The mains voltage is a sinusoidal AC voltage of 50 Hz or 60 Hz. This is a frequency that is in the lower audio frequency range and corresponds to a low hum. Mains hum is an annoying effect where the mains frequency interferes with audio or video signals.
There are several reasons how the mains frequency is superimposed on the audio or video signal. One is that current-carrying lines are always surrounded by magnetic fields whose field lines are induced into other lines. This can be the case if lines or transformers carrying mains voltage are located in the vicinity of high-impedance audio or video lines. In such a case, the mains frequency is induced into the audio or video signal and superimposes it.
A mains hum can also be caused by sieve switching of power supplies. The residual voltage of an insufficient filtering superimposes the internal supply voltage and thus causes the disturbing mains hum. Depending on the rectification method, this can be the single or double mains frequency.
Another cause of mains hum can be potential differences over which equalizing currents flow. This is often due to the grounding and is known from hum loop. ettz hum is not only disturbing in audio and video systems, but also affects the display on televisions, personalcomputers, medical measurements and measurement technology in general.