${{\sf S \text{-}UbiTap}}$S-UbiTap: Leveraging Acoustic Dispersion for Ubiquitous and Scalable Touch Interface on Solid Surfaces.

As various computing devices, such as smartphones, IoT devices, smart speakers etc, becomes omnipresent in our daily lives, interest in ubiquitous computing interfaces is increasing. In response to this, various studies have introduced on-surface input techniques that leverage the surface of surrounding objects as touch interfaces. However, most of them struggle to support ubiquitous interaction due to their dependency on specific hardware or environments. In this work, we propose S-UbiTap, an input method that turns any flat solid surface into a touch input space by listening to sound (i.e., with microphones already present in the commodity devices). More specifically, we develop a novel touch localization technique that leverages the physical phenomenon, called dispersion, which is the characteristic of sound as it travels through solid surfaces, and address the challenges that limit existing acoustic-based solutions in terms of portability, accuracy, usability, robustness, scalability, and responsiveness. Our extensive experiments with a prototype of S-UbiTap show that we can support sub-centimeter accuracy on various types of surfaces with minor user calibration effort. In addition, the accuracy is maintained even when the size of the touch input space increases. In our experience with real-world users, S-UbiTap significantly improves usability and robustness, thus enabling the emergence of more exciting applications.