Link

Using Ultrasonic Beacons to Track Users – Schneier on Security

Quoting Schneier:

The idea is for devices like televisions to play ultrasonic codes in advertisements and for nearby smartphones to detect them.

Quoting from the linked article:

We spot ultrasonic beacons in various web media content and detect signals in 4 of 35 stores in two European cities that are used for location tracking. While we do not find ultrasonic beacons in TV streams from 7 countries, we spot 234 Android applications that are constantly listening for ultrasonic beacons in the background without the user’s knowledge.

Source: Using Ultrasonic Beacons to Track Users – Schneier on Security