What happens when you touch the surface ?

- The mechanical energy propagates inside the material

How can the piezo find out where the finger landed ?
- Using the time of flight :

The energy wave arrives at the different sensors at time t1, t2 and t3.
The only problem is that you cannot determine t1 as there is no t=0s origin.
If there was a way to find t1, you would only need two piezo.

The only information we can find are the time differences. But that's more than enough because every time difference describes a hyperbola which goes through the point where the energy wave came from. The intersection between the 3 hyperbolas that can be drawn gives you an unique point : the origin of the energy wave.