# What is the longest distance pair of acoustic mirrors? **URL:** \ **Category:** Factual Questions\ **Tags:** science-math\ **Created:** [January 22, 2026, 5:04pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930 "2026-01-22T17:04:21Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [January 22, 2026, 5:04pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/1 "2026-01-22T17:04:22Z")
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?](https://boards.straightdope.com/t/how-far-apart-could-usable-sound-reflecting-dishes-be/446303) .) ---
**Author:** ![xtenkfarpl](https://avatars.discourse-cdn.com/v4/letter/x/c2a13f/32.png) [@xtenkfarpl](https://boards.straightdope.com/u/xtenkfarpl)\ **Post date:** [January 22, 2026, 5:42pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/2 "2026-01-22T17:42:00Z")
Not quite the same geometry, but the whispering gallery in St Paul’s in London exhibits the same phenomenon. > **[Whispering gallery](https://en.wikipedia.org/wiki/Whispering_gallery)** > > A whispering gallery is usually a circular, hemispherical, elliptical or ellipsoidal enclosure, often beneath a dome or a vault, in which whispers can be heard clearly in other parts of the gallery. Such galleries can also be set up using two parabolic dishes. Sometimes the phenomenon is detected in caves. > A whispering gallery is most simply constructed in the form of a circular wall, and allows whispered communication from any part of the internal side of the circumference to any other part. Th... I don’t know of any other examples using parabolic mirrors, but I suspect that quite a few exist. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [January 22, 2026, 8:03pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/3 "2026-01-22T20:03:09Z")
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? With a loudspeaker, they’ve reached 100+ meters: [Parabolic loudspeaker - Wikipedia](https://en.wikipedia.org/wiki/Parabolic_loudspeaker) Weaponized/commercialized, 3 kilometers: [Long-range acoustic device - Wikipedia](https://en.wikipedia.org/wiki/Long-range_acoustic_device) Related: [Sound from ultrasound - Wikipedia](https://en.wikipedia.org/wiki/Sound_from_ultrasound#Parametric_array) and [Sound amplification by stimulated emission of radiation - Wikipedia](https://en.wikipedia.org/wiki/Sound_amplification_by_stimulated_emission_of_radiation) The Chinese have also sent encoded acoustic messages 375 miles underwater: [China enables 375-mile submarine messaging with acoustic breakthrough](https://interestingengineering.com/innovation/china-375-mile-underwater-messaging-acoustic-breakthrough) ---
**Author:** ![MacDoc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/macdoc/32/18696_2.png) [@MacDoc](https://boards.straightdope.com/u/MacDoc)\ **Post date:** [January 22, 2026, 8:40pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/4 "2026-01-22T20:40:32Z")
> **[Acoustic mirror](https://en.wikipedia.org/wiki/Acoustic_mirror)** > > An acoustic mirror is a passive device used to reflect and focus (concentrate) sound waves. Parabolic acoustic mirrors are widely used in parabolic microphones to pick up sound from great distances, employed in surveillance and reporting of outdoor sporting events. Pairs of large parabolic acoustic mirrors which function as "whisper galleries" are displayed in science museums to demonstrate sound focusing. > Between the World Wars, before the invention of radar, parabolic sound mirrors were used ... I’d guess the UK holds the record from WWII - there is a pretty big one at The Dish in Australia - there for fun > **[Inside the big dish that brought us TV from the Moon](https://www.bbc.com/future/article/20161207-inside-the-big-dish-that-brought-us-tv-from-the-moon)** > > In a quiet corner of rural Australia sits a giant radio dish that helped ensure the most famous space mission of all could be seen on TV. Richard Hollingham went to visit it. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 22, 2026, 11:21pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/5 "2026-01-22T23:21:35Z")
> [@xtenkfarpl](#): > > Not quite the same geometry, but the whispering gallery in St Paul’s in London exhibits the same phenomenon. 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). ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [January 22, 2026, 11:28pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/6 "2026-01-22T23:28:29Z")
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. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [January 22, 2026, 11:32pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/7 "2026-01-22T23:32:24Z")
> [@MacDoc](#): > > there is a pretty big one at The Dish in Australia - there for fun Yes, and they work well. Measuring on google maps it looks like they are around 50m (150ft) apart. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 22, 2026, 11:54pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/8 "2026-01-22T23:54:22Z")
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 Wikipedia page on the topic now – [Acoustic mirror - Wikipedia](https://en.wikipedia.org/wiki/Acoustic_mirror) ---
**Author:** ![velomont](https://avatars.discourse-cdn.com/v4/letter/v/8491ac/32.png) [@velomont](https://boards.straightdope.com/u/velomont)\ **Post date:** [January 23, 2026, 12:12am UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/9 "2026-01-23T00:12:58Z")
> [@CalMeacham](#): > > They used them in Britain (and elsewhere, like Malta) during WWI to listen for German aircraft before the advent of radar. 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. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 23, 2026, 2:35am UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/10 "2026-01-23T02:35:48Z")
