8-6 Acoustic Calibration Model 831 Manual
Model 831 with 1/2” Free-Field Microphone
The CAL200 provides a nominal pressure level of 94 dB or
114 dB. The exact levels are printed on the Larson Davis
calibration sheet that came with the calibrator. When using a
free-field microphone, the pressure level at the microphone
diaphragm will be slightly different. Thus, a free field
correction of -0.12 dB should be applied to either of these
levels. Pressure and random incidence microphone do not
require a correction of this type. If the calibrator and
instrument are near room temperature (23° C) and near sea
level (101.3 kPa) then no other corrections need to be made.
If the calibration sheet for the CAL200 indicates 113.98 dB
for it's level when set to 114 dB then set the Cal Level in the
Model 831 to 113.86 dB and 1kHz.
When the microphone and instrument are at a temperature
other than near room temperature or static pressures not near
sea level, then corrections will need to be added for the
ambient temperature and the prevailing static pressure.
Check the calibration data shipped from Larson Davis with
the CAL200 to get these corrections. The corrections can be
added to the level obtained in the previous paragraph to get
the actual level of the CAL200.
The microphone's sensitivity varies with static pressure. If
the instrument is calibrated in one environment and moved
to another, then the sensitivity will change (after
stabilization) depending on the change of temperature and
pressure. The coefficient of static pressure is -0.01 dB/kPa.
If the system is calibrated at 85 kPa for instance then it will
be 0.16 dB less sensitive at sea level. The sensitivity of the
microphone and Model 831 varies slightly with temperature
also. The coefficient of temperature is -0.009 dB/°C. If the
system is calibrated at 18 °C then it will be 0.05 dB less
sensitive at 23 °C.
The Larson Davis 3" Wind Screen has less than 0.05dB
effect on the system response at 1 kHz.
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