> and uses the identifiers to track which channels are getting played.
I don't think they're using infrasound for that. I think they use a technology similar to Shazam where it just analyzes the sound to determine what's on.
They are, it's one of the reasons they [Nielsen] bought Arbitron back in 2013. The PPM devices use low-frequency tones encoded into the audio stream at the time of broadcast.
https://en.wikipedia.org/wiki/Portable_People_Meter