Tapping on different points on a desk also produces sound characteristics that are unique to those points. One thing that could help to randomize that is a sur

Tapping on different points on a desk also produces sound characteristics that are unique to those points. One thing that could help to randomize that is a surface with tightly-coupled [truly] randomly-moving masses underneath, so that the resonances of the surface also change randomly. (And if done very right or very wrong, you get a free Van De Graaf generator!) --- Or: A laser projected keyboard where the keys are always shifting to different locations. (Bonus: Free psychosis!)