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Can you eavesdrop on a speaker by watching its power LED?

Answer: Yes.

Google Home smart speaker on a book on a shelf Ci
If you thought the fact that someone couldn’t hear what was coming out of your speakers would prevent them from eavesdropping, think again. It turns out, as long as they can still see your speaker, there’s a way that they can figure out exactly what’s coming through it.

The discovery comes from a research team from Ben-Gurion University, who discovered that the power LED indicators on speakers flicker ever so subtly based on the sounds being put out. These changes in brightness are indetectable to the human eye, or even a high-resolution video camera. But there is one thing that can see them: an electro-optical sensor, which converts light into an electrical signal.

When they coupled the electro-optical sensor with a telescope, the team found that they could decipher exactly what was coming out of a speaker from up to 115 feet away. While the results aren’t crystal clear, they’re certainly clear enough that you might want to consider covering your speaker’s power LED before you discuss sensitive material on a conference call.
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