Acoustic mirror is probably too wide a term to capture the intent. Lots of uses for elements that reflect acoustic energy. Here we are concerned with focusing as well as just reflecting. A large scale example of a single element is an arch dam with a close to elliptical profile, people stood at each end can converse perfectly. Where I live, the nearby [Barossa](https://en.wikipedia.org/wiki/Barossa_Reservoir) dam, of 144 metres (472 ft) length does this brilliantly. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [January 23, 2026, 5:59am UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/11 "2026-01-23T05:59:33Z")
There was a pair set up at Afrikaburn once, that was at least 100m apart, and worked OK. You had to speak very loudly, though, it didn’t do mere whispers. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [January 23, 2026, 3:24pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/12 "2026-01-23T15:24:44Z")
> [@CalMeacham](#): > > These dishes form the ends of ellipses. Is that correct? I thought the dishes were parabolas; put your mouth at the focus and speak, the sound reflects off the parabola and travels linearly to another parabolic dish, where it’s reflected back to the focus (and hopefully someone’s ear). For an ellipse, stand at one focus and speak and the sound bounces off the ellipse and directly to the other focus. Two reflections when using parabolic dishes, one reflection in an ellipse. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [January 23, 2026, 3:52pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/13 "2026-01-23T15:52:02Z")
In practice, for portions of the reflector that are much closer to the nearest focus than the distance between foci, the two shapes would be almost identical, probably closer than the manufacturing tolerances of the dishes. The difference is most relevant when you’ve got the complete shape, like with a dome or the face of the dam @Francis_Vaughan mentioned, where you have parts of the reflector that are far from either focus. In those cases, you probably want an ellipsoid. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [January 23, 2026, 4:12pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/14 "2026-01-23T16:12:37Z")
> [@Robot\_Arm](#): > > Is that correct? I thought the dishes were parabolas; put your mouth at the focus and speak, the sound reflects off the parabola and travels linearly to another parabolic dish, where it’s reflected back to the focus (and hopefully someone’s ear). > > For an ellipse, stand at one focus and speak and the sound bounces off the ellipse and directly to the other focus. > > Two reflections when using parabolic dishes, one reflection in an ellipse. As far as the shape goes, it’s probably a toss-up– spheres, ellipsoids, and paraboloids are pretty similar in shape. According to my sources, the concrete acoustic mirrors they used in Britain were actually spheroids, even though paraboloids would technically be better. But you couldn’t “steer” one of those huge, heavy concrete structures, so they effectively directed them by moving the location of the pickup microphone. So using a spheroid was the overall best solution. As for the museum devices, the ones that indicated what they were claimed they were ellipsoids. You’re correct that the operation is different – one bounce for an ellipse vs. two for a paraboloid. But I think the difference in angle for real sound waves over that distance is negligible. You can argue with the gods about whether it ought to be a paraboloid or an ellipsoid. Technically, if it’s an ellipsoid you’d get a sound at the ocus whether the second device is there or not, but in practice I’ll bet it won’t make a difference. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [January 24, 2026, 12:36am UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/15 "2026-01-24T00:36:27Z")
They are all conic sections. An ellipse has two focii. A parabola has one. You could regard a parabola as an ellipse having a second focus at infinity; which if you look at how the sections are formed cutting a cone, makes intuitive sense. So if you have a single face that focuses at each end, you have an ellipse. If you have two separate small reflectors, you have two parabolas facing one another. The orientation of the sections are different. The ellipse is side on, with the reflecting surface forming one long side of the ellipse, whereas the parabolas are end on, and the reflectors form the ends of truncated parabolas. The ellipse could be viewed as taking two parabolic reflectors facing one another, then because the focii are not infinitely far apart, slightly changing the shape so that each section has a second focus at the other end’s focus point. Do that and you have re-created the ellipse. You don’t need the entire reflector, and cutting it in half along the long axis still leaves an effective system. Which is what the dam wall is. If you wanted the best possible shape for the two parabola system you would actually make it from the ends of an ellipse. In really you would be hard pressed to measure the difference in shape. ---
**Author:** ![JacobSwan](https://avatars.discourse-cdn.com/v4/letter/j/a6a055/32.png) [@JacobSwan](https://boards.straightdope.com/u/JacobSwan)\ **Post date:** [January 24, 2026, 2:21pm UTC](https://boards.straightdope.com/t/what-is-the-longest-distance-pair-of-acoustic-mirrors/1026930/16 "2026-01-24T14:21:52Z")
We used to do a related demonstration at my Uni physics dept on open days. A weather balloon filled with heavier than air gas could be used as an acoustic lens to focus sound across the atrium. I coined the phrase ‘acoustic refraction’ to try and explain it to people, as it worked the same way as light refraction. The sound travelled slower through the balloon and got bent and focussed as a result.