Credit: Peter Griffin/Public Domain

Consumer privacy advocates find this seriously annoying: Some ads use inaudible sound to link your phone, TV, tablet, and PC. Dan Goodin of Ars Technica issued a "Beware" to readers earlier this month and the matter has concerned other technology-watching sites as well.

In October, the Federal Trade Commission got a letter about this from the Center for Democracy & Technology. The letter discussed "cross-device " of users by some marketing firms. The letter aimed to raise awareness of online companies that deploy "that squeal high-frequency sounds from the devices they're loaded on," said Softpedia.

The CDT's concern is that "users are often unaware of the wealth and detail of information that is being collected about their online and offline activities and the significant privacy invasions that result."

What type of tracking technology is this? The tech, said Goodin, is in the form of "inaudible, high-frequency sounds." He said "ultrasonic pitches are embedded into TV commercials or are played when a user encounters an ad displayed in a computer browser."

What is more, "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."

CDT said, "The average American owns four digital devices and spends sixty hours per week viewing content across devices, more than half of which is viewed using smartphone applications."

Just how does the tracking play out? The CDT offered a scenario that demonstrates why this may be of concern. A company could see that a user searched for sexually transmitted disease symptoms on her PC, it said, looked up directions to a Planned Parenthood on her , visited a pharmacy, then returned to her apartment. "The cross-device tracking allows companies to infer that the user received treatment for an STD. The combination of information across devices not only creates serious privacy concerns, but also allows for companies to make incorrect and possibly harmful assumptions about individuals."

Concluding their remarks in the letter, the group said, "We applaud the FTC for considering this important topic. Consideration of issues like cross device tracking is something that a majority of Americans want."

The group said it was calling for a solution that involves "increased transparency and a robust and meaningful opt-out system."

Right now, said Goodin, "there are no easy ways for average people to know if they're being tracked by it and to opt out if they object. Federal officials should strongly consider changing that."

The CDT said cross-device tracking has been put into use by "more than a dozen" marketing firms. Many consumers search on mobile devices but buy on computers; tracking them across multiple screens has been the advertising incentive. "By tracking individuals across devices, marketers can create complete and detailed profiles of each individual user and recognize long-term shopping or behavioral patterns," said the group.

The CDT's Greg Norcie, staff technologist, blogged earlier this year on the topic of unsanctioned web tracking. "Unsanctioned tracking is unknown to the user, without consent, and unable to be blocked by purely technical means. This means we must establish standards, policies, and laws which ensure that trackers obtain consent."

He said that users must be able to control their data with easy to use opt-outs. "Advertisers and service providers should not utilize tracking technologies which are designed to perform unsanctioned tracking without providing a usable method for users to opt out."