Larson Davis CAL150 Manual - Calibrating With ½"
CAL150 Technical Reference Manual
Microphone Calibration
2-2
position for 94.0 dB, as indicated by the label on the lower
left of the front panel.
Calibrating with ½"
Free-Field Microphone Corrections
The CAL150 provides a nominal pressure level of 94 dB or
114 dB. The exact levels are printed on the Larson Davis
calibration sheet that comes 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 CAL150 indicates
113.98 dB for its level when set up 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 CAL150 to get these corrections. The corrections can be
added to get the actual level of the CAL150.
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
0.21 dB 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.