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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 Transformer s, particularly when the transformers are handling large amounts of current. In the realm of sound reinforcement (as in Public Address System s and Loudspeaker s), electric hum is often caused by Induction . This hum is generated by oscillating Electric Current s induced in sensitive (high Gain ) Audio circuitry by the alternating Electromagnetic Field s emanating from nearby mains-powered devices like power transformers. The audible aspect of this sort of electric hum is produced by Amplifier s and loudspeakers. It is often the case that electric hum at a venue is picked up via a the power plug used at the mixing desk. Audio gear should never be operated without its AC safety ground; performers have died under such conditions.http://www.thisislocallondon.co.uk/display.var.842516.0.electric_shock_kills_teacher_at_gig.phphttp://www.gearwire.com/forum-guitar-electrocution.htmlhttp://www.cbsnews.com/stories/2005/10/31/national/main995829.shtmlhttp://jaynir.wordpress.com/2006/12/11/she-died-with-a-microphone-on-her-mouth/ The other major source of hum in audio equipment is shared Impedance s; when a heavy current is flowing through a conductor (a Ground trace) that a small-signal device is also connected to. As room-temperature Superconductor s 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 Converter s 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 . SEE ALSO REFERENCES EXTERNAL LINKS
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