Back in the late 80s, I needed to go into a room where a bunch of old engineers were laying out circuit boards on computers with CGA monitors.  They couldn't hear the whine but I could since I was only 30.  Every time I went in there I had to stick my fingers in my ears.  CGA monitors were at the 15.7 KHz frequency but better monitors like my EGA were at 21.8 KHz which I couldn't hear very well.