The attack uses a technique called ultrasound cross-device tracking (uXDT) which made its way into advertising circles as early as 2012. Marketing companies running uXDT campaigns will play an ultrasonic sound, inaudible to the human ear, in a TV or radio ad, or even in an ad delivered via a computer browser. Although the user wont hear it, other devices such as smartphones will be listening. When the app hears the signal, it will ping the advertising network with details about itself. The attack could unmask more than just Tor users.”]