Cross-device tracking (XDT) technologies are currently the "Holy Grail" for marketers because they allow to track the user's visited content across different devices to then push relevant, more targeted content. For example, if a user clicks on a particular advertisement while browsing the web at home, the advertisers are very interested in collecting this information to display, later on, related advertisements on other devices belonging to the same user (e.g., phone, tablet).
Currently, the most recent innovation in this area is ultrasonic cross-device tracking (uXDT), which is the use of the ultrasonic spectrum as a communication channel to "pair" devices for the aforementioned tracking purposes. Technically, this pairing happens through a receiver application installed on the phone or tablet. The business model is that users will receive rewards or useful services for keeping those apps active, pretty much like it happens for proximity-marketing apps (e.g., Shopkick), where users receive deals for walk-ins recorded by their indoor-localizing apps.
I know of traditional media ad-tech that uses ultrasound markers embedded in ads to track/verify if the ads were really broadcast as promised (number of times & in the correct time slots.
So the steps are:
1. Inject ultrasound markers into ad during post-production.
2. Have a server with multiple tuner cards to monitor multiple stations, grab the audio.
3. Filter audio on specific ultrasound frequencies, search for the pre-injected patterns.