Researchers at MIT have come up with an ingenious "visual microphone" technique for reconstructing sound by analysing video clips of the tiny vibrations induced by the sound in nearby objects.
Suppose you're having a conversation near to a plastic bag (or the leaves of a plant, a glass window, some tin foil - anything light enough to have significant vibrations induced in it by the sound waves from your conversation). Then by recording high-speed video of the vibrating object, the researchers would be able to reconstruct your conversation - even if they were far away from you, or even shooting the video through sound-proof glass! Creepy, but impressive.
Here's a cool video of the technique in action: https://www.youtube.com/watch?v=FKXOucXB4a8
The visual microphone has obvious applications in the fields of law enforcement, forensics, and general snooping. But I've even heard some people suggest that an improved version of this technique might allow the recovery of some sound from silent films! Wouldn't that be cool?
The technique exploits the rolling shutter effect in digital cameras [...] to detect sound-related movements that might otherwise be invisible; the only gear you need is a camera that can record at high frame rates. It's good enough to capture singing from a bag of potato chips, and musical tones from a potted plant.