Ok I think you misunderstood something
My iPhone App is a acoustic measurement. But the measurement via Audacity, seen in the video, is a "real" optical measurement !! I use a IR-LED and an IR-Fototransistor, which is connected to the audio-input of my computer. When light hits the transistor, I can see this on the audacity signal. This method of measuring is very exact, so it can be used as a reference to the acoustic measurement from my App.

I thought this was clear to see in the video, apparently it isn't...