When a microphone receives external power, there is a positive polarization voltage on the back plate. This positive polarization of the plate causes positive changes in air pressure to send a negative electrical signal from the microphone, and negative air pressure to send positive electric signals. Frequency measurements are accurate but the polarity of the result will look negative.
Prepolarized Microphones do not exhibit this behavior. A positive acoustic input gives a positive electrical output, and a negative acoustic input gives a negative acoustic output.
In SoundCheck, there is an easy way to convert the polarity of these signals. Insert a Unary type Post Processing step, set Operand A to your recorded time waveform, and select Change Sign for the operation.
The stimulus to be played:
The Recorded Time Waveform:
Run this waveform through a Post Processing step to change the sign:
The modified Recorded Time Waveform: