BackDoor: Sounds that a microphone can record, but that humans can't hear

Microphones are like human ears. If we play an audible sound, both humans and microphones will be able to hear it. We design sound that appears just as any other sound to microphones, but remains inaudible to humans. This sound can be regulated to carry data bits, thereby enabling an acoustic (but inaudible) communication channel to hundreds of smart devices that already have built-in microphones. Other applications include jamming spy microphones in the environment, live watermarking of music in a concert, and even threats like acoustic denial-of-service (DoS) attacks on phone calls or inaudible command attacks on voice-enabled devices.