A normally undesired low-pitched noise that can be heard in an audio signal. The source of this noise can be a ground loop or AC electrical induction from power lines that are too close to audio lines. This phenomenon is often referred to as “60-cycle hum.” Here’s why:
In the Americas and most of Asia alternating current is generated with a frequency of 60Hz. This also creates strong 2nd and 3rd harmonics at 120Hz and 180Hz. Europe and the rest of the world use 50Hz mains, so the harmonics fall at 100Hz and 150Hz.
We say “normally undesired” because some contemporary musicians – most of them electric guitar players – consider hum to be a part of their signature sound. In fact, some digital effects units and amp modeling devices include a digitally recreated hum in their presets.