Mains hum
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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.
It is often the case that electric hum at a venue is picked up via a ground loop. In this situation, an amplifier and a mixing desk are typically at some distance from one another. The chassis of both items are grounded via the mains earth pin, and are also connected along a different pathway via the shield conductor of a shielded cable. As these two pathways do not run alongside each other, an electrical circuit in the shape of a loop is formed. One gets the same situation occurring between musical instrument amplifiers on the stage and the mixing desk. To fix this, stage equipment often has a "ground lift" switch which breaks the loop. A more dangerous but frequently used option is to snap the earth pin off the power plug used at the mixing desk.
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. As room-temperature superconductors have yet to be discovered, all practical conductors will have 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 converters that rectify the mains waveform. See also ground loop.
Assuming a tempered scale with A=440Hz, a 60Hz tone is almost exactly halfway between A# and B two octaves below Middle C, and a 50Hz tone is between G and G# two octaves below Middle C, but slightly sharper than the quarter-tone. These notes fall within the range of a 4-string bass guitar.
- Mains hum (file info) — play in browser (beta)
- 5 seconds of 60 Hz mains hum
- Problems listening to the file? See media help.
- Mains hum (file info) — play in browser (beta)
- 5 seconds of 50 Hz mains hum
- Problems listening to the file? See media help.
[edit] See also
[edit] External links
- Bruce Carter. Audio Applications for Op Amps, Part II
