Expressive touch: studying tapping force on tabletops

This paper investigates users' ability to perform force-sensitive tapping and explores its potential as an input modality in touch-based systems. We study force-sensitive tapping using Expressive Touch, a tabletop interface that infers tapping force from the sound waves created by the users' finger upon impact. The first part of the paper describes the implementation details of Expressive Touch and shows how existing tabletop interfaces can be augmented to reliably detect tapping force across the entire surface. The second part of the paper reports on the results of three studies of force-sensitive tapping. First, we use a classic psychophysic task to gain insights into participants' perception of tapping force (Study 1). Results show that although participants tap with different absolute tapping forces, they have a similar perception of relative tapping force. Second, we investigate participants' ability to control tapping force (Study 2) and find that users can produce two force levels with 99% accuracy. For six levels of force, accuracy drops to 58%. Third, we investigate the usability of force tapping by studying participants' reactions to seven force-sensitive touch applications (Study 3).