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Sometimes old faulty CRT monitors generate nasty high-frequency squeal sound. What element might be responsible for generating such sound? I have heard that it might be dry electrolytic capacitor; is it? What is the physics of generating this sound (how it is generated)?

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up vote 4 down vote accepted

Such noise is generated by the so called "line" (= horizontal deflection or flyback) transformer. This transformer does several things in a CRT or TV at the same time: it couples the horizontal output stage to the horizontal deflection coils and recovers the energy from the coils field during flyback, and generates the high voltage for the CRT (about 20 kV).

The transformer has a core made from ferrite material. Such materials have a small and unwanted magnetostriction effect (There a varieties of ferrites which have a strong effect). In case of failiure the core of such a transformer can be driven into saturation, and the frequency of oscillation comes down to the region where our ears are much more sensitive. Then You hear that vibration of the core.

BTW in the 50ties, when I was much younger (including my ears) I could hear the 15 625 Hz tone of TV sets out in the street when I passed houses at nighttime (less other noise). Whether the transformers were louder in those times, or I just had better ears, I don't know.

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Children and teenagers can hear the noise of televisions which adults can't. –  delete Mar 15 '11 at 14:40
    
Oh Yes, the most astonishing thing was, that most friends of comparable age didn't hear that frequency, and I heard it clearly, but not the sound/music of the TV sets. Maybe that 15 kHz pass easily through window panes? –  Georg Mar 15 '11 at 14:47
    
Maybe this article is interesting: en.wikipedia.org/wiki/The_Mosquito –  delete Mar 15 '11 at 14:57
    
@Master... really interesting! Now I remember that I read about this some years ago, but I forgot about it. –  Georg Mar 15 '11 at 15:36
    
It used to be a real pain in physics experiments, the 15-20Khz flyback transformer produces lots of RF noise that is too low a frequency to easily screen and too high a frequency to easily chop. Fortunately we now have LCDs - with their own range of noise problems –  Martin Beckett Mar 15 '11 at 15:39
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its down to the pizza-electric effect where faulty electric signals in the audio range cause components to wiggle mechanically in the audio range.

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Isn't the pizza-electric effect what causes the cheese to go runny in the microwave? –  Martin Beckett Mar 15 '11 at 16:55
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Is this a joke? if yes, please explain to a curious foreigner. –  Georg Mar 16 '11 at 10:56
    
nope its not a joke. i meant piezoelectric effect, but i'll keep in the misspelling so that people understand what Martin means.anyway, looking into it further it's mainly inductors that squeal and im wrong. –  Larry Harson Mar 19 '11 at 21:55
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