[MarianaM (OP)]

Paul is wondering...

I once read that Russian spies would eavesdrop by picking up sound from glass windows through small vibrations. Does good double-glazing stop that?
And a related question: can we pick up speech by observing the vibrations in the cheeks or by the movement of the cheeks?

Get those lips moving, and fingers typing... can you help?

[alancalverd]

If you can see lips and cheeks, a good lipreader can give you an excellent rendition of speech. I saw an amazing demonstration on a recent "Yesterday" program where they had discovered silent films of soldiers in the WWI trenches. A profoundly deaf lipreader was voicing the conversations from very grainy and jerky film, but at some point she said "I can't make out what those blokes are saying." The historian said "They are from the Lancashire regiment - does that help?" whereupon she carried on, rendering their 100-year-old silent accent and dialect into Received Pronunciation.

If you can't see the speaker, the inner pane of the double glazing will still respond to the sounds inside the room, and in fact will be clearer because street noise will be suppressed by the outer pane. You will need some very clever optics to separate the two, but I'm pretty sure it can be done.

[RD]

Paul is wondering...

I once read that Russian spies would eavesdrop by picking up sound from glass windows through small vibrations. Does good double-glazing stop that?...?

https://youtu.be/1MrudVza6mo?t=141

[Bored chemist]

You will need some very clever optics to separate the two, but I'm pretty sure it can be done.

If you can see the window from two different locations the optics becomes trivial.
It would be hard if you were in the house across the street, but if you had access to two houses across the street it's simple.

[evan_au]

You will need some very clever optics to separate the two, but I'm pretty sure it can be done.

Instead, maybe you need some fairly clever audio processing to learn the resonance characteristics of the glass cavity, and deconvolve the resonance from the speech?

[alancalverd]

You can make a fourier transform of a single impact, either by throwing a stone at the window or waiting until somebody drops something or knocks on the door to the room, and use this to re-weight the speech signal you pick up from the glass pane. Hi-fi isn't really necessary: we used to get by with about 2 kHz bandwidth on telephones and according to the European Union, 4 kHz is more than enough for airborne radiotelephony.