relation between flow rate and frequency response of mobile microphone

I have dyson blade less fan, I'm recording wav files using mobile microphone at different fan levels (I'm measuring fan flow rate and recording using mobile simultaneously). I'm trying to find the relation between flow rate and frequency response of mobile microphone. In matlab I should do the processing for wav files. my question: what is the best normalization in my case. Shall I do the normalization in time domain (before fft) or in frequency domain (after fft), the purpose of normalization is to compare the recorded audios regardless the loudness of the fan while recording since fan sound increases at higher levels and we should compare the air flow not the loudness of the fan.