Joseph A. Paradiso, Che King Leo

Abstract

We describe a system that locates and characterizes knocks and taps atop a large sheet of glass using four contact piezoelectric pickups located near the sheet's corners to simultaneously record the structural-acoustic wavefront coming from the impacts. A digital signal processor extracts relevant characteristics from these signals, such as amplitudes, frequency components, and differential timings, which are used to estimate the location of the hit and provide other parameters, including a degree of confidence in the position accuracy, the nature of each hit (e.g., knuckle knock, metal tap, or fist bang – our system responds to any kind of impact), and the strike intensity. As this system requires only simple hardware, it needs no special adaptation of the glass pane, and allows all tracking transducers to be mounted on the inner surface, hence it is quite easy and inexpensive to deploy as a retrofit to existing windows. This opens many applications, such as an interactive storefront, with content controlled by knocks on the display window.