It takes a lot of work, but it can be done. The Mongol Yam, a horse-based mail system, is probably the best example that has been done before. But after visiting the local science museum recently, I saw these.

Acoustic Mirrors, you speak in front of one, and a person at the other one, some distance off, can hear what you say. In between them, you cannot hear anything from them.

I was wondering if a system of these could be set up, to have rapid transportation of information from one point of a kingdom to another, especially when I don't have horses.

If possible, what's their max range? How would weather affect them?

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    $\begingroup$ You might be interested in a similar system that was used successfully before the invention of the electrical telegraph, the semaphore line. $\endgroup$ Aug 18, 2018 at 6:16
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    $\begingroup$ Are you asking about how to achieve fast long-distance communication of any kind in a world where people have not (yet) harnessed electricity, or are you asking specifically about acoustic mirrors? If the former, what properties are you after? For example, do you require that the message cannot be overheard (or whichever other term applies) by people along the route? $\endgroup$
    – user
    Aug 18, 2018 at 6:19
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    $\begingroup$ Yes to the first one, the max range is what I'm interested in, and it's a fact that you cannot hear anything between them. $\endgroup$
    – GoingFTL
    Aug 18, 2018 at 6:23
  • $\begingroup$ @GoingFTL So you should rephrase your question: was that possible in medieval age? (technology level, cost-effectiveness...) $\endgroup$
    – Kepotx
    Aug 18, 2018 at 6:24
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    $\begingroup$ Another optical means, similar to the semaphore line, is the heliograph. You might particularly be interested in the property that the heliograph flashes can only be seen on relatively close to the line between sender and target. (Though they could still be intercepted, so secret codes were quite valuable.) $\endgroup$
    – Cadence
    Aug 18, 2018 at 6:52

8 Answers 8


Bull's Eye Lighthouses

Lighthouses have been in use since Antiquity.

The idea is relatively simple:

  • a stone tower, build at the top of a hill, guarantees that the light at the top is visible from afar,
  • a fire, at the top, is fueled by whatever is handy: wood, turf, etc...

On top of that, we apply the Bull's Eye Lantern principle: the light source is encased in an opaque (reflecting) material which only lets the light out in a particular direction, somewhat focusing the beam.

And then we put a movable panel in front of the light exit, allowing the operator to modulate the output.

That's it. Make a network of such lighthouses across the countryside, teach morse code to the operators, and you have messages propagating at the speed of light (minus delays incurred at each hop).


  • Requires good visibility conditions, heavy fog could be crippling.
  • Requires a lot of fuel.
  • Works best on moonless nights.

Your major problem is wind. Sound travels through the air, when the air is moving the velocity of the sound and the velocity of the air sum to get a new net velocity relative to the ground. This is a problem if your just shouting across a large field but is much more of a problem in your system, especially in crosswind.

In order for your system to work sound from one dish must hit the other but wind will likely blow that sound off course. In order to ensure you have some sound hitting the other dish you may need a dish whose length in ratio to the length of the distance across which the message is being sent is greater than the ratio of the wind speed to the speed of sound.

Doing some back of the envelope maths and using 8m/s as a good wind speed then we find that your dish length needs to be 2.3% the length of your route AT least. The longer the better if you want a strong signal in high wind. This means for a five mile route you have a dish about two hundred metres across this is hard but not impossible. Even then however a strong breeze is all it takes to render your relay system useless.

You might be able to solve this by moving your sound source or acoustic mirror, but that requires the sort of quick, reliable precision you get with lasers that would make for much more reliable communications technology.

You could put everything in a tunnel, but those sorts of tunnel systems would be incredibly hard to maintain. A more reliable system might be using acoustic mirrors as telescopes to listen in on extremely loud sounds, such as gongs, bells, organs or large firecrackers, similar to acoustic aircraft detection.


I am aware that this question asks about acoustic mirrors, but there is a far better alternative. Normal mirrors! you can make aqueduct style stone tubes that are a few centimeters wide, bury them between points, using mirrors to turn, and then flashing a light from a lantern down the tube will cause the light to be reflected off of the mirrors all the way to the other end. Use a Morse code-like system, and you have a light based telegram system.

