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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The camera flash uses high voltage to produce the flash of light. But it is usually powered by a low voltage battery. In order to convert the low voltage DC current to high voltage in order to charge the capacitor a flyback transformer is used. This is a transformer that uses a switch on the input side to generate the changing current that a transformer need. Each time the switch closes a current spike is produced on the output which is used to charge the capacitor. When the capacitor is out of charge it is able to accept most of this spike but as the capacitor charges up the output will soon match the voltage over the capacitor and it does not charge any more. So in a camera flash the switch is controlled by how much current is flowing. At first each current spike takes a long time to discharge into the capacitor but over time the spike discharges sooner and therefore the switch is switching faster. And each time the switch is toggled the magnetic field generated by the transformer is deforming the wiring of the transformer and other nearby components which is what makes the sound.
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