do you need to join them together with string?
I don’t believe so Leon, that would be a telephone. If you’ve been to Parkes and seen “the Dish” out the front you will notice two dishes facing each other placed at least 50 metres apart. When standing in the focal point of this dish you face it (the dish) and whisper very softly and the person standing 50 metres in the focal point of the opposing dish can hear you as if you were standing next to them. It works on focal point and sound waves. If you haven't seen it you've got to see it!
I’m guessing the coffee tins are a crude adaptation of this science. I don’t know if it works for sure, the tins aren’t even parabola shaped so the focal point would be ropey if it even existed at all. It would be interesting to bust this myth…
Cheers
Ed