New technology is making targeted advertising sneakier than ever. Ultra high-pitched audio inaudible to human ears can now allow cross-device tracking. Ars Technica reported that the Center for Democracy and Technology (CDT), a digital advocacy group, complained about the use of this hidden online behavior tracking method.

The letter addressed to the United Federal Trade Commission said that these signals “are a more accurate way to track users across devices.” This is compared to traditional methods like browser fingerprinting which already does not rely on cookies to track activity.

The letter also singled out a company called SilverPush, the leading firm developing “ultrasonic audio tracking.” According to the CDT, SilverPush technology allows communications between computers and smart devices, including connected TVs.

Here is an excerpt of the letter that explains how the technology works:

“When a user encounters a SilverPush advertiser on the web, the advertiser drops a cookie on the computer while also playing an ultrasonic audio through the use of the speakers on the computer or device. The inaudible code is recognized and received on the other smart device by the software development kit installed on it. SilverPush also embeds audio beacon signals into TV commercials which are “picked up silently by an app installed on a [device] (unknown to the user).” The audio beacon enables companies like SilverPush to know which ads the user saw, how long the user watched the ad before changing the channel, which kind of smart devices the individual uses, along with other information that adds to the profile of each user that is linked across devices.”

PC World reported that SilverPush has been embedded into some apps and up to 18 million smartphones in April of this year. The technology only listens for certain signals at the right frequency and users cannot opt out of this tracking technology.