An increasing number of Android applications are attempting to track users without their knowledge, according to a new report.

Over recent years, companies have started hiding “beacons”, ultrasonic audio signals inaudible to humans, in their adverts, in order to track devices and learn more about their owners.

Electronic devices equipped with microphones can register these sounds, allowing advertisers to uncover their location and work out what kind of ads their owners watch on TV and which other devices they own.

The technique can even be used to de-anonymise Tor users.