Apologies for the over-the-top headline. The technique works with any voice-driven assistant and, yes I know, Alexa doesn’t have ears. The idea is simple and it seems fairly effective. It allows you to get voice commands to Alexa that that are inaudible to people in the same room. The argment goes that voice responsive assistants don’t need much in the way of security because if someone in the room says then the chances are you would hear them and put a stop to the evil plan. There are times when this goes wrong, of course. What if someone on the radio says that same thing, but mostly we tend to assume that voice input cannot be covert. We might have to rethink this assumption due to some interesting experiments at Princeton Department of Electrical Engineering.  Liwei Song and Prateek Mittal have managed to get Alexa to respond to a voice input that no-one else can hear. The trick is to use an ultrasound source which is so high pitched that it is outside of the range of human hearing. Of course you might object that if humans can’t hear it then neither can Alexa, but as stated in the intorduction Alexa doesn’t have ears. Instead there is a microphone, an...