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:
“While we were thus engaged in the twilight, we heard faintly, from far down the stream, what sounded like two strokes of a woodchopper’s axe, echoing dully through the grim solitude.... When we told Joe of this, he exclaimed, “By George, I’ll bet that was a moose! They make a noise like that.” These sounds affected us strangely, and by their very resemblance to a familiar one, where they probably had so different an origin, enhanced the impression of solitude and wildness.”
—Henry David Thoreau (1817–1862)
“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)