a directional microphone away
from a loudspeaker, or reduce the input gain on the microphone or amplifier
to reduce the level of the echo picked up by the microphone.
-1
-1
-1
Good
Acceptable
Not Acceptable
-10 -4
-4
-4
-7
-7
-7
8
8
8
11
11
11
14
14
14
5
5
5
2
2
2
17
-10 17
-10 17