Microphones that are of the external polarized type and therefore normally need 200V polarization voltage
(like the Nor1225) can be used with reduced polarization voltage. In the Nor145 the polarization voltage can
be set to 70V and the result is that the sensitivity is reduced with approximately 10 dB. When this is done,
the strain in the membrane is lowered and therefore there is a small influence on the frequency response.
The "High Level" correction also numerically brings the response back to a nice flat curve.
This correction is strictly made for the Nor1225, and should not be used with other microphones as the response
correction will differ.
The reduced sensitivity is fine, but the distortion at high levels will be approximately unchanged, so this method
is not good for measuring on sine waves.
When high level is selected you need to use a sound calibrator to verify the absolut level / sensitivity again.