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Resonating frequencies of tubes

  1. Jan 9, 2015 #1
    Greetings, didn't know where to post this. To which category it fits, if even any.
    Anyway, I'm studying sound production and am doing a work on resonating frequencies of a certain tube.
    Please bare in mind I'm not a physician or in the studies of physics, just a musician trying to understand resonance in acoustics. Please move thread if in wrong part of forum

    I've done a test on how to calculate the actual resonating frequency of a certain tube with OPEN ends.
    I'm using a brass tube of 200mm with a diameter of 0.7 cm.
    In the calculation I came to the conclution of 847 Hz, but the actual frequency that's resonating while hitting the tube is 1687 Hz. I hit the tube while holding approximately 25 % of the tube to hold it on a node. So I don't disturb the resonance. It's the same frequency that's heard when dropping the tube on the ground without holding it. So it shouldn't be that I'm manipulating the frequency response by holding it on 25%. The deviation from 843.5 is small but I think what I'm hearing is the second harmonic (first overtone) and not the fundamental. But why does not the fundamental resonate? In all literature I read it says that it should be the fundamental frequency that is heard.

    So the fundamental frequency should be 843.5, why am I then only hearing 1687 while making it resonate?
    f1 =843.5 =fundamental frequency = first harmonic
    f2 =1687 =first overtone = second harmonic
    f3 =2530.5 =second overtone = third harmonic
    and so forth.
    What I then began to think was that when hitting the tube the "first overtone" is the one that's heard it must be that, but I can not explain why. When the calculus tells me the fundamental is 843.5 and the literature says it's the frequency that should be heard.

    I'd be glad if someone has a answer for this.
     
    Last edited: Jan 9, 2015
  2. jcsd
  3. Jan 9, 2015 #2

    mfb

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    The motion of air in the tube won't change the tube itself so much, so the position where you hold it should not matter much. At 1/4 the length you do not have a node for the fundamental frequency.
    Probably.

    How do you start the oscillation? What hits the tube where?
    How do you measure the frequency?

    The brass could have its own resonance frequencies, but it is unlikely that they are so close to an resonance frequency of the air inside.
     
  4. Jan 9, 2015 #3

    Quantum Defect

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    How are you calculating the allowed modes? Are you modeling the vibration of the air column (like a flute or organ pipe) or the brass tube (like chimes) ? I don't think it is the former, since the value for f1 is way too low.

    If the latter (which I suspect), you should be modeling the vibrations of a "bar" with one end clamped. For these vibrations, you will not get a harmonic series, like you would for a string clamped at both ends.

    Something is odd with your numbers...
     
  5. Jan 10, 2015 #4

    CWatters

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    How long is the tube?
     
  6. Jan 10, 2015 #5

    mfb

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    200mm according to the first post.

    The calculated resonance frequencies look fine for air, but if the brass tube itself oscillates we will get completely different frequencies.
     
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