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How are nodes and antinodes added to harmonics of open air columns?

  1. Aug 3, 2012 #1
    Hi everyone!
    I was reading up on sound waves and I got to the point where this website was describing how instruments produce standing waves through the interference of different frequencies but then it started to talk about how the length of air columns can affect the wavelengths of the sound and therefore change the frequency and I've seen this through real life experiences but this website doesn't seem to provide a good explanation. All they say us that nodes and antinodes are added to the column and that affects the frequency but how does one add antinodes and nodes? From what I've interpreted through other things I've learned is that the wavelength would remain constant regardless of how long the air column is, right?
    Thanks for all your help!
    Here's a link to what I was reading: http://www.physicsclassroom.com/Class/sound/u11l5c.cfm
     
  2. jcsd
  3. Aug 3, 2012 #2
    Firstly your weblink does not say anything like your post.

    For instance they give a formula and plenty of figures to show that the wavelength (thus frequency) of the sound varies with pipe length.

    However you are right they do not make it clear how to produce the harmonics.

    The basic method relies on the fact that the energy is lowest with the fundamental and increases with each higher harmonic.

    So if you only input enough energy to excite the fundamental, that is all you will get.

    Blow harder, as any musician knows, and you will get some harmonics. Blow too hard and you will get mainly harmonics.
     
  4. Aug 4, 2012 #3

    CWatters

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    I've not looked at this before but your link suggests that antinodes form at the open ends of a pipe. So I imagine that antinodes can be "added" by opening holes in the side of a pipe (eg like a flute, recorder).
     
  5. Aug 4, 2012 #4
    Good point!

    :biggrin:
     
  6. Aug 4, 2012 #5

    CWatters

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    Actually I think I was wrong :-(

    I think the holes on a flute change the effective length so that's not quite what you were asking.

    If you want to make a pipe of fixed length vibrate at a harmonic I think you have to change the "exciter frequency" which would be the lips for a trumpet player..

     
    Last edited by a moderator: Sep 25, 2014
  7. Aug 4, 2012 #6
    I realise that this changes the effective length, but it's still a good point.
     
  8. Aug 5, 2012 #7
    In many woodwind instruments there are holes for changing the effective lenth (changing the position of the final antinode) and one hole (sometimes two or three) for "adding an internal antinode". These holes are called register holes and on more complicated instruments they are opened by caps operated by register keys.
    A second register considerably extends the range of the instruments, and playing in the higher register(s) is easier (or made possible at all) by using register holes than by relying on overblowing alone.
    In a recorder, the "hole 0" for the left thumb will even serve both purposes. (depending on the open/close state of the other holes)
    http://en.wikipedia.org/wiki/Register_key
     
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