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Low tech telecommunications (sound)

  1. Apr 5, 2015 #1
    I'm a historian. I've struck on a curiosity about telecommunication technology in pre-modern times. I've read about a number of methods, and I'd like to ask some physics questions that got beyond my ken. This information is for my curiosity and creativity, not for any rigorous study, so I may be mentioning Wikipedia.

    Thank you for humoring me, and I hope I'm not asking too many questions :) I've now separated them into multiple threads by subject; this one ought to be about sound. To conform to PF custom, I'll be adding the other questions to threads after the first ones have been answered. Let anyone feel free to answer only those questions he prefers to.

    My main curiosity is about acoustic mirrors (big concrete, dishes) and echoes. I've read about sound mirrors on several sites, many of those concerning devices built between the World Wars (and right before radar) with the idea of amplifying the noise of incoming planes (to forewarn of bombers). Cf. Japanese war tubas, as well: huge tuba-looking things aimed at the sky and attached to stethoscopes for monitoring.

    Question 1. Is there any acoustic property of concrete which makes concrete better than other types of common stone? Some pre-modern cultures had sorts of concrete, but it'd still be a technological limitation since they all had big stones?
    Last edited: Apr 5, 2015
  2. jcsd
  3. Apr 5, 2015 #2


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    Welcome to PF!
  4. Apr 5, 2015 #3


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    Nah, as long as the parabolic reflector is rigid to sound (so it doesn't vibrate with it and introduce loss to the reflections), anything hard is fine.
  5. Apr 5, 2015 #4


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    Yeah, I think using concrete meant you could shape it into a parabolic and smooth its surface enough to improve the signal.
  6. Apr 5, 2015 #5
    That's very exciting to know, thanks! Onto question 2.

    This one is inspired by West African "talking drums," as reported by Wikipedia and in various reviews of a book "The Information." A Wiki talkpage discussion offered me figures on why that poster didn't believe a drum could ever carry very far. Multiple reviews suggest that the book furthers the claim that relays could send messages hundreds of miles (presumably without dozens and dozens of stops, but that is a presumption). I say that's the inspiration and wanted to give fair representation, but whether drums by themselves can carry a long way isn't what I mean to ask.

    Q2. Suppose you combined a loud instrument and big stone acoustic mirrors into a relay. To start, never mind drumming phrases, but let me just ask about alerting people with the sound of distant drums. Can we theorize how far a pair of big acoustic mirrors could communicate a drumbeat well enough for sentries to be sure they were hearing the signal?
  7. Apr 5, 2015 #6


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    For other posters benefit, I'm sharing my PM posts to Vir27:

    An article on how sound affected the outcome of some Civil War battles:


    Also there's a book called Blip, Ping and Buzz about various methods of detection.

    I remember it had a discussion on Britains methods of detecting German Air attacks using the acoustical mirrors and the chainholm system (precursor to modern radar).

    Last edited by a moderator: May 7, 2017
  8. Apr 5, 2015 #7


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    Interesting question. I remember an exhibit at the Exploratorium in San Francisco where there were 10' parabolic surfaces set up about 30 yards apart. There were chairs to put two people at the foci of the two parabolas facing away from each other. You could sit in those chairs and hold a quiet conversation, which was pretty amazing to me.

    So it does seem like you can get a lot of gain (40-60dB?) by a carefully-placed acoustic reflector antenna at the transmitter (TX) for sound communications -- not so sure at the RX end for long-distance comm...
  9. Apr 5, 2015 #8


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  10. Apr 5, 2015 #9


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    Don't forget the sound refraction problem which greatly limits its distance of travel.

    I imagine you could use a parabolic mirror to listen for the sound and then drum the message to the next station but I don't think it was ever done (someone would need to discover the parabolic mirror feature).
  11. Apr 5, 2015 #10

    Suggest the 200 ft. wall could "hear" 20-30 miles away. But there are several things I don't know:

    1. These walls had a microphone and I don't know how much difference the microphone was making to the range. Even if it were only equally useful with a human ear, they don't want to have to keep some guy sitting out there by himself around the clock--possibly projecting his own noises across the water ;) So I was hoping we had some way to figure out how the size of the wall related to the distance an instrument was receivable at another wall.

    2. Here are some other examples which might give us other data points for how the size of the mirror relates to the distance, but not all have enough data (like is their placement due to a maximum range or the convenience of that site within the parks) and some have water between them (which I think might amplify?).


