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Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the frequency of the mains electricity. The fundamental frequency of this sound is usually double that of the local power-line frequency: that is to say, 100Hz in areas with 50Hz power, and 120Hz in areas with 60Hz power. The sound often has heavy harmonic content above 50/60Hz. Due to the presence of mains current in mains-powered audio equipment as well as ubiquitous AC electromagnetic fields from nearby appliances and wiring, 50/60Hz electrical noise can get into audio systems, and is heard as mains hum from their speakers. Mains hum may also be heard coming from powerful electric power grid equipment such as utility , caused by mechanical vibrations induced by magnetostriction in magnetic cores. Onboard aircraft (or spacecraft) the frequency heard is often higher pitched, due to the use of 400 Hz AC power in these settings because 400Hz transformers are much smaller and lighter.
Around high-voltage power lines, hum may be produced by corona discharge.
In the realm of sound reinforcement (as in public address systems and ), electric hum is often caused by induction. This hum is generated by oscillating induced in sensitive (high gain or high impedance) Sound 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 and loudspeakers (note that this is not to be confused with Audio feedback).
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. All practical conductors will have a finite, if small, resistance, and the small resistance present means that devices using different points on the 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 power supplies that Rectifier the mains waveform. (See also ground loop.)
In vacuum tube equipment, one potential source of hum is current leakage between the heaters and cathodes of the tubes. Another source is direct emission of electrons from the heater, or magnetic fields produced by the heater. Tubes for critical applications may have the heater circuit powered by direct current to prevent this source of hum.Robert B. Tomer, Getting the most out of vacuum tubes, Howard W. Sams, Indianapolis, USA 1960, Library of Congress card no. 60-13843, available on the Internet Archive. Chapter 3
Leakage of analogue video signals can give rise to hum sounding very similar to mains hum.
Humbucking is a process in which "hum" that is causing objectionable artifacts, generally in audio or video systems, is reduced. In a humbucker electric guitar pickup or microphone, two coils are used instead of one; they are arranged in opposing polarity so that AC hum induced in the two coils will cancel, while still giving a signal for the movement of the guitar strings or diaphragm.Tom Hirst, Electric Guitar Construction, Hal Leonard Corporation, 2003 , page 126
In certain vacuum-tube radio receivers, a winding on the dynamic speaker field coil was connected in series with the power supply to help cancel any residual hum.
Some other common applications of this process are:
In the mid-1990s, as part of the Beatles anthology series, the three surviving members, Paul McCartney, George Harrison, and Ringo Starr, regrouped to record initially incidental music for the albums, but decided to rework some John Lennon demos instead. Several demos were given to McCartney from Ono, the most notable being "Free as a Bird", "Real Love", and "Now and Then".
Of the demos received, only the aforementioned three were worked on. Of the three, "Real Love" and "Now and Then" were more difficult to work on than "Free as a Bird," as both contained a prominent 60-cycle mains hum as a result of the cheap recording equipment Lennon used to record the demos. While the mains hum was removed from "Real Love",Maclauchlan, Paul (1998). Gobnotch's Recording Sessions Update – February 1995 . Retrieved 24 June 2005. it was noticeably louder on "Now and Then", which made it much harder to remove. This, and to a much bigger extent, Harrison's distaste for that particular demo, lead to it being scrapped altogether, although reports circulated in the years since that McCartney was hoping to finish it. www.inthenews.co.uk In 2009, a version of Lennon's demo, supposedly without the mains hum that hampered the Beatles version, appeared as a bootleg. In 2023, the mains hum was finally removed thanks to Peter Jackson's sound source separation technology, and the track was released on November 2, 2023.
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