"Researchers from the Brunswick Technical University in Germany discovered that many Android applications have used ultrasonic beacons to track users without their knowledge. Over the past few years, advertisers have started taking advantage of a technology called ultrasound cross-device tracking (uXDT) to track mobile users and build extensive profiles on them. The way uXDT works is that advertisers embed ultrasounds in the ads they play on TV or radio in the 18-20kHZ frequency range, and then smartphones or PCs with microphones pick up those sounds. This will tell the advertisers what kind of ads people are watching on TV, what type of phones they use, their location, and other advertising-relevant type of information such as user behavior. According to the researchers, the ultrasonic tracking can also be used to de-anonymize Tor users. The devices the Tor users own could give them away when some of the apps they install contain code that can intercept ultrasounds with the phone’s microphone." Source: http://www.tomshardware.com/news/android-apps-track-users-ultrasounds,34324.html Good thing that i bought an extension to my tinfoil hat, so i can block ultrasounds too now.