Hundreds of privacy-invading apps are using ultrasonic sounds to track you

Apps are using ad-tracking ultrasonic tones – audio signals that your phone can hear, but you can’t – build up a profile about you: what you’ve seen, where, and in some cases even the websites you’ve visited. While this technology is still in its early stages of development, it’s becoming popular.

Source: Hundreds of privacy-invading apps are using ultrasonic sounds to track you