Audible Noise and Interference
AC-powered appliances can give off a characteristic hum, often called "mains hum", at the multiples of the frequencies of AC power that they use (see Magnetostriction). It is usually produced by motor and transformer core laminations vibrating in time with the magnetic field. This hum can also be evident in poorly-constructed audio amplifiers, where the power supply is inadequately filtered.
Most countries chose their television vertical synchronization rate to approximate the local mains supply frequency. This helped to prevent power line hum and magnetic interference from causing visible beat frequencies in the displayed picture of analogue receivers, but is of diminishing importance in modern digital display systems.
Read more about this topic: Utility Frequency
Famous quotes containing the words noise and/or interference:
“You may say a cat uses good grammar. Well, a cat does—but you let a cat get excited once; you let a cat get to pulling fur with another cat on a shed, nights, and you’ll hear grammar that will give you the lockjaw. Ignorant people think it’s the noise which fighting cats make that is so aggravating, but it ain’t so; it’s the sickening grammar they use.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)
“Adolescent girls were fighting a mother’s interference because they wanted her to acknowledge their independence. Whatever resentment they had was not towards a mother’s excessive concern, or even excessive control, but towards her inability to see, and appreciate, their maturing identity.”
—Terri Apter (20th century)