Ultrasound could waken a sleeping smart home

The home of the future, we are assured, will be swarming with tiny sensors: security cameras, carbon monoxide detectors, speakers and everything else. Few need to be running all the time — but how do you wake them up when they’re needed if they’re off in the first place? Ultrasound. That’s the idea being pursued by Angad Rekhi and Amin Arbabian at Stanford, anyway. Their approach to the problem