Riding the Non-linearities to Record Ultrasound with Smartphones

We demonstrate that high frequency ultrasonic sounds can be designed to become recordable by unmodified smartphone microphones, while remaining inaudible to humans. The core idea lies in exploiting non-linearities in microphone hardware with a combination of ultrasound frequencies. These frequencies can be regulated to carry data bits, thereby enabling an acoustic (but inaudible) communication channel to today's microphones. Other applications include jamming spy microphones in the environment, live watermarking of music in a concert, and even acoustic Denial-of-Service (DoS) attacks.