I saw thesde once in a science museum, though smaller models. They had two in opposite corners of the huge exhibit room. You could stand in one and speak and hear the person in the other just fine in a normal voice, while everyone else in the room heard nothing.
What I do with MY kids is slice as much as I want to give them. If I get any complaints, I say "Life isn't fair. If you don't like the cake, I'll eat it for you."