London College of Communications graduate Hon Lam Li created this Wall Piano that lets users play music by tapping on the wall.
There are two microphones attached on the wall surface. Those microphones are acting like human ears for the computer. Therefore, the computer is able to hear people who tap/bang on the wall. The program that I made could translate those hits into piano keys.
It all depends on how hard you hit the wall. The lower key will be produced when you hit it harder; and the higher key will be produced when you hit it softer. Just like every other modern piano, it has 88 keys.
http://www.creativereview.co.uk/crblog/the-wall-piano/ - Thanks Patrick Burgoyne!