Cross-device tracking can also be performed through the use of ultrasonic inaudible sound beacons. [...] When a user encounters a SilverPush advertiser on the w

Cross-device tracking can also be performed through the use of ultrasonic inaudible sound beacons. [...] When a user encounters a SilverPush advertiser on the web, the advertiser drops a cookie on the computer while also playing an ultrasonic audio through the use of the speakers on the computer or device.