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This method allows you to decode the audio of a speaker just by analyzing the LED flicker that it produces and meters away

When one thinks of spy on audio conversations he comes up with hidden microphones or some malware that allows calls to be intercepted. However, when it comes to ingenuity, the possibilities are vastly greater. An Israeli researcher has shown what is possibly the most curious method to date: analyzing the flickering of the speaker LEDs.


The idea is not entirely new, the researcher himself presented a similar method last year where he analyzed how a light bulb vibrated in a room. From there, with a telescope tens of meters away, he could decipher the conversation. This time there is no need to go to a light bulb, just analyze the speaker LEDs.

That “seemingly useless” speaker LED

Speakers (like many other devices) generally have an indicator LED to warn that the equipment is working or is connected to the mains. These LEDs however can reveal much more information for a malicious attacker. As described by Ben Nassi and his colleagues, it is useful for analyzing what the loudspeaker is playing.

The idea is that As an electromagnet rises and falls within the speaker to create the vibrations that the device produces, the energy available to the LED also rises and falls. That is, the activity that the speaker itself produces to generate the sound is reflected in the energy available to power the speaker’s LED. This sometimes causes it to reduce or increase the intensity of the shine it produces.

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Evidently this slight change in brightness is not noticeable to the human eye. However, it is a subtle enough change to be detected by an electroscopic sensor. From there it is a matter of deciphering. Depending on the power with which the LED is lit or dimmed, the vibrations of the speaker are higher or lower, meaning one wave or another. It is only necessary from there to decode the wave signal in audio.

According to the researchers, with their experiments have successfully captured and deciphered sounds played on loudspeakers at distances of up to 35 meters. You don’t have to turn the speaker to maximum volume or anything like that, just use a remote telescope to clearly see the brightness of the LED. The result is obviously not perfect. The decoded signal is not entirely clear, but enough to easily understand what is being said if it is a conversation.

Van Eck Interference: A "wave escape" used since the 70s to spy remotely and without a trace

The solution to avoid this hack? As simple as covering the LED indicator with an opaque tape or anything else. On the part of the manufacturers there are not really many options to avoid it. One of the safest would be to feed the LED with a totally different power source from the rest of the speaker, or directly not to place any LEDs.

Via | Hackday
More information | Ben nassi