Smartwatches can now track your finger in mid-air using sonar !

As mobile and wearable devices such as smartwatches grow smaller, it gets tougher for people to interact with screens the size of a matchbook.

That could change with a new sonar technology developed by University of Washington computer

scientists and electrical engineers that allows you to interact with mobile devices by writing or gesturing on any nearby surface -- a tabletop, a sheet of paper or even in mid-air.

FingerIO tracks fine-grained finger movements by turning a smartphone or smartwatch into an active sonar system using the device's own microphones and speakers.

Because sound waves travel through fabric and do not require a line of sight, users can even interact with a phone inside a front pocket or a smartwatch hidden under a sweater sleeve.

In a paper to be presented in May at the Association for Computing Machinery's CHI 2016 conference in San Jose, California, the UW team demonstrates that FingerIO can accurately track two-dimensional finger movements to within 8mm, which is sufficiently accurate to interact with today's mobile devices. The work was recognized with an honorable mention award by the conference.

"You can't type very easily onto a smartwatch display, so we wanted to transform a desk or any area around a device into an input surface," said lead author Rajalakshmi Nandakumar, a UW doctoral student in computer science and engineering. "I don't need to instrument my fingers with any other sensors -- I just use my finger to write something on a desk or any other surface and the device can track it with high resolution."

Using FingerIO, one could use the flick of a finger to turn up the volume, press a button, or scroll through menus on a smartphone without touching it, or even write a search command or text in the air rather than typing on a tiny screen.

FingerIO turns a smartwatch or smartphone into a sonar system using the device's own speaker to emit an inaudible sound wave. That signal bounces off the finger, and those "echoes" are recorded by the device's microphones and used to calculate the finger's location in space.

Using sound waves to track finger motion offers several advantages over cameras -- which don't work without line-of-sight when the device is hidden by fabric or another obstructions -- and other technologies like radar that require both custom sensor hardware and greater computing power, said senior author and UW assistant professor of computer science and engineering Shyam Gollakota.

"Acoustic signals are great -- because sound waves travel much slower than the radio waves used in radar, you don't need as much processing bandwidth so everything is simpler," said Gollakota, who directs the UW's Networks and Mobile Systems Lab. "And from a cost perspective, almost every device has a speaker and microphones so you can achieve this without any special hardware."