eli5 what is this iconic high pitched sound you hear, when a movie or game character activates night vision goggles? Is this a real thing?
In: Technology
People talking about capacitors and high-voltage thyristors are in the ballpark, but missing key details.
There are a few different night vision technologies, but the classic one you are referring to uses a device called an “[image intensifier tube](https://en.wikipedia.org/wiki/Image_intensifier)”. This has some similarities to a CRT (cathode ray tube, a kind of display that used to be used in TVs and other devices) but is a totally separate device. It does (like a CRT) require a high voltage to operate, and has special circuitry to generate that high voltage from a low-voltage source like a battery.
It is the circuitry that creates this high voltage that you might hear and that is exaggerated in the sound effects you hear. The actual source of the sound is where no one has quite hit the mark.
An easy way to convert one voltage to a different one is a transformer. But transformers only work with alternating current, not the direct current from a battery that always flows in the one direction. This is because of some fun physics that relate electricity and magnetism.
So the small voltage from the battery powers an oscillator circuit that chops it up into pulses of a high frequency, but you wouldn’t hear that because it’s electricity, not sound. Then that signal is put through a transformer, consisting of two coils of wire very close together, so the magnetics effects of one also affect the other. The second coil is many more “turns” or loops of wire and this relates to the increase in voltage.
Because the transformer and its coils are made of real-life materials, and there are many turns of small wire being shoved around by constantly changing currents and magnetic fields, those coils sometimes move a little as they operate. This movement is how a small fraction of the electrical signal gets converted (accidently) to sound you can hear. After all, current in coils near magnets are how things like motors turn wheels. Oh, and speakers intentionally convert electrical signals to sound using this same relationship between electricity and magnetism.
The output of the transformer is then “rectified” or converted back to direct current (DC), now at the much higher voltage and used to power the image intensifier. Yes, it is probably stored in a capacitor along the way, but this is far less likely to create sound, because it’s construction is far less likely to have parts that can vibrate, and also because its function here is to store the DC voltage, not transmit AC current which has that high frequency. (But this is why many people mentioned capacitors.)
In a night vision device, this step-up circuitry might be working extra hard just after its switched on, to rapidly get everything up to working voltage, including charging that capacitor that acts as a kind of reservoir to keep a constant supply to the device. Yes, capacitors often act for electricity like a town’s water tower acts for the water supply. Filling one quickly from empty would be a massive effort, but once filled you only need to put back as much as is used.
In a certain type of photographic flash, a similar circuit is used to charge up a capacitor (or a bank of them) as a store of energy that gets dumped super-rapidly into a xenon flash tube. This is often controlled by a device called a thyristor, which is a kind of electronic switch that once turned on stays on until the current flow ends. This makes it ideal to dump that energy REALLY fast into the xenon tube to be converted to light. Then you hear the sound again as that oscillator circuit recharges the capacitor for the next shot. (This all to explain why someone thought it was related to a thyristor.)
TLDR; the sound is related to generating high voltages, and likely comes from a small transformer rattling VERY fast.
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