https://www.vice.com/en_us/article/bvg5dv/ultrasonic-waves-can-make-siri-share-your-secrets

Researchers at the Washington University in St. Louis have discovered a new method of attacking mobile devices via inaudible voice assistant commands, using a novel method that’s effective from up to 30 feet away. While the attack requires such a specific environment that we’re unlikely to see it used much in the wild, it nevertheless represents a previously unknown vulnerability that affects virtually all mobile devices, including all iPhones running Siri and a plethora of Android devices running Google Assistant.

The research team presented its full report on the vulnerability at the Network and Distributed System Security Symposium on February 24, and has subsequently published a summary on the university website. The gist of the main finding is that these voice assistant programs listen to a frequency far wider than the human voice is capable of producing, and thus can be fed ultrasonic waves that will be interpreted as voice commands while remaining completely inaudible to the human ear.