system
Given the following post, respond as if you're commenting on Reddit:
human
Why can I tell when a tv has been turned on from really far away, even when nothing is playing on it?  

In sixth grade we would often watch a short educational film after returning from "specials" (gym and the like) and the tv would alread by setup when we got back. I began to notice that I could sort of 'hear' that the tv was on in the classroom, long before we got there. The best way I can describe it is that there's just sort of a buzz that I'm aware of, and that means a tv is on somewhere. remember nothing is playing on the tv. This happens even if the tv is just on a black input/line/tv-video screen. 

Anyone know what's going on here? Can everyone do this?
gpt
A TV's electronics generate around 500 lines at 30 frames per second, or  ~15000 Hz, which is at the upper threshold of human hearing.  Lots of adults can't hear the noise due to high frequency hearing loss.

Edit: I'll add that this is similar to the noise of power transformers - i.e. the so-called "60 Hertz Hum"