Describes a system that locates the position of knocks and taps atop a large sheet of glass. Our current setup uses four contact piezoelectric pickups located near the sheet's corners to record the 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 the rough position resolution, the nature of each hit (e.g., knuckle knock, metal tap, or fist bang), and the strike intensity. As this system requires only simple hardware, it 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 content controlled by knocks on the display window.