Cirrus Optimus Green Series, Instrument Hanbook

Page: 47 / 56

Optimus sound level meter - instrument handbook
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9.5.1 Free Field Correction
When calibrating a microphone that is to be used for free field measurements, a
small correction may be necessary to compensate for the difference between the
microphone’s free field response at ‘zero degrees’ or ‘head-on’ incidence, and the
pressure level generated by the calibrator.
The correction is typically -0.3 dB for Cirrus ½ inch microphones (making the effective
calibration level 93.7dB).
An example of the procedure used to calculate the value for an MK:224, MK:229 or
MK:216 microphone is shown below:
Level = 94.0 dB + microphone correction
Level = 94.0 dB + ( -0.3 dB)
Level = 93.7 dB