Project Website http://ttt.media.mit.edu/research/tap.html
The Responsive Environments research group has designed, built, and fielded a system that locates the position of knocks and taps on a large sheet of glass.
Digital signal processors interpret relevant characteristics from a knock on the glass to estimate the location of the hit and provide other data, 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 uses simple hardware and does not require special adaptation of the glass pane. Nor does anything need to be mounted on the outside of the glass. Therefore, it is easy to deploy onto existing windows. This opens many applications, such as an interactive storefront, with dynamic content controlled by knocks on the display window.