The key is the 'Doppler effect'. That's the way sounds appear to change based on the distance between a moving source and the listener. You hear it all the time: it's what makes fire engine and ambulance sirens, car stereos and airplane propellers seem to change pitch as they go past.
What's clever here is that Colsto has focused on the sounds made by aircraft in the backgrounds of videos. In many parts of the world the skies are very busy, and a faraway airplane that's inaudible to you or other listeners might still leave a sonic footprint on your video or audio that can be isolated and analyzed. That's definitely the case in the US data Colsto used.
