Larson Davis CAL200 Manual - page 8
2-2
Microphone Calibration
CAL200 Manual
Select the Output Level
The output level is user-selected to be 94.0 or 114.0 dB
using the slide selector on the side of the CAL200. Slide the
switch to lower position for 114.0 dB or to the upper
position for 94.0 dB, as indicated by the label on the lower
left of the front panel.
CAL200 Used to Calibrate an Instrument with ½" 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 its level when set to 114 dB then set the Cal Level in the
instrument to 113.86 dB and 1 kHz.
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 typically -0.013 dB/kPa for PCB
®
½” free-field
microphones. If the system is calibrated at 85 kPa, for
instance, then it will be 0.21dB less sensitive at sea level.
The sensitivity of the microphone varies slightly with
temperature also. The coefficient of temperature is typically
-0.009 dB/°C for PCB ½” free-field microphones. If the
system is calibrated at 18 °C then it will be 0.05 dB less
sensitive at 23 °C.