Inaudible Sounds Being Used to Track Users Across Multiple Devices

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High-frequency sounds are being used to track people's behavior across
multiple devices. The sounds, which are inaudible to humans, are
embedded in television commercials and online advertisements. Tablets
and smartphones detect the sounds. The US Federal Trade Commission (FTC)
held a Cross-Device Tracking workshop on Monday, November 16, to address
the issue.
https://www.ftc.gov/news-events/events-calendar/2015/11/cross-device-tracking

embedded in television commercials and online advertisements...