One tube is stacked on top of the other with the solid piece of Plexiglas in between. The audience is told to both watch and listen as they will actually see and here the block penetrate the glass. The solid block is dropped into the top tube; the audience sees it past the first window. It hits the Plexiglas and they hear it. Finally they see it pass the window in the bottom tube and hear it hit the table.
This is wonderful visual magic which is very baffling and yet very, easy to do.