A sound completely inaudible to humans but audible to any microphone It has been designed by researchers from the University of Illinois.
We are not able to hear at levels above 20 kHz and the microphones reach 24 kHz.
This sound combines multiple tones that, when interacting with the mechanics of the microphone, originates what researchers call a "shadow," and It is transmitted from ultrasonic speakers.
But what can something like that serve? Rommit Roy, one of the authors of this research, explains it like this:
Imagine having a private conversation with someone, you can transmit this inaudible signal, which translates into a white noise in the microphone, to prevent any spy microphone from recording voices.
The signal can also be used to send communications between Internet of Things (IoT) devices, such as Amazon Echo or Google Home, which would reduce Bluetooth's burgeoning load.