There's in fact quite a number of sources of noise in electronics:
Magnetic fields impinging on ferrous metals (try turning on an old amplifier with a mild steel case, it goes *woomnnngg* from the inrush current through the transformer). These magnetic fields are caused by current flowing through coils of some sort.
One of the more unusual effects i've observed is capacitors "singing" or humming in time with a signal. This is caused by large current fluctuations causing magnetostriction in the dielectric (my word of the day). I had it once in an amplifier, i could sort of hear the music with no speakers attached... right before the cap exploded and splattered me with gunk.
TVs generate a couple of frequencies, but the one most heard within the audible range is at 18khz. I can usually hear it when i enter a house, it's more a "presence" than a sound. I've heard of people who can hear the higher frequency sounds of flourescent lights (not the noise from the ballast). But don't worry, your ears' frequency range drops with age. Fan noise is my pet hate.
If it's really quiet, i can hear the switched mode supply in my palm m105, esp. with backlighting on.
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There's in fact quite a number of sources of noise in electronics:
Magnetic fields impinging on ferrous metals (try turning on an old amplifier with a mild steel case, it goes *woomnnngg* from the inrush current through the transformer). These magnetic fields are caused by current flowing through coils of some sort.
One of the more unusual effects i've observed is capacitors "singing" or humming in time with a signal. This is caused by large current fluctuations causing magnetostriction in the dielectric (my word of the day). I had it once in an amplifier, i could sort of hear the music with no speakers attached... right before the cap exploded and splattered me with gunk.
TVs generate a couple of frequencies, but the one most heard within the audible range is at 18khz. I can usually hear it when i enter a house, it's more a "presence" than a sound. I've heard of people who can hear the higher frequency sounds of flourescent lights (not the noise from the ballast). But don't worry, your ears' frequency range drops with age. Fan noise is my pet hate.
If it's really quiet, i can hear the switched mode supply in my palm m105, esp. with backlighting on.