Brilliant Products

Sounds That Mics Can Hear And Humans Can't Could Protect You From Electronic Eavesdropping

It seems like every time there's a convenient new gadget released, a few months later you find out that it's been collecting your personal information or recording your conversations. In the future, that might not be so easy — not if University of Illinois researchers have anything to say about it. They've created a sound that's inaudible to humans, but can be picked up by any microphone, making it a powerful shield against spying electronics.

Advertisement

An Audio Disillusionment Charm

The human ear can hear sounds in the range of roughly 20 Hz to 20 kHz, or 20–20,000 Hz. Since we record sound so we can hear it, microphones have the same "hearing" range as humans — mics generally max out at around 24 kHz. But in 2017, researchers at the University of Illinois Coordinated Science Laboratory discovered a loophole in the way microphones operate. With the right combination of tones, they found, you can make a microphone pick up a sound that's above the range of human hearing. They call this sneaky system BackDoor.

The paper gives an example of how it works: BackDoor might play two tones together, one 40 kHz and one 50 kHz, both way beyond the range of what a human and a microphone can pick up. The tones arrive at the microphone's power amplifier. But because a microphone's design isn't perfect, the power amplifier not only amplifies the frequencies, but multiplies them too, resulting in two more "shadow" tones: 40 kHz + 50 kHz, or 90 kHz, which is still inaudible; and 40 kHz - 50 kHz, or 10 kHz, which is perfectly audible, if a little ear-splitting.

Importantly, that 10 kHz tone isn't broadcast; just recorded. That means that humans wouldn't hear anything strange in the moment, but if they played the sound back, they'd hear a piercing whine. The best part? You don't need any special equipment for it to work. Any microphone can record the tone.

As part of the BackDoor experiment setup, a 2Watt speaker array system for jamming applications.

Alexa, Earmuffs!

This could be huge for electronic privacy. "Imagine having a private conversation with someone. You can broadcast this inaudible signal, which translates to a white noise in the microphone, to prevent any spy microphones from recording voices," says coauthor Nirupam Roy. "Because it's inaudible, it wouldn't interfere at all with the conversation." And forget warnings about recording devices before concerts and plays — venues could just play these special frequencies to make anything recorded unusable. It could also help smart home devices communicate with one another in lieu of Bluetooth or WiFi.

But like a lot of technology, this also has its dark side. "Inaudible sound jammers, could, for example, affect someone wearing a hearing aid because the internal microphone would pick up that sound," said Roy. Or worse, criminals could jam phones during a robbery, keeping victims from contacting the police. But facing the downsides head-on will protect people in the long run. If the researchers are aware of the ways BackDoor can be used for ill, they can take steps to prevent that from happening.

Love getting smarter? Sign up for our newsletter to learn something new every day!

Microphones as Fast As Possible

Share the knowledge!
Advertisement