6.2 Mains Hum
Anything electrical that is plugged into the mains can be susceptible to hum. Mains electricity is an alternating current, whereas something like a battery provides direct current. In the UK the mains voltage is about 240 V, with a frequency of 50 Hz. Most of Europe has a voltage of 220 V at 50 Hz, while the USA has a voltage of 110 V and a frequency of 60 Hz.
The voltage doesn’t cause noise, but the fact that it is alternating and not direct current can cause problems. The 50 Hz frequency (in the UK and Europe) can interact with some components inside electrical devices such as transformers, and can cause a vibration which is audible as a hum. The fundamental frequency of this hum is usually 50 Hz, (or 60 Hz in the USA), but just like a musical instrument, this can have a number of additional harmonics.
If you notice any hum in your recording environment, if possible, try to turn off any problematic device before recording.