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Comment by ilamont | Hacker News Reader

In the 80s one of my high school teachers said her husband was an engineer who worked on surveillance technology and described how windows could be used as recording devices. People's voices inside caused the window to have minute vibrations, which could be measured by a laser pointing at the window's exterior and reflected back and then turned into audio output. Essentially, turning a window into a microphone diaphragm. It blew my mind.

I just looked it up, the technique was apparently invented by a Soviet engineer 40 years prior - Leon Theremin, who is also known for the musical wand heard on Led Zeppelin II and various 1960s horror movie soundtracks.

https://people.ece.uw.edu/nikitin_pavel/papers/RFID-vox_2013...

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Vaguely similar, since electrical wires emit EM fields that are disturbed by human bodies, you can use a special camera to see who's in a building based on those disturbances. I'm not sure how high-fidelity or easy to spoil it is, but this was real technology at least 15 years ago.

xfinity is now selling a security product based on seeing people in your home with Wi-Fi signals

Security or surveillance product?

This is a feature in modern wifi routers and every single one is capable of detecting presence of multiple people within an apartment. With some training, movement can be distinguished from the data.

Philips Hue light bulbs can also do person detection if you have a few of them and the right kind of hub.

I saw a video at some point of someone building one of these, I'm not sure it would have worked terribly well for actual use in the outdoors but the sound quality was much better than I expected at close range. I wonder how you defend against this, short of having your important meetings in depressing windowless basements?

Maybe double or triple glazed windows?

Similar technique around the same time presented at SIGGRAPH:

https://www.youtube.com/watch?v=FKXOucXB4a8

https://people.csail.mit.edu/mrub/VisualMic/

I recall a HN thread (or maybe it was a github project) about using a similar tactic to figure out keypresses in offices. Very interesting stuff.

Wow, keypresses?

I would love a source if you have one.

https://github.com/ggerganov/kbd-audio is probably the best-known one.

The amusing aspect is that this is so easy that multiple good ways someone could hobby program this long before AI was a thing. The sound of a bunch of keypresses may just blur together to our human ears, but if you try, you can tell that different keys make different noises, which is really the key insight. Between the different noises, the possible directionality you can get from a stereo (or more) pickup, and characteristic delay patterns during typing various bi-grams, there's a ton of information leakage in a recording of a keyboard.

Oh, I thought we were talking about being able to distinguish which key was pressed using wifi-radar, like one can do with deanonymizing someone via wifi gait analysis.

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