A question for rowuk

Discussion in 'Trumpet Discussion' started by Sofus, Jul 4, 2009.

  1. Sofus

    Sofus Forte User

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    Robin!

    After the thread "Hidden slot" another question has come to mind:

    We have learned that the mouthpiece end is the open end,
    and from the laws of standing waves in a tube this should
    mean that the wave pressure is low in a mouthpiece, since
    the wave pressure is low in an open end. (unless I´ve
    misunderstood, of course!).

    Some people, who usually know what they talk about, say
    otherwise, so my question is:

    * is wave pressure low as I expect in the mp end?
    * if not, then why not?


    The part of the mp that I´m thinking of is the part immidiately
    in front of the lips. The fact that the pressure will vary with
    the size of the cross-section is not what I´m after. Instead
    I´m just asking whether the mp follows the same laws as any
    open end would do regarding pressure maxima/minima.
     
    Last edited: Jul 4, 2009
  2. Bear

    Bear Forte User

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    Hmmm... Me thinks we need NickD on this one!
     
  3. Markie

    Markie Forte User

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    Hi sofus,
    I'm not rowuk but clarify if you would. Are you saying that a mouthpiece is an open ended device verses a close ended device?
     
  4. Sofus

    Sofus Forte User

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    Yes!

    Surprising, but this was clarified by Robin in the thread
    "Hidden slot", found further down in this forum.
    I also would have guessed that the bell end was an open
    end and the mp end was a closed end, but the days of
    surprises are not yet over! I have even seen the opposite
    being stated by some professor in an article on internet!

    :-)
     
    Last edited: Jul 4, 2009
  5. Pseudonym

    Pseudonym Pianissimo User

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    Even though this is pretty intriguing, what are you looking to gain by learning this?
     
  6. A.N.A. Mendez

    A.N.A. Mendez Utimate User

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    Sunny Ca.
    Yawn, .........Snore.........:-?
     
  7. Sofus

    Sofus Forte User

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    When it comes to the question of how beneficial it is to add mass
    to a mouthpiece, this has a lot to do with it, I should think.


    Not sure what you mean by this, A.N.A. Mendez.
    Would you care to explain?
     
    Last edited: Jul 4, 2009
  8. rowuk

    rowuk Moderator Staff Member

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    Sofus,
    actually with a gedeckt/gedackt pipe, the high pressure is at the open end.
    Let me try and explain the trumpet better.
    The first mistake that most people make is that they think that a trumpet is like a horn speaker, the flare just being for amplification. This is not true.

    We blow air through our lips which makes them open and close like a switch (not vibrate like a reed - this is why the whole aperature argument is nonsense), a standing wave is set up in the horn and the wave reflection in the instrument makes our chops more or less vibrate at the tuning frequency of the horn. Some of the energy "escapes" from the instrument due to the impedance at the bell not being infinite. This is what the audience hears.

    If we want to visualize that standing wave principle, we blow across the top of a beer bottle. A note will come out which is based on the length of a straight pipe or the volume of an irregular shaped object. The note is a standing wave in the bottle. The highest pressure is where we blow into the bottle.

    In the case of the trumpet, the sound that "escapes" IS amplified by the bell and the soundwave at the bell is low velocity, high impedance.

    If we didn't have that standing wave and the horn "just" amplified, the input would be high velocity, low impedance and at the bell low velocity, high impedance. Then the trumpet would sound dramatically different for each player as we would only hear the amplified lip-flapping instead of the leaky standing wave.

    If you want to try some of this stuff, glue a small speaker element (like from headphones) to a mouthpiece. Then run a sweeping sine wave and measure the output at the bell. It will sound NOTHING like a trumpet, but you will be able to see what the system does to that waveform and what resonances are present.
     
    Last edited: Jul 4, 2009
  9. Sofus

    Sofus Forte User

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    Great!

    As usual you have once again clarified things, Robin!

    Thank you!
     
  10. Bear

    Bear Forte User

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    Ok Rowuk, now I need some interpretation! HA. First off, what is the "aperature arguement"? Secondly, our lips do vibrate... we can't just blow into the horn and have it sound... or are you saying the reflection of the air/wave off the nodes/mpc cup/whatever cause that vibration? And if that's the case...

    lol,
    Tim
     

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