https://www.theverge.com/2017/9/7/16265906/ultrasound-hack-siri-alexa-google

Is your digital assistant taking orders behind your back? Scientists from China's Zheijiang University have proved it’s possible, publishing new research that demonstrates how Siri, Alexa, and other voice-activated programs can be controlled using inaudible ultrasound commands. This provides a new method of attack for hackers targeting devices like phones, tablets, and even cars. But don’t get too worried — the technique has a number of key limitations that means it’s unlikely to cause chaos.

Using ultrasound as discreet form of digital communication is actually pretty common. As pointed out in a FastCompany report on the topic, Google’s Chromecast and Amazon’s Dash Buttons both use inaudible sounds to pair to your phone. And advertisers take advantage of these secret audio freeways too, broadcasting ultrasonic codes in TV commercials that work like cookies in a web browser; tracking a user’s activity across devices.