Denge sound mirrors (andrewgrantham.co.uk)

At the Goonhilly Earth Station visitor centre (currently closed), there are two sound mirrors set up at opposite ends of the centre indoors, allowing people to communicate across the whole room. The effect is really quite weird.

http://emlia.org/pmwiki/pmwiki.php?n=Tripocalypse.Prologue Shameless plug: I used this effect in a story, to mute Jesus so that he cannot pass judgement on humanity and end the world.

As featured in San Francisco's Exploritorium.

And

This amazing video of the same name by Coldcut: https://vimeo.com/29148950