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June 13, 2020 12:02 am

Spies Can Eavesdrop By Watching a Light Bulb's Vibrations

An anonymous reader quotes a report from Wired: Researchers from Israeli's Ben-Gurion University of the Negev and the Weizmann Institute of Science today revealed a new technique for long-distance eavesdropping they call "lamphone." They say it allows anyone with a laptop and less than a thousand dollars of equipment -- just a telescope and a $400 electro-optical sensor -- to listen in on any sounds in a room that's hundreds of feet away in real-time, simply by observing the minuscule vibrations those sounds create on the glass surface of a light bulb inside. By measuring the tiny changes in light output from the bulb that those vibrations cause, the researchers show that a spy can pick up sound clearly enough to discern the contents of conversations or even recognize a piece of music. In their experiments, the researchers placed a series of telescopes around 80 feet away from a target office's light bulb, and put each telescope's eyepiece in front of a Thorlabs PDA100A2 electro-optical sensor. They then used an analog-to-digital converter to convert the electrical signals from that sensor to digital information. While they played music and speech recordings in the faraway room, they fed the information picked up by their set-up to a laptop, which analyzed the readings. The researchers found that the tiny vibrations of the light bulb in response to sound -- movements that they measured at as little as a few hundred microns -- registered as a measurable changes in the light their sensor picked up through each telescope. After processing the signal through software to filter out noise, they were able to reconstruct recordings of the sounds inside the room with remarkable fidelity: They showed, for instance, that they could reproduce an audible snippet of a speech from President Donald Trump well enough for it to be transcribed by Google's Cloud Speech API. They also generated a recording of the Beatles' "Let It Be" clear enough that the name-that-tune app Shazam could instantly recognize it. There are some limitations. "In their tests, the researchers used a hanging bulb, and it's not clear if a bulb mounted in a fixed lamp or a ceiling fixture would vibrate enough to derive the same sort of audio signal," the report adds. "The voice and music recordings they used in their demonstrations were also louder than the average human conversation, with speakers turned to their maximum volume." With that said, the teams says that "they also used a relatively cheap electro-optical sensor and analog-to-digital converter, and could have upgraded to a more expensive one to pick up quieter conversations," reports Wired. "LED bulbs also offer a signal-to-noise ratio that's about 6.3 times that of an incandescent bulb and 70 times a fluorescent one."

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