Not too long ago, you might have realised that some screens, whether it be a PC monitor or TV, made a strange high pitched noise that became more prominent when you turned them on, but why exactly was the reason for this?
Older TVs and PC monitors made use of CRT screens which stood for Cathode Ray Tubes. A CRT screen works by a vacuum encloser that had an electron gun at the back of the display that would fire electrons towards the back of the screen which was coated in phosphors that would light up from the energy of the electrons. In order to control where the electrons are being fired, an electromagnet is used.
A flyback transformer controls the hertz rate of each horizontal scan which totals to 625 lines per screen and this happens at the frame rate of 25 times per second which totals to 15625 times per second and as a result, magnetostriction occurs which results in a very high pitched sound: a pitch of exactly 15625Hz.
Take a look at the video below by the YouTube channel: Tom Scott on Why Old Screens Make A High Pitched Noise...