Not so much anymore I guess, because LEDs and phone cams and all that, but for a long time, electronic xenon-tube flashes (strobes) made this high-pitched whining tone when you turned them on and their internal capacitors were charging up. When the ready light comes on, the whine tone stops rising in pitch. Or maybe it’s the other way around, when the whine tone stops rising in pitch, the ready light comes on? Anyways, was this some unintentional by-product of charging up that kind of capacitor with that kind of circuit (I think I remember it being called a “thyristor”)? Or was it a deliberate noisemaker inside the flash so the user could tell its charging-up/ready status without looking? I also think I might remember that some (or all?) flashes that could operate on batteries or off an AC wall plug adaptor made the noise when using batteries, but not when using the power adaptor.
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It technically wasn’t the capacitors charging that made the noise, it was the transformer that was charging the capacitors. Camera flashes tended to be much higher voltage than the rest of the camera so they could be bright enough, so they needed to use a transformer to turn the battery voltage into the light’s voltage. The specific kind of transformer they used had that distinctive whine as a side effect of cycling the current back and forth to boost the voltage.
The ones that could plug into mains power and didn’t have the whine would either use a different kind of transformer or maybe the used a flash bulb that used that voltage natively and so didn’t need a transformer at all.
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