Also known as “whsper dishes” according to Wikipedia, but I have never heard that phrase.
This is where a pair of parabolic dishes are set up fairly far apart from each other, and if you talk quietly into one, a person standing by the other one can hear you loud and clear. I’ve seen one at the former Toronto Science Center, and it projected one’s voice across the large room which was as big as a convention hall. The only other one I’ve seen was on the shorefront at Baltimore, but the mirrors were close enough that you weren’t quite sure you weren’t just hearing them speak regularly.
(Searching for this gets search engines and AI confused with another use of “acoustic mirror”, namely the use of parabolic dishes to detect noises, specifically aircraft, rather than to both send and collect sound.)
(Also, Discourse reminds me that in 2008, I asked how far away they could be in this thread, but I didn’t ask how far they actually had been made. For once it was accurate in noticing that there were threads that were “similar!” How far apart could usable sound-reflecting dishes be? .)
Interesting question… would love to experience the furthest one in person!
Side thought: Does it have to be strictly the demonstration kind where you only have two parabolic dishes and humans talking, or can it be more technologized?
There’s also one in the US Capitol Building. Though neither of those uses a pair of paraboloids; they’re both ellipsoids (which is slightly better, though the difference is probably negligible).
It seems the max length of these are about 100 yards apart (give or take a little).
Can they be longer? I do not know. Probably but doubtless will lose fidelity.
I have been to the one at the Museum of Science an Industry in Chicago. I am not sure but guessing they are about 100 feet (not yards) apart. Really cool to experience though.
There’s a big one at the Exploratorium in San Francisco, and I think they drag one out at the Boston Museum of Science now and then. There are such “ears” in the Chicago Museum of Science and Industry, and the Ontario Science Center outside Toronto.The last time I was at Squam Lake Nature Center in New Hampshire they had a set of them outdoors. I don’t recall how far apart they were.
These dishes form the ends of ellipses. But there was a case where the whispering ear was supposed to be a parabola. They used them in Britain (and elsewhere, like Malta) during WWI to listen for German aircraft before the advent of radar. They put a microphone at the focus and someone listened very carefully, trying to block out wind noise and the like. It was claimed that they could detect airplane or zeppelin motors from as far as twenty miles away. They did demonstrate a range of 12-15 miles. It was enough to get a warning out to seek shelter.
Eventually (after the war) they had some that could detect aircraft at 30 miles. There was a proposal in 1935 to build more “Acoustic mirrors”, but by that time radar WAS available, and had a range of 40 miles, and wouldn’t exhaust the listeners (who were restricted to 40 minute shifts).
The reason I know about this is that I did an article about these for Optics and Photonics News back in 2011.
There’s a sf/alternate history short story, “The Receivers” by Alistair Reynolds, in which these play a role. It’s a really nicely written and somewhat touching story.