I love it when somebody takes a concept as simple as a calculator and rethinks it. Rather than considering it something that manipulates numeric symbols, this box turns math into a haptic/audio experience. I can imagine some kids finding this much more engaging than manipulating symbols on a display. For example, I bet kids who are dyslexic would find this more interesting than a regular calculator. Check out the video.
It looks like a great maker project to me. Now, how do you figure out where the user is knocking?