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B Can-and-string phone

  1. Aug 30, 2018 #1
    I haven't tried using cans and strings to hear someone's voice at the end.. does it sound like a person voice or more tin like?

    Any equations anywhere to compute for the frequency and propagation of the wave?
  2. jcsd
  3. Aug 30, 2018 #2
    You will not be surprised to hear that it sounds awful. The frequencies of vibration from the air couple into vibration of the bottom of the tin can. The tin can bottom has modes and frequencies it vibrates well with little damping. These frequencies couple strongly into the tin can. The tin can does not vibrate well at other frequencies and these frequencies do not readily couple into the tin can. These modes can be described by the elastic constants of the can and the boundary conditions which for the bottom of the can are that the circular edge cannot vibrate (much). Look up drum modes. For a tin can some higher frequencies tend to vibrate more readily. If you would like to know which frequencies, thump the bottom of a can. That high metallic ring is what your voice will sound like and is the very definition of tinny.

    The bottom of the can acting like a diaphragm picks up the sound from the air. The vibrating diaphragm then couples to the string or wire. The wire should be under tension. The string also has preferred modes determined by the length and tension. Look up modes on a string. These too selectively enhance some frequencies and suppress others so the voice is further distorted. The string also has dispersion. Different frequencies travel at different rates. The laser blaster sound from Star Wars was made by recording the sound of hitting a telephone pole guide wire with a screw driver. The high frequencies travel faster so you hear the high frequency echo before the low frequency. That’s what makes the distinctive downward chirp. Well the same thing happens on the tin can phone. High frequency travels faster, and that further distorts the voice smearing the consonants.
  4. Aug 30, 2018 #3

    Is there commercially available can phone I can try (best model).. what would be other words for "can phone" that I can google at ebay?
  5. Aug 30, 2018 #4


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    Don't people buy food in "tin cans" in your country? :eek:

    More pedantically, "steel cans plated with zinc (or whatever) to resist corrosion".

    Try a Google search for "tin can telephone" for many DIY examples. I've never made one myself, but they were a common "meme" (in modern language) when I was a kid (in the US) more than 50 years ago.
  6. Aug 30, 2018 #5
    This makes me laugh so hard!
  7. Aug 31, 2018 #6


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    I have used plastic cups, single use, too. They are very light weight and the base acts as a good diaphragm to pickup the air vibrations. (Well matched.)
  8. Aug 31, 2018 #7


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    My recollection of my can phone experiments is hazy because it was a long time ago. I do remember that string was almost useless, while a piece of thin copper wire worked much better.

    Forget the equations until you have built and tested one. This is a case where an experiment is worth a thousand hypotheses, opinions, and equations.
  9. Aug 31, 2018 #8


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    It's all a matter of matching the transducer to the transmission medium. (As always :smile:)
  10. Aug 31, 2018 #9

    If one uses a special alloy can (what is it?).. can it reproduce all frequencies and in combination with special string.. can it produce more natural voice?

    Does one have any recording of any tin can phone sound so we can hear what it sounds like?
  11. Aug 31, 2018 #10


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    Good grief MAKE ONE and play with it …. it's not rocket science !
    1) get 2 empty food tins
    2) a 15 metre piece of string
    3) punch/drill a hole in the centre of the end of each tin
    4) thread one end of string through that small hole in the tin and tie a knot on the string INSIDE the can
    5) do the same with the other tin
    6) give one tin to a friend
    7) the two of you move apart till string is tight and start talking/listening

    as jtbell said

    I made one when I was 6 yrs old and has fun with it with my friends

  12. Aug 31, 2018 #11
    tin can is avoided in my place so I have to order one at amazon.. and since i'll order it.. may as well get an alloy... is there no computer simulation or calculations of it so I can study it while awaiting the order
  13. Aug 31, 2018 #12
    The thing about the tin can is the shape. The bottom acts as a diaphragm that can be vibrated by the sound waves. Lots of other things with similar shape will work fine and maybe even better. Someone above mentioned plastic cups. The important thing is that it vibrates well, so give some can/cup like objects a thump and find something that sounds pretty live. Also string isnt the only choice. As Sophie suggested you want something that rings about the same as your can/cup. Try some different strings/wires and different tensions and see what works.
  14. Aug 31, 2018 #13
    I read in wikipedia that 3 miles were the range of some tin can phones.. that's very long.. how did they do it?

