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There is nothing new about eavesdropping based purely on sound waves without the use of microphones. But up until now, these methods have all had their disadvantages. They could not be conducted in real-time. Some devices had to be programmed with malware. Or a laser beam had to be activated. Scientists at Ben-Gurion of the Negev University in Israel have now developed a novel method that does not require any of these tools. All it takes is an ordinary lamp. This lamp can be used to listen in on audio without the need for any modifications.

For their Lamphone project, the researchers use the tiny vibrations on the lamp’s glass surface that are caused by nearby sounds. They can record this flickering of light from a certain distance and reconstruct the sound in real-time. All they need is visual contact with the lamp. It’s even possible through a closed window. Then they can record the flickering of the light (which is imperceptible to the eye) with a telescope and an electro-optical sensor. The information is immediately processed with the help of an analog-to-digital converter. In turn, a computer converts it into audio in real-time. However, it does not work if the lamp is covered by a lampshade.

“Lamphone shows that high-resolution data obtained by the visual microphone is not required when using a hanging light bulb,” Ben Nassi, one of the developers of the surveillance method, wrote in Cryptology magazine. “Instead, we show that an electro-optical sensor that outputs information at a lower resolution (a one-pixel sample) is sufficient to recover sound. As a result, Lamphone can be applied in real-time scenarios.”

Listening in at a distance of 25 meters

In one experiment, the researchers recorded music and speech at a distance of 25 meters using an LED lamp (E27/12 watts). The Shazam music recognition app was able to easily recognize the songs from Coldplay and the Beatles. Google speech recognition converted a speech given by US President Donald Trump into text.

“We tested the performance of Lamphone attack in terms of its ability to recover speech and songs from a target location when the eavesdropper is not at the same location,” Ben Nassi added. “The target location was an office located on the third floor of an office building. Curtain walls, which reduce the amount of light emitted from the offices, cover the entire building. The target office contains a hanging E27 LED bulb (12 watts).”

The eavesdropper was located on a pedestrian bridge 25 meters as the crow flies from the target office. “The experiments described in this section were performed with three telescopes with different lens diameters (10, 20, 35 cm). We mounted an electro-optical sensor (the Thorlabs PDA100A2, which is an amplified switchable gain light sensor that consists of a photodiode that is used to convert light to electrical voltage) to one telescope at a time, ” Nassi describes the experiment in more detail. “The voltage was obtained from the electro-optical sensor via a 16-bit ADC NI-9223 card. And was processed in LabVIEW script that we wrote. The sound that was played in the office during the experiments could not be heard at the eavesdropper’s location.”