Electric hum, mains hum, or power line hum is an audible oscillation at the frequency of the mains alternating current, which is usually 50 or 60 hertz depending on the local electric utility configuration (see Mains electricity). The sound often has heavy harmonic content.

The most common cause of electric hum is magnetostriction, wherein ferromagnetic materials change shape minutely when exposed to magnetic fields. Magnetostrictive electric hum is most often noticed around large linear transformers, particularly when the transformers are handling large amounts of current.

In the realm of sound reinforcement (as in public address systems and loudspeakers), electric hum is often caused by induction. This hum is generated by oscillating electric currents induced in sensitive (high gain) audio circuitry by the alternating electromagnetic fields emanating from nearby mains-powered devices like power transformers. The audible aspect of this sort of electric hum is produced by amplifiers and loudspeakers.

The other major source of hum in audio equipment is shared impedances; when a heavy current is flowing through a conductor (a ground trace) that a small-signal device is also connected to. No conductor is perfect, and the small resistance present means that devices using points on that conductor as a ground reference will be at slightly different potentials. This hum is usually at the second harmonic of the power line frequency (100 Hz or 120 Hz), since the heavy ground currents are from AC to DC converters that rectify the mains waveform. See also ground loop.

Electrical phenomena