I had a question about sound measurement in air using a B&K microphone. In the acquired time resolved sound measurement signal I can see both negative and positive voltages/ pressure values. I was wondering does the negative values associates with the rarefaction events in time at a fixed position in space and correspondingly positive value associates with the compression events ? If this is correct the mean value of the time signal should represents the ambient pressure in the absence of the sound?
A microphone usually measure pressure variation or pressure gradient of the acoustic sound wave at the transducer location. That means that the atmospheric pressure (what you possibly call ambient pressure) does not generate output on a microphone.
What you are actually measuring at the output of the microphone is the acoustic wave pressure oscillating around the atmospheric (quasi) constant pressure.
To measure atmospheric pressure, you need a barometer.