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
It is the buzzing of capacitors. A lot of electronics will make that sound but most of them do not, and night vision goggles are not really ones that would. Much like how swords being drawn from a scabbard almost always make a metallic scraping sound in TV and movies, that is just a sound effect that people have become accustomed to so they keep putting it in.
When Peter Jackson was making The Lord of the Rings at first he made sure the unsheathing of swords did not make that sound. But test audiences hated it and thought it was not believable.
It just goes to show that believability and realism are not the same thing when it comes to movies.
The sound is the energizing of high voltage coils. It is reminiscent of the iconic sound of turning on a CRT television. There would have been a time that night vision goggles had tiny CRTs in them like the ones found in old camcorders’ viewfinders and these sort of night vision goggles, worn on the head, may have significantly amplified that sound but in general the sound is used to suggest “screen turning on”.
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early analog night vision devices used a vacuum-tube type video display to amplify the signal. This was the days before LCDs were everywhere. Pure analog amplification of light. The tubes required high voltage to run, so the sounds you are hearing are the capacitors charging that run the amplification tubes. Yes, they made that sound, kinda. But only the older ones. New ones don’t anymore, they are all digital and use LCD displays.
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.
The singing is from the charging coil being pumped. Old night vision used a photo sensitive vacuum tube that required a lot of dc voltage but not much current. Driving a step up transformer was the way to do this. But transformers only work with ac and batteries are dc. So an oscillator circuit generates a small ac wave from the batteries and that wave is fed to the transformer. High voltage from the transformer is sent to a rectifier and a very high voltage capacitor. Typically the result was around 20000 volts.
The singing sound is the magnetic field of the transformer making the transformer core vibrate. As the high voltage cap becomes charged the current in the transformer shrinks in the same way a ball bounces as it drops energy. (It is called a sync function.)
The end result was to put this high voltage tube up against your eye ball and go “ooo, look at that”.
Capacitor charge, and no, night vision goggles don’t actually make that noise. But that sound effect has been burned into the public consciousness thanks to Hollywood and video games, so they have to put it in there otherwise people wouldn’t make the right association. It’s like shooting a gun with a silencer/suppressor screwed on the barrel being a complete shock when you hear it for the first time. You expect that quiet, *”phewt!”* sound, only to hear a gunshot, just a little quieter (but still loud as hell).
It’s not nearly as noticeable in real life with most night vision nods you would see. You can hear it if you’re the person wearing them as they first turn on, but it’s very faint and there’s little to no noise while they’re running. My experience is limited to a handful of night vision nods from late 90s to pre 2010.
Source: I used AN/PVS-7 NVGs in the US Army in the 90s and 2000s, and at one point was instructing drivers in driving under NVGs.
Yes they make that sound. I do not know why. I can only speak to the PVS-7s.
It’s a capacitor charging – I may be dating myself, but older inexpensive and disposable cameras made the same sound when charging the flash.
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