In one set of experiments, the team was able to recover intelligible speech from the vibrations of a potato-chip bag photographed from 15 feet away through soundproof glass.
Researchers also extracted useful audio signals from videos of aluminum foil, the surface of a glass of water, and even the leaves of a potted plant.
"When sound hits an object, it causes the object to vibrate," said Abe Davis, a graduate student in electrical engineering and computer science at Massachusetts Institute of Technology (MIT) and first author on the research paper.
"The motion of this vibration creates a very subtle visual signal that's usually invisible to the naked eye. People didn't realize that this information was there," said Davis.
Reconstructing audio from video requires that the frequency of the video samples the number of frames of video captured per second be higher than the frequency of the audio signal.
In some of their experiments, the researchers used a high-speed camera that captured 2,000 to 6,000 frames per second.
That's much faster than the 60 frames per second possible with some smartphones but well below the frame rates of the best commercial high-speed cameras, which can top 100,000 frames per second.
In other experiments, however, they used an ordinary digital camera.
Because of a quirk in the design of most cameras' sensors, the researchers were able to infer information about high-frequency vibrations even from video recorded at a standard 60 frames per second.
While this audio reconstruction wasn't as faithful as it was with the high-speed camera, it may still be good enough to identify the gender of a speaker in a room; the number of speakers; and even, give accurate enough information about the acoustic properties of speakers' voices, their identities.
The researchers' technique has obvious applications in law enforcement and forensics, but Davis is more enthusiastic about the possibility of what he describes as a ‘new kind of imaging.’
"We're recovering sounds from objects. That gives us a lot of information about the sound that's going on around the object, but it also gives us a lot of information about the object itself, because different objects are going to respond to sound in different ways," he said.

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