German researchers have demonstrated that microphones and speakers built into laptops can be used to covertly transmit and receive data through inaudible audio signals. Their experiments showed that messages can be transmitted using audio signals in the low ultrasonic frequency range at around 20,000 Hz between two Lenovo T410 business laptops over a maximum distance of 19.7 meters and at a rate of 20 bit/s. It’s still enough to transmit small pieces of valuable information like log-in credentials and encryption keys stolen by malware running on air-gapped computers.”]