BackDoor: Making Microphones Hear Inaudible Sounds

Drs. Choudhury and Hassanieh, and Mr. Roy from the Electrical and Computer Engineering Department at the University of Illinois at Urbana-Champaign have developed a system enabling microphones to record inaudible sounds called BackDoor. This system can be used for data communication, communication security and privacy, and acoustic watermarking. BackDoor operates at frequencies of 40 kHz and higher, and is not limited with the 20-24 kHz bandwidth as other ultrasound devices. Reported results achieve data rates of 4 kbps at a distance of 1 m, and 2 kbps at 1.5 m, as well as room-level (3.5 m radius) privacy protection against electronic eavesdropping.