If this is true, I’m going to knit myself a tinfoil hat. And one for my laptop, and one for my tablet, and one for my cellphone.

Ars Technica: Beware of ads that use inaudible sound to link your phone, TV, tablet, and PC. “Privacy advocates warn feds about surreptitious cross-device tracking.” By Dan Goodin.

“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. Dan Goodin reports for Ars Technica on cross-device tracking software already in use today.”

Link via and discussion over at MetaFilter.