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Back to basics

  1. Oct 4, 2006 #1

    Office_Shredder

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    I'm interested in making a tin can telephone kind of thing. A quick google search reveals standard materials will only give me about 15 feet max of range (which I had anticipated from the start). My crazy idea is to make something about 40 or 50 feet long that works, so it looks like I'll need to be innovative. The way I'm figuring it for now is to get the strongest fishing line I can find (lots of tension, and no internal friction), and experiment with that. I really have no idea what would work for the tin-can replacements (one website said paper cups, but those wouldn't be able to handle the tension in the fishing line).

    So if anybody has any ideas on how to increase efficiency, or maybe a design trick to give me some extra length, I'd love some advice. Thanks
     
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  3. Oct 4, 2006 #2

    berkeman

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    Maybe try making some sound-powered microphone --> headphone links, like they used to use on ships.
     
  4. Oct 4, 2006 #3

    Office_Shredder

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    I'm not quite sure what you're talking about, is there a picture of this on google or something?

    EDIT: Oh, the speaking tube kind of thing? I don't think that's a good idea.... first, I don't have 40 feet of pipe, second, I kind of don't want this to be overly conspicuous. Third, I don't have forty feet of construction space. Essentially, I want to string this up between dorm rooms on opposite side of a lawn.
     
    Last edited: Oct 4, 2006
  5. Oct 4, 2006 #4
    Go get the fishing line - it would be very surprising if you couldn't manage to get this to work. Afterwards, you might want to try a simple laser link (have the person with a stereo playing stick a mirror shard to their window, bounce a laser pointer via that onto a calculator's solar cell, and wire the cell straight to an earphone). Of course, sound quality likely won't get as good as if you could just send it over the college ethernet..
     
    Last edited: Oct 4, 2006
  6. Oct 4, 2006 #5

    Office_Shredder

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    Well, after this, I want to connect multiple cans on a sort of network. The way it would work is instead of a single hole punched with the wire threaded through and knotted, I would punch two holes and loop the wire through, allowing it to continue on to another can
     
  7. Oct 4, 2006 #6

    Danger

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    Although I have no scientific explanation for this, I would recommend staying with single-strand steel wire for your set. I think (intuitively) that it will give a better signal than plastic (which most fishline is). This is based solely upon the fact that steel wire doesn't have the flexibility of plastic and should therefore transmit a more prestine signal.
     
  8. Oct 4, 2006 #7

    berkeman

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    Okay, okay. If you have your heart set on using physical wire (Danger is correct), then maybe you can boost the efficiency by converting the sound energy (longitudinal waves) into transverse waves to send down the taut wire, and then convert the transverse waves back into logitudinal waves at the headphones on the other end. One of the biggest inefficiencies of the cans and string method is that you are trying to propagate longitudinal sound energy directly down the string. String/wire is best at propagating transverse signals.

    http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/sound/u11l1b.html


    EDIT -- changed "speaker" to "headphones" in the spirit of efficiency
     
  9. Oct 4, 2006 #8

    Integral

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    What do you mean "used to use" are you saying they have gotten away from sound powered phones!

    A sound powered phone used the current generated by a simple microphone to drive a headset. Generally they had a few (~5) sets on a line. So you could hear everyone on the loop. We all talked to somebody setting in a central location. These systems were completely self powered so as long as your line was intact you could communicate. The the ship could be dead in the water without any power and the sound powered phones would still work. Sound quality was not so good, but it was about communication not fidelity.

    I have been out of the Navy for over 30yrs so perhaps they have a new system now, but the old sound powered phones were simple and they worked I would be very surprised to learn that they have been abandoned.
     
  10. Oct 4, 2006 #9

    Danger

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    Hey, Integral;
    While I have no faith whatsoever in the US military (or any other aspect of your government), this would seem to be something that even they couldn't screw up. Anything that doesn't rely upon external power is best.
     
  11. Oct 5, 2006 #10

    Office_Shredder

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    Is there a simple way to do this? Obviously that would be great, but I can't think of anything easy.

