Hum

A constant tone, usually with a frequency of 50 or 60 Hz, or some harmonic thereof, which is heard from electrical equipment that is improperly grounded, or as a result of mechanical vibration in any electrical device.

The sound is an example of a pitched drone, and is similar (except in duration) to the human vocal activity of the same name. In countries using alternating current, hums form a common keynote sound. As the frequency of alternating current varies from country to country, the pitch of this hum varies as shown on the world hum map (see alternating current).

Compare: Internal Dynamics, Narrow Band Noise, Redundancy, Rumble, Stationary Sound, Tempo.