Ars Technica:

Privacy advocates are warning federal authorities of a new threat that uses inaudible, high-frequency sounds to surreptitiously track a person’s online behavior across a range of devices, including phones, TVs, tablets, and computers.

The ultrasonic pitches are embedded into TV commercials or are played when a user encounters an ad displayed in a computer browser. While the sound can’t be heard by the human ear, nearby tablets and smartphones can detect it. When they do, browser cookies can now pair a single user to multiple devices and keep track of what TV commercials the person sees, how long the person watches the ads, and whether the person acts on the ads by doing a Web search or buying a product.

And the ad industry wonders why we hate them and happily use ad blockers.