There's a spy in your bag of chips

Researchers at the Massachusetts Institute of Technology have developed an algorithm that can reconstruct an audio signal by analysing minute vibrations of objects depicted in video.

In one set of experiments, the researchers recovered intelligible speech from the vibrations of a chip packet filmed from about 5m away through soundproof glass.

The signal caused the chip packet to move only a minuscule amount - about a tenth of a micrometre, or five thousandths of a pixel.

But the researchers reconstructed the sound that caused it - a recitation of Mary Had A Little Lamb - by analysing the vibrations using a high-frame-rate camera.

They extracted useful signals from videos of aluminium foil, the surface of a glass of water, even leaves .

"When sound hits an object, it causes the object to vibrate," said Abe Davis, a graduate student in electrical engineering and computer science at MIT and first author on the paper.

"The motion of this vibration creates a very subtle visual signal that's usually invisible to the naked eye. People didn't realise that this information was there."

The technique has potential applications in policing and forensics, but Davis is more enthusiastic about the possibility of what he describes as a "new kind of imaging".

"It also gives us a lot of information about the object itself, because different objects are going to respond to sound in different ways."