Mains hum

Source: Wikipedia, the free encyclopedia.

Mains hum, electric hum, cycle hum, or power line hum is a sound associated with

400 Hz
AC power in these settings because 400 Hz transformers are much smaller and lighter.

50 Hz hum
60 Hz hum
400 Hz hum

Causes

Electric hum around transformers is caused by stray magnetic fields causing the enclosure and accessories to vibrate. Magnetostriction is a second source of vibration, in which the core iron changes shape minutely when exposed to magnetic fields. The intensity of the fields, and thus the "hum" intensity, is a function of the applied voltage. Because the magnetic flux density is strongest twice every electrical cycle, the fundamental "hum" frequency will be twice the electrical frequency. Additional harmonics above 100/120 Hz will be caused by the non-linear behavior of most common magnetic materials.

Around high-voltage power lines, hum may be produced by corona discharge.

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 or high impedance) 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 (note that this is not to be confused with acoustic feedback).

The other major source of hum in audio equipment is shared

AC to DC power supplies that rectify 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.[1]

Leakage of analogue video signals can give rise to hum sounding very similar to mains hum.

Prevention

It is often the case that electric hum at a venue is picked up via a

AC ground lift adapter or by breaking the earth pin off
the power plug used at the mixing deck. Depending on the design and layout of the audio equipment, lethal voltages between the (now isolated) ground at the mixing desk and earth ground can then develop. Any contact between the AC line live terminals and the equipment chassis will energize all the cable shields and interconnected equipment.

Humbucking

Humbucking is a technique of introducing a small amount of line-frequency signal so as to cancel any hum introduced, or otherwise arrange to electrically cancel the effect of induced line frequency 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.[2]

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:

  • Humbucking transformers or coils used in video systems.
  • Telephone (and other audio) system and computer communications wiring.

Consequences

In music

In musical instruments, hum is usually treated as a nuisance, and various electrical modifications are made to eliminate it. For instance, humbucker pickups on electric guitars are designed to "buck" or reduce the hum.[3] Sometimes hum is used creatively, for example in dub and glitch music.

John Lennon demos

Main articles: Real Love (Beatles song) and Now and Then (Beatles song)

In the late 1970s, former

Beatle John Lennon recorded some demo songs at his and Yoko Ono's Dakota apartment. These demos did not see any official release at the time, nor were they properly recorded for Double Fantasy or its follow-up Milk and Honey
, but they did spread as bootlegs amongst Lennon fans.

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 the most difficult to work on as, compared to "Free as a Bird"; 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",

sound source separation technology,[9] and the track was released on November 2, 2023.[10]

In audio systems

Power line hum can be alleviated using a band-stop filter.[11]

In video systems

In

cathode ray tube
.

In forensics

Main article: Electrical network frequency analysis

Electrical network frequency (ENF) analysis is a

digital watermark that can be used to find when the recording was created, and to help to detect any edits in the sound recording.[13][14][15]

See also

References

  1. ^ 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
  2. ISBN 1-57424-125-7
    , page 126
  3. ^ Thompson, Art. "Bench Tests: Cool Blues Gear". Guitar Player. 26 (8): 118.
  4. ^ Maclauchlan, Paul (1998). Gobnotch's Recording Sessions Update – February 1995 Archived 4 November 2007 at the Wayback Machine. Retrieved 24 June 2005.
  5. ^ Remnick, David (11 October 2021). "Paul McCartney Doesn't Really Want to Stop the Show". The New Yorker.
  6. ^ www.inthenews.co.uk Archived 27 September 2007 at the Wayback Machine
  7. ^ "BBC Four – Mr Blue Sky: The Story of Jeff Lynne and ELO".
  8. ^ Remnick, David (11 October 2021). "Paul McCartney Doesn't Really Want to Stop the Show". The New Yorker.
  9. ^ https://deadline.com/2023/06/paul-mccartney-john-lennon-beatles-now-and-then-new-record-1235422760/
  10. TheBeatles.com. Archived
    from the original on 26 October 2023. Retrieved 27 October 2023.
  11. ISBN 81-88152-10-2. Retrieved 2009-08-10.{{cite book}}: CS1 maint: multiple names: authors list (link
    )
  12. ISBN 0750696605
    page 372
  13. ^ Cooper, A.J: "The electric network frequency (ENF) as an aid to authenticating forensic digital audio recordings – an automated approach". June 2008., Conference paper, AES 33rd International Conference, USA (2008)
  14. ^ Grigoras, C.: "Digital audio recording analysis – the electric network frequency criterion". Archived from the original on 2012-03-07. Retrieved 2010-10-11., International Journal of Speech Language and the Law, vol. 12, no. 1, pp. 63-76 (2005)
  15. ^ Mateusz Kajstura, Agata Trawinska, Jacek Hebenstreit. "Application of the Electrical Network Frequency (ENF) Criterion: A case of a digital recording". Forensic Science International, Volume 155, Issue 2, Pages 165-171 (20 December 2005)
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