    While the can-string method isn't essential, it's the bes thing I can think of that fits what I'm trying to do.... essentially, I want to be able to let two people in different buildings across a lawn speak to each other. At the same time, there can't be a glaringly obvious physical connection between the two windows. While I liked the laser idea, I don't have a way of trying that out at the moment (maybe when I get back for break I can do it with my dad, he'd love this kind of project).

    danger, what kind of shop (if any) would have steel wire?
     
  12. Oct 5, 2006 #11

    Danger

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    Pretty much any decent hardware store or automotive supplier should have it. If you get the stuff made for hanging paintings, it's pretty thin but still strong. Now that I know what you're up to, though, I'd suggest getting a wireless intercom. Some of them can be had for about $20.
     
  13. Oct 5, 2006 #12

    DaveC426913

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    Wouldn't that sort of spoil the fun?
     
  14. Oct 5, 2006 #13

    DaveC426913

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    Could this be done simply by turning both cans 90 degrees to the wire?

    That way, the vibration of the transmitting can would be transferred into transverse waves in the wire.

    Seems too easy.
     
  15. Oct 5, 2006 #14

    Danger

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    Okay... now I have 4 jobs. Add 'party-pooper' to the resume. :tongue:
     
  16. Oct 5, 2006 #15

    berkeman

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    Hey, that's not a bad idea. Another improvement on that would be to pick a center frequency for the audio that you want to transmit (say, 500Hz?), and adjust the tension in the wire and the placement of the cans in from the ends of the wire so that the base of the can can drive an antinode at that frequency in the transverse direction. So you need to support the ends of the wire with some other mechanism, and the cans are placed 1/4 wave in from the ends. Interesting!
     
  17. Oct 5, 2006 #16

    Office_Shredder

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    I also thought of this after posting, but there could be a problem. Since the wire will have to lay across the bottom of the can for this to work, the can may actually simply muffle the waves created on the wire.

    My other idea was to connect the wire to a third object (something immovable), and have a piece of metal placed at a 45 degree angle to the wire, so that the metal connects to the can. I'll have to try both of them out to see what works. Unfortunately, there's no organized recycling in Oxford, so I'm having trouble finding cans.

    And for those of you suggesting I just go buy a headset thing, that's really not why I'm doing this :tongue2:
     
  18. Oct 6, 2006 #17

    DaveC426913

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    The can only has to have one point of contact with the wire. If you hold the can at 89 degrees, so the wire only contacts at the centre, it should be fine.
     
  19. Oct 6, 2006 #18

    Office_Shredder

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    I had a better idea..... just put the hole on the edge.

    I got an assortment of plastic cups to experiment with (including one designed for champagne, with a thick bottom that I could run a wire through parallel to the bottom of. It seems like a promising design, but I don't know if I can get it in a metal variety easily)
     
  20. Oct 7, 2006 #19

    DaveC426913

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    No. The whole point of the tin can arrangement is that the vibrational deflection is greatest in the centre of the bottom. That's where you want the wire to be attached.
     
  21. Aug 27, 2007 #20
    Sound powered phones are still in use today in the military and are much more effective than a tin can and string. The principle of operation is listed below:

    Sound powered telephone communication technology uses electro-mechanical transducers which enable audio communication over a single wire pair without the use of external power or batteries. The sound pressure created from a user's voice when talking into the transducer is all that is required to power the system.

    There are several types of terminating equipment for the system from telephone sets to headsets.

    The company Dynalec Corporation (http://www.dynalec.com/SoundPowered.html) sells all required gear to set up a sound powered system. These systems are used not only by the military but at many industrial sites as well as mine sites.

    If someone was trying to set this up by putting the components together themselves you would still have to get the electro-mechanical transducers to convert the voice waves into current.

    Wires for such a system would not have to be kept taunt, and could be as small as telephone cross connect.

    The other benefit of a system like this is the ability to connect several phones in parallel and all users could communicate to all others in the chain.

    The problem of the tin can and string method is that the principle of wave propagation (vibration) along the string. The longer the string, the more loss of propagation and more outside enviromental effects (wind, noise, ect..) can degrade your signal. If this is more of a "see if I could do it", you would need to shield your string from outside influences (ie string in a vacuum tube), but you already said your trying to remain inconpicuous.

    Good Luck
     
    Last edited: Aug 27, 2007
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