    "Their maximum range was very limited, but hundreds of technical innovations (resulting in about 300 patents) increased their range to approximately a half mile (800 m) or more under ideal conditions.[5] An example of one such company was Lemuel Mellett’s 'Pulsion Telephone Supply Company' of Massachusetts, which designed its version in 1888 and deployed it on railroad right-of-ways, purportedly with a range of 3 miles (4.8 km)."
    Last edited: Aug 31, 2018
  15. Sep 1, 2018 #14
    So I assume the 3 mile tin can acoustic phones described at wikipedia uses some sort of electricity to amplify the vibration of the tin can? Meaning the 3 mile wires have longitudinal movements to transmit the sound? Is this correct?
  16. Sep 2, 2018 #15


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    Why not order some canned food on Amazon? That way, you can at least enjoy the food, even if the experiment does not go well.
  17. Sep 2, 2018 #16


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    There are lots of things you could use instead of a tin can. Anything that's roughly cylindrical with one end sealed, or hemispherical, or conical, should do, as long as it's the right size to hold to your mouth and to your ear, and made from a material you can punch a hole through. Examples:
    • paper cup
    • disposable plastic cup
    • yoghurt pot, or other similar-shaped thin-plastic pot
    • plastic bottle cut in two
    • small cardboard box, with one end open
    • inner cardboard tube from a toilet roll, with card or paper stuck across one end
  18. Sep 2, 2018 #17
    i'm no longer inquiring about the simple tin can telephone but one commercial used in the 1900s that uses springs... the description I found says:

    "However, later in the 19th century, “mechanical phone” systems — essentially elaborate versions of “two cans and a string” were actually used in areas not served by Bell’s phone system (and which evaded any of Bell’s patents). Some were complete exchanges with operator switchboards, such as Lemuel Mellett’s “Pulsion Telephone” invented in 1888 for use on American railways.

    A newspaper article from 1889 reads as follows: “The Pulsion Telephone was a mechanical telephone which really seems as if it might rival the ordinary telephone, at any rate for moderate distances, has been recently exhibited on a 3 miles line, between Finchley-road and Hendon, on the Midland Railway [Editor’s Note: near London], under the name of the ‘Pulsion Telephone.’ It is the invention of Mr. Lemuel Mellett, of Boston, U.S. [Editor’s Note: Actually he was from Newton and then Somerville, Massachusetts.] The principle seems to be to have on the resounding-plate a number of small coiled springs held at one end only; these respond to various harmonic vibrations, and the vibration of the wire is taken up and reinforced, giving great distinctness of utterance. It seems not to matter much whether the wire is twisted or passes through loose earth. One peculiarity is that it can be tapped at any point by resting a hat upon the wire, a useful quality in case of accident, though evidently unfitting it for private messages.”

    I can't find illustration of it. Anyone got any picture of "the resounding-plate a number of small coiled springs held at one end only"... this is just for historical curiosity.
  19. Sep 4, 2018 #18


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    It may well sound awful, but 'telephones' made with string and plastic (sorry!) cups still sound good enough to astound middle/high school students.

    And you can kind of cross connect them - if you have two pairs of students, tying the two strings together in the middle means that when one speaks, the other three can all hear.
  20. Sep 4, 2018 #19


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    My experience is that I can shout further than getting the cans to convey a message....
  21. Sep 4, 2018 #20


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    Yeah, but then mum can hear it too...
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