The SilverPush technology allows advertising firm to track user habits with near-ultrasonic sounds. The technology assigns letters of the alphabet to high-pitch tones. The combination of letters is used to identify TV ads, for example the AP is associated to a Geico ad and display an image and link to the insurance biz. A research has identified 30 applications using the SilverPush SDK, including shopping apps developed by companies in India and Far East. In US and Europe end-user must explicitly give its consent, in Europe and in the US.”]
Source: https://securityaffairs.co/wordpress/42129/hacking/silverpush-technology.html