You could probably do something like this, but accuracy is going to be a problem. Sound waves are not uniform events. If you could rely on precise clicks at pre

You could probably do something like this, but accuracy is going to be a problem. Sound waves are not uniform events. If you could rely on precise clicks at precise frequencies, with minimal echo, you could get acceptable accuracy, but relying on finger thumps in varied substrate is going to be difficult. I think the reason you don't see more of this is that there are better solutions to the problem of establishing touch interfaces on suitable surfaces.