one. One such tactic for cross-device tracking is called browser fingerprinting, and occurs when browsers, which are modifiable to the users' tastes, produce a unique signal that companies or advertisers can use to single out the user. Browser fingerprinting has been a cause for concern because of its effectiveness and also since it does not allow for users to opt-out of the tracking. Another tactic used by Google is called AdID and works on smartphones in tandem with cookies on a user's computer to track behavior across devices. Now, cross-device tracking has evolved into a new, radical form of surveillance technology which enables users to be tracked across multiple devices, including smartphones, TVs, and personal computers through the use of audio beacons, or inaudible sound, emitted by one device and recognized through the microphone of the other device, usually a smartphone. In addition, cross-device tracking may presage the future of the Internet of things (IoT), in which all types of devices––such as offices, cars, and homes––are seamlessly interconnected via the internet. Ultrasonic tracking Humans interpret sound by picking up on different frequencies. Given the variety of sound waves that exist, humans can only hear frequencies that are within a certain range––generally from 20 Hz to 20 kHz. By the age of 30, most humans cannot hear sounds above 18 kHz. Ultrasound, which is shorter wavelengths greater than or equal to 20 kHz, enables the rapid transmission of data necessary for cross-device tracking to occur. Another integral component of cross-device tracking is the usage of audio beacons. Audio beacons are beacons that are embedded into ultrasound, so they cannot be heard by humans. These audio beacons are used to surreptitiously track a user's location and monitor online behavior by connecting with the microphone on another device without the user's awareness. In October 2015, the Center for Democracy and Technology submitted comments to the Federal Trade Commission (FTC) regarding cross-device tracking technology, specifically mentioning SilverPush. Audio "beacons" can be embedded into television advertisements. In a similar manner to radio beacons, these can be picked up by mobile apps. This allows the behavior of users to be tracked, including which ads were seen by the user and how long they watched an ad before changing the channel. In March 2016, the FTC issued warning letters to 12 app developers using cross-device tracking in their apps. The FTC warned these developers that they may