Acoustic mirrors were used in the British coast for early detection of incoming planes over the channel. They are parabolic concrete structures in which the focus is occupied by a person who listens the sounds emited from far distances bouncing in the mirror and being redirected to that focus. Those structures never came to be useful in war times.
Nowadays there's a project from Lise Autogena a Danish artist living in the United Kingdom in order to build a pair of these acoustic mirrors, one each side of the channel so two people could speak from side to another.
Link
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.
Abusive comment hidden.
(Show it anyway.)