Dan Seifert reports in The Verge that researchers at China's Zheijiang University have found that it is possible to use inaudible ultrasound commands to control Siri, Alexa, and other voice-activated computer programs. The ultrasound commands use frequencies greater than 20 kHz, the type that dolphins and bats can hear. Google’s Chromecast and Amazon’s Dash Buttons also use those frequencies to link to cell phones, and advertisers also broadcast ultrasonic codes in TV commercials to follow a user’s activity across devices. The researchers tested 16 voice control systems, including Siri, Google Now, Samsung S Voice, Cortana, Alexa, and several in-car interfaces. They were successful in, “activating Siri to initiate a FaceTime call on iPhone, activating Google Now to switch the phone to the airplane mode, and even manipulating the navigation system in an Audi automobile.” The researchers could also cut up syllables and rearrange them for voice command systems that respond to only one person’s voice. That's the bad news. The good news is that to use ultrasound voice command hacking, one has to be no more than a few feet away from the device to be hacked. And it has to be quiet -- asking, "Siri to turn on airplane mode was 100 percent successful in an office; 80 percent successful in a cafe; and just 30 percent successful in the street," according to The Verge. And they need a particular type of speaker, and sometimes needed multiple attempts to locate the exact frequency that worked on the device in question. So, at this point in history, ultrasound voice command hacking is not very practical, or something to be worried about. But that may change as technology evolves.