« The beacons are frequencies from 18kHz to 20kHz, a range that is inaudible to most humans but can be reliably detected by most phone microphones. By embedding them into audio, marketers can track the whereabouts of shoppers as they move throughout a large department store.
The tracking can also be used for purposes that are decidedly less ethical. Advertisers, for example, may use the beacons with no disclosure at all to measure how often a particular TV ad is viewed. The technology can also be covertly used to perform cross-device tracking that allows marketers to tie a single person to the multiple media devices she uses. The researchers said the beacons could similarly be used to identify people using the Tor anonymity service ».
Source : More Android phones than ever are covertly listening for inaudible sounds in ads | Ars Technica