    3. Would the big curvy wall model be able to transmit to a counterpart like the parabolic models do?

    I don't think the West African drummers used acoustic mirrors. To my knowledge, that's my idea, which is why I'm wondering how to figure out how far away it could work with what size stone acoustic mirror. To give you perspective on my curiosity, on other occasions I've spent a lot of time trying to work out things like exactly what technology is required before you can make a hot air balloon, although I found someone had followed this question through to practice: See http://juliannott.com/nazca/ .
  12. Apr 6, 2015 #11
    Ok, so I get one bump, here it goes. Feel free to add anything to the conversation above, but in case these questions might be easier to answer, I'll add them.

    Would the kind of noise-maker (drum, horn, screamer, whatever) make any difference to a project of relaying sound long distances between big acoustic mirrors?

    Would it work better in a tunnel?
  13. Apr 7, 2015 #12


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    lower frequency travels further
  14. Apr 7, 2015 #13
    Great. I've found tables for what sort of modern instruments (at least) hit what frequencies. Thanks.
  15. Apr 7, 2015 #14
    One of the earliest distance communication devices was a Greek idea of two water filled jars
    with coded messages at different levels. Torch patterns were used to tell the remote tower
    when to open and close the valve, the remaining level was the message.
  16. Apr 7, 2015 #15
    I think I mention that one in the light thread, but haven't been discussing it because it seemed to me its advantage is more data rather than more distance. Though if you wanted more data you could work out some other signal than a torch. But now you bring me to consider it further, the water serves mainly as encryption. You could communicate a coded number (corresponding to a part of the rod) by whatever means, and if the enemy had your codebook somehow they could perhaps intercept that. The water makes it so they'd need at least your approximate spigot size (and shape) to receive the sent number.

    That's interesting to consider because while I have been aware of the device, I hadn't sufficiently thought about its components. I imagine you might even use it in a sound-based scenario, being that while the sound doesn't transmit over a long distance practically infinitely, a Start and Stop signal should have about the same delay. In the event of an established relay where sentries have to run around and blow their horns or whatever into each of the next pair of acoustic mirrors--which I'm taking the impression can work as a relay over big spaces...?--your delays are probably regular enough that you know about what they are and can space the messages on your rod to limit misunderstanding.

    So thanks for bringing it back up.
  17. Apr 9, 2015 #16


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    I don't understand the use of the Greek water system at all. The containers are just simple devices to measure time, with different times corresponding to different messages. You need better than 10% accurary to be able to send 10 different messages, sending arbitrary (not pre-determined) messages is not possible at all and the system cannot be too fast as the timing is important.
    Where is the advantage over "raising the torch 5 times means signal X"? That is probably faster (you can easily raise and lower a torch in the time tolerance for opening and closing those valves and their precision of outflow), it is easy to add a code "raising the torch N times means I transmit a message symbol by symbol" for arbitrary messages, and counting is much less error-prone in general.
    If you get tired by moving torches, add a pendulum and raise the torch only at the end of a sequence ("okay, got raised again after 5 oscillations").
  18. Apr 9, 2015 #17
    Right, that's just what I was saying. You could communicate a coded number for your recipient to match to a rod by whatever means: a number of torch raises, in your example. As I see it, the water clock appears to increase the entropy in the system. This has the remotely conceivable advantage of either making your enemy think that your messages are too difficult to crack or at least putting one more obstacle between him and cracking it (e.g., an extra level of encryption). The far more definite cost of increasing the likelihood that something goes wrong with your message. The sources don't attribute any such encryption advantage to the system--by all appearances the idea was to make things clearer--and the sort of messages discussed are not so detailed that an enemy who somehow had your rod of possible messages could on his worst day narrow down which ones could be the important news. The disadvantage seems much more likely to obtain than does the advantage, unless the idea of your using it is entirely a bluff. The primary sources absolutely do not suggest they had bluffing in mind.

    Now that this discussion has prompted me to keep looking into the topic, I see that our primary source agrees with us. Polybius assesses the device himself immediately after his passage which Wikipedia cites, and he does not find it practicable. He recommends an alternative scheme he had improved upon himself involving using combinations of torches to communicate letter by letter. The author to whom Polybius attributes the invention was supposedly trying to improve on simple signal beacons, though his idea only survives through Polybius and one other source (citation "a" on p211 at right at LacusCurtius). The secondary source whom Wikipedia cites (ctrl+F 'Livius') makes the claim that Phillip II of Macedon used the water clock system, but I read his source and see nothing more specific than "signal fires" on mountains. I personally don't understand connecting that description with Polybius' specific description. According to the editor's comment in that latest passage the Romans appear not to have used the hydraulic telegraph, either. Not to make an argument from silence, but perhaps they had the best of reasons.

    All that said, it's still true that johnbbahm's contribution made me learn more about that subject than I otherwise would have. So thanks.
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