Larson Davis SoundAdvisor 831C Reference Manual page 54

5.1.3 Understanding Calibration Stability
SoundAdvisor Model 831C
can be accurately measured without being influenced by the operational noise from the
meter (Noise Floor).
Noise Floor Level
The noise floor is calculated as the energy sum of the noise generated by the microphone,
preamplifier, and instrument circuitry. The appropriate noise floor, and nominal sensitivity
compute automatically in the 831C with the following preamplifiers and their commonly
paired microphones:
• PRM831
• PRM2103
• ICP with ADP074
Overload Level
When using a microphone having a sensitivity of 50 mV/Pa, the input overload may occur
approximately as shown in Table 5.1.
Table 5.1 Input Overload Levels
Input Gain, dB
Under Range Level
The Under Range level is the higher of the following levels:
• Noise Floor + 9.14 dB (the under range value shown when self noise contributes ≥
0.5 dB to the readings).
• Actual point where the log-linearity exceeds maximum permitted value.
Except for very low-noise-level microphones, the under range level is usually determined
by the noise floor + 9.14 dB.
For more information about measurement range, noise floor, linearity
LEARN MORE
range, and max peak levels as they relate to different microphones and gain
settings, see "SLM Performance (Nominal Values)" on page A-3.
The 831C maintains a stable value of sensitivity over long periods of time. Significant
changes in sensitivity, or a pattern of small but regular sensitivity changes, are indicative of
problems with the measurement system. To assist you in identifying these situations, the
831C provides 2 notifications:
• Calibration History Table
This table includes data and date/time for the 10 most recent calibrations. It's a good idea
to periodically review the instrument's Calibration History, which is found on the meter
3

under Tools Calibration
• Large Change Notifications
During calibration, the meter makes an automatic comparison between the sensitivity
determined by the calibration and the previous value. A pop-up warning displays when the
difference between these two values exceeds 3 dB.
0
20
//*

Cal Hist (
Understanding Calibration Values
Overload Level, dB Peak
143
123
)
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