Pretty soon, mobile devices such as smartwatches will be the size of a pebble.

Or at least, that was the scenario in Gary Shteyngart's novel "Super Sad True Love Story," set in a near-future dystopian period.

In any case, they get smaller every day, and it is harder for people to type on the tiny screens. University of Washington (UW) electrical engineers and computer scientists have developed new sonar technology that they say allows a person to interact with the device - including a smartphone - just by writing or moving a hand on a nearby surface - or even mid-air.

The technology, called FingerIO, follows the tracing movement of your hand and tracks finger movements at a fine-grained level. This is done by turning the device into an active sonar system by employing the existing microphones and speakers in the machine.

The movement is still picked up through sweater sleeves and other fabrics because sound waves can go through fabric and don't require a line of sight.

The UW team's study paper will be presented in May at the Computing Machinery's CHI 2016 conference in San Jose, Calif. The researchers will show that FingerIO is able to efficiently track finger movements in two dimensions to a degree within 8mm. This makes it more than able to interact with contemporary mobile devices. The conference has already awarded an honorable mention to their work.

"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," noted Rajalakshmi Nandakumar, lead author and 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."

There are lots of possibilities with the technology. FingerIO can allow one to use a quick finger movement to increase volume, push a button or run through a menu, all without touching the device at all. It's also possible to write a search command or text in mid-air.

Using the system, the device's speaker picks up an inaudible sound wave. The signal from this bounces off a finger, and echoes from it are "heard" by the device's microphones and used to find the finger's location in space.

"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 Shyam Gollakota, study senior author and UW assistant professor of computer science and engineering. "And from a cost perspective, almost every device has a speaker and microphones so you can achieve this without any special hardware."

Follow Catherine Arnold on Twitter at @TreesWhales.