ALTERNATING CURRENT (AC)
Any type of electrical transmission where the current repeatedly changes direction, and the voltage varies between maxima and minima. Therefore, any electrical AUDIO signal may be called an AC signal. Whereas the voltage range of an audio signal is less than a few volts, that of electrical current is standardized in the 110-120 volt or 220 volt range with a frequency of 60 or 50 Hz (see map).
When electrical equipment is improperly grounded, a HUM is heard at one of those PITCHes, or a multiple thereof. Alternating current was introduced in the 1890's, and is now used almost universally to distribute electricity on a large scale.
Compare: DIRECT CURRENT, OSCILLATION. See: GENERATOR, PULSE WAVE, RECTIFICATION.
The World Soundscape Project's Hum Map of the world showing countries using 60 Hz or 50 Hz alternating current.
Sound Example: Electrical hum in a North American restaurant.Sound Example: A DC elevator in an old building in Vancouver. Note the absence of electrical hum.