Polycom SoundStructure C16 Design Manual page 215

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.
Typical Room Gain Measurements
Good
-10
Acceptable
-10
Not Acceptable
-10
Tabletop microphone applications typically have room gains that are 0 or less while ceiling microphone
applications typically have room gains that are 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 reference 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 is increased by the amount the amplifier gain is increased. The convergence
of the AEC can slow down when the room gain exceeds approximately +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 also is 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.
Polycom, Inc.
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