Polycom C16 - Manual (Page 187)

Installing SoundStructure Devices
7 - 11
sense, the meter segment label represents the minimum signal level required
to light the meter segment. The clip indicator at +20 will illuminate when the
signal exceeds +20dB.
Signal Meters
The meters on the SoundStructure devices show a VU average signal level
with a peak meter overlaid on the VU meter. The VU meter drives the meter
segment display while the peak meter shows the maximum amplitude. The
peak meters conform to the IEC 60268-18 standard and have a 12dB/second
decay from the peak signal levels.
To properly adjust the levels for microphones, adjust the analog input gains so
that during normal speech at the desired distance from the microphones two
yellow LEDs are reached by the peak meter and occasionally additional LEDs
are flickering above that. The VU meter should show a solid green LED at the
0dB level.
The following figure shows examples of peak signal levels that are too low,
just right, and too high during normal conversational speech at the desired
distance from the microphone.
If the meter levels are too low for a given microphone and for the desired dis-
tance from the microphone, increase the input gain slider to add more gain to
the signal in the analog domain. As a starting point for adjusting gains, con-
sider the following table that lists microphone sensitivities with the analog
input gain required to create a 0 dBu nominal signal level in the SoundStruc-
ture products assuming a 72dB SPL audio signal at the microphone. The
sensitivity information includes both dBV/Pa and mV/Pa formats and the
microphone gains in this table have been rounded to the nearest 0.5dB.
SoundStructure devices provide up to 64dB of analog gain to support micro-
phones with sensitivities as low as -44 dBV/Pa (or 6.3 mV/Pa). Microphones
that have a lower sensitivity may require additional external signal gain to
provide enough gain to get to the 0 dBu nominal signal level. A microphone
with higher sensitivity means that less gain is required to achieve a 0dBu nom-
inal signal when a 72dB SPL signal is present at the microphone.
Bad
(Too Low)
Good Bad
(Too High)
-20
-16
-12
-8
-4
0
4
8
12
16
+20