What is hum?
Hum is produced when there is an alternation in the current at the frequency of the mains electricity. The fundamental frequency of this sound is usually double of fundamental 50 Hz or 60 Hz, 100 Hz or 120 Hz depending on the local power line frequency. This sound often has heavy harmonic content that is above 50 Hz or 60 Hz. The presence of mains-powered audio equipment as well as ubiquitous AC electromagnetic fields from nearby appliances and wiring, 50/60 Hz electrical noise can get into audio systems and is heard as mains hum from their speakers. They can also be heard coming from powerful electric power grid equipment such as utility transformers that are caused by mechanical vibrations induced by magnetostriction in a magnetic core.
When it comes to musical instruments, the hum is usually treated as a nuisance and various electrical modifications are made to remove it. In analog videos, the mains hum can be seen as hum bars scrolling vertically up the screen. Broadcast television frame rates are chosen to match the line frequency, to decrease the disruption these bars cause to the picture. A hum bar can be caused by a ground loop in cables carrying analog video signals, poor power supply smoothing or magnetic interference with the cathode ray.