Cross-device tracking refers to technology which enables the tracking of users across multiple devices such as smartphones, television sets, smart TVs, and personal computers. More specifically, cross-device tracking is a technique in which technology companies and advertisers deploy trackers, often in the form of unique identifiers, cookies, or even ultrasonic signals, to generate a profile of users across multiple devices, not simply one. For example, one such form of this tracking uses audio beacons, or inaudible sounds, emitted by one device and recognized through the microphone of the other device. This form of tracking is used primarily by technology companies and advertisers who use this information to piece together a cohesive profile of the user. These profiles inform and predict the type of advertisements the user receives. Background There are many ways in which online tracking has manifested itself. Historically, when companies wanted to track users' online behavior, they simply had users sign in to their website. This is a form of deterministic cross-device tracking, in which the user's devices are associated with their account credentials, such as their email or username. Consequently, while the user is logged in, the company can keep a running history of what sites the user has been to and which ads the user interacted with between computers and mobile devices. Eventually, cookies were deployed by advertisers, providing each user with a unique identifier in his or her browser so that the user's preferences can be monitored. This unique identifier informs the placement of relevant, targeted ads the user may receive. Cookies were also used by companies to improve the user experience, enabling users to pick up where they left off on websites. However, as users began using multiple devices––up to around five––advertisers became confused as to how to track, manage, and consolidate this data across multiple devices as the cookie-based model suggested that each device––whether a phone, computer, or tablet––was a different person. Other technologies such as supercookies, which stay on computers long after the user deletes his or her cookies, and web beacons, which are unique images from a URL, are also used by trackers and advertisers to gain increased insight into users' behavior. However, advertisers were still limited in that only one device was able to be tracked and associated with a user. Thus, cross-device tracking initially emerged as a means of generating a profile of users across multiple devices, not simply