An app, 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.
Since 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.

Researchers at University of Washington (UW) showed that FingerIO can accurately track two-dimensional finger movements to within 8mm, which is sufficiently accurate to interact with today's mobile devices.
"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, doctoral student at UW.
"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," Nandakumar said.