  The exchange of data between mobile devices in the near-ultrasonic frequency
band is a new promising technology for near field communication (NFC) but also
raises a number of privacy concerns. We present the first ultrasonic firewall
that reliably detects ultrasonic communication and provides the user with
effective means to prevent hidden data exchange. This demonstration showcases a
new media-based communication technology ("data over audio") together with its
related privacy concerns. It enables users to (i) interactively test out and
experience ultrasonic information exchange and (ii) shows how to protect
oneself against unwanted tracking.
