How A Lightbulb Can Be Used To Spy On Your Conversations

Excuse us while we go invest in black-out blinds and candles... Image credit: Audio und werbung/

Forget the Alexa lurking in the corner of your room or the Roomba quietly skirting around your house, it’s the humble lightbulb that could have been spying on you all this time. A team of Israeli cybersecurity experts has claimed that it's possible to use a standard lightbulb, as well as a telescope and some other relatively fancy scientific equipment, to listen in on a conversation from afar. 

The theory goes that it’s possible to closely watch the surface of a hanging lightbulb while it vibrates in response to sound waves. By observing the bulb's subtle movements with a telescope and a sensitive electro-optical sensor, it’s possible to work out the soundwaves that caused the bulb’s vibration and understand what was being said in that room.

A team of Israeli cybersecurity researchers from the Ben-Gurion University of the Negev and the Weizmann Institute of Science published a report in 2020 investigating this technique, dubbed the “Lamphone Attack”. 

To test out the idea, the team ran an experiment that saw a bunch of eavesdroppers attempt to listen in on a sealed room at least 25 meters (82 feet) away through a window. An electro-optical sensor was mounted on the telescope and pointed towards the lightbulb in the room, which would be able to convert the light into an electrical current. This is then passed through an analog-to-digital converter and studied by an algorithm. 

According to the paper, the technique manages to recover the sound with remarkable clarity. The researchers played a clip of former President Donald Trump saying one of his many infamous catchphrases — "We will make America great again!” — and the team of eavesdroppers was able to capture it. It certainly sounded a little fuzzy, like a drunk Trump speaking via radio on a submarine, but it was distinguishable enough to be recognized by Google's Automatic Speech Recognition. They also managed to recover the signals of a few songs played in the distant room, including "Let It Be" by the Beatles, and it was recognized by the music-identifying app Shazam. 

"How small are these vibrations? The experiments conducted show that extreme slight vibrations of just a couple of millidegrees are sufficient for recovering speech and non-speech audio,” the researchers said in an accompanying video to the paper.

The technique is not totally foolproof, however. While incandescent and LED light bulbs produce enough vibration for the sound to be recovered, fluorescent bulbs do not. Additionally, the light bulb must be switched on, hanging near to the sound, and should be clearly visible without being obscured. Those caveats aside, the relatively straightforward technique appears to be surprisingly effective. Now, excuse us while we go invest in black-out blinds and candles...


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