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    $\begingroup$ You'll have trouble keeping your tubes straight (and keeping them from shifting over time), as well as with the fact that lantern light (or sunlight) will not be perfectly collimated like a modern laser beam, and so will disperse pretty rapidly unless you somehow e.g. silver the whole inside of the tubes, turning them into some kind of medieval optical fibers. But that's a pretty challenging thing to create and maintain. Also, depending on the tech level involved, ancient mirrors were a lot poorer than what we have today. $\endgroup$ Aug 18, 2018 at 10:12
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    $\begingroup$ [...] It would be much easier to dispense with the tubes and just have an open-air heliograph system. At night, you could still use an artificial light source, although it would have to be a pretty bright one unless you had a lot of relay stations close together (but then, the same is true of your tube system). It would be less effective in heavy rain or snow, but potentially still usable if the lights were bright enough. And of course you'd better encrypt any sensitive messages, because eavesdropping would be pretty easy. $\endgroup$ Aug 18, 2018 at 10:16

Such a system, assuming it was feasible (which isn't) would not be practical, let me explain you why.

First, the principle behind it is to have the speaker and the listener being positioned at the focal points of an ellipse, similar to the whispering gallery.

ellipse and property of its focal points

The properties of the ellipse make it that the acoustic waves emitted in one focal point will converge into the other. But this mean that, once you have built your elliptical mirrors, you are bound to a fixed distance. Not one meter more, not one meter less.

But first you need to be able to build two elliptical surfaces with narrow enough tolerances that the waves are properly focused, which is rather challenging.

You can bypass the problem by building two parabolic surfaces (you just need to be able to spin a liquid fast enough while it becomes solid), but that would be less efficient, as you would lose part of your signal.

parabolic profile and property of its focal point

But at this point you would need an extremely precise alignment to be able to capture the acoustic waves. In practical this could be, at best, a though task, since you have to compensate for the motion of the air in between the two surfaces. Any shear in the wind flow will surely destroy the synchronization between the various paths, destroying the communication channel.

Late addendum: apparently the approach with parabolic surfaces was used in UK around 1920, to detect airplanes crossing the Channel. They could hear an airplane from 25 miles, giving 15 minutes of warning. More info here (page is in Italian). Mind that, being just a receiver, they were less annoyed by wind: they just needed the airplane to be at the suitable location.

  • $\begingroup$ And it won't work at all on windy days. Wind carries the sound beam away, so it won't hit the receiving mirror. $\endgroup$
    – AlexP
    Aug 18, 2018 at 9:30

Others already said why acoustic mirrors are hard to use. My suggestion is a system of flags, not unlike the maritime flags:


The communication system would be composed of many flag stations, each one with 4 to 6 flag sets (for communication with several stations at the same time) and an equal number of telescopes (to see other stations' flags).

There would be flags for specific meanings, for letters, numbers and symbols, for start/end character, start/end word, start/end message, and signal incoming message.

The system would be slow to transmit, from the taking out and putting on several flags at once, for each character/word; but the bigger distances traveled (several kilometers with almost no delay) would make up for it.


I think the idea is brilliant, but without using magic as a means of facilitating it, you'd be hard-pressed to truly explain it without some system of underground tubes or wires - even only if left behind by a previous civilization.

In my own story, I've borrowed from HTTYD's tv series and created messenger dragons that work like a little like messenger pigeons only a little more intelligent. They can be told to seek, much like a scent dog. Or told to go somewhere specific. I can't say they're the same level of intelligence as humans, but there are some dogs that can be trained to read or count, or differentiate colors, to a certain extent. And my mom is always telling me: "I'm sure you'll figure it out, with a little time and some patience, towards yourself if nothing else."

I certainly always expect too much of myself and get frustrated when I don't live up to my own expectations. Me thinks it is a weakness in all of us writers.

PS: Although I still think the idea is plausible, I looked at the link you provided. It was not the 'mirror' I expected it to be, I guess I was thinking of the mirror on the wall type. :-) silly me. Still a neat concept though. I will be following this conversation for future input.

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    – Secespitus
    Aug 23, 2018 at 13:39

As others have noted, this is not going to be a very practical system, in terms of cost or reliability.

But if you are willing to live with these issues, then maybe go for something mildly interesting such as the whistle language of the Canary Islands to either boost range (requiring less reflectors, and therefore cheaper) or reliability of the overall communications system.


A possible solution can be the African drum language 1. It was probably the fastest means of communication before the invention of telegraph.


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