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[1] by Angad Rekhi and Amin Arbabian at Stanford, anyway. Their approach to the problem of devices that can’t stay on, yet can’t be all the way off, is to minimize the amount of energy necessary to send and receive a “wake” signal. That way the internet of things really only consumes power when they’re actively in use.
Radio, which of course all these tiny sensors use to transmit and receive information, is actually pretty expensive in terms of power and space. Keeping the antenna and signal processor ready and listening uses more energy than these devices have to spare if they’re to last for years on a charge.
Ultrasonic sensors, on the other hand, are incredibly power-efficient and require very little space. Ultrasound — soundwaves above the human range of hearing, 22KHz or so — is a much more physical phenomenon, and detecting it is easier in many ways than detecting radio frequency waves. It’s a bit like the difference between a sensor that’s sensitive to nearly intangible x-rays versus one that detects ordinary visible light.
Rekhi (left) and Arbabian looking natural in the lab.