https://www.theregister.co.uk/2017/09/07/dolphins_help_pwn_electronics/

Voice control is all the rage these days, but a team of Chinese researchers has come up with a way to subvert such systems by taking a trick from the natural world.

Apps like Google Assistant and Siri are set to always be listening and ready for action, but shouting into someone else's phone is hardly subtle. So the team from Zhejiang University decided to take a standard voice command, convert it into the ultrasonic range so humans can't hear it, and see if the device could.