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
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