A Matter of Touch
Touch & Activate: Adding Interactivity to Existing Objects using Active Acoustic Sensing (UIST2013).
full paper:
http://dl.acm.org/citation.cfm?id=2501988.2501989&coll=DL&dl=ACM&CFID=254518961&CFTOKEN=60671742
TapSense: Enhancing Finger Interaction on Touch Surfaces
full paper:
Touché: Touch and Gesture Sensing for the Real World.