Passive acoustic knock tracking for interactive windows

Abstract

We describe a novel interface that locates and characterizes knocks and taps atop a large glass window. Our current setup uses four contact piezoelectric pickups located near the sheet's corners to record the acoustic wavefront coming from the knocks. A digital signal processor extracts relevent 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 the rough accuracy of this estimate, the nature of each hit (e.g., knuckle knock, metal tap, or fist bang), and the strike intensity. This system requires only simple hardware, needs no special adaptation of the glass pane, and allows all transducers to be mounted on the inner surface, hence it is quite easy to deploy as a retrofit to existing windows. This opens many applications, such as an interactive storefront, with projected content controlled by knocks on the display window.