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.