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Homework Help: Sound wave in a tube of air with holes (lab)

  1. Nov 26, 2011 #1


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    1. The problem statement, all variables and given/known data
    I had a Physics lab with the following setting:
    A signal generator is connected to a speaker which is located on one end of a tube. This creates the sound wave. The tube is open on both ends.
    A micro is movable along the tube and it is connected to an occsilator which displays the sinusoidal wave. With the micro I could find the nodes (no sound) of a standing wave.
    My length of the tube is L=0.9m
    Now we are in mode n=2 where I have my node in the middle at about 0.45m at a frequency of about 360Hz. I can calculate the wavelength which is 2L/n = 0.9m and the speed of sound in air v = wavelength * frequency. So far I understand.
    The tube has 3 holes, one at the center and the other two in the middle of the center and the ends. So the holes divide the tube in 4 parts.

    Now I have to open the hole in the center with the result that the sound is louder (amplitude goes up).
    I close the center hole and open the other 2 holes. The sound becomes softer and the frequency it throws off the frequency.

    My question why is that? Why does it become louder when I open a hole at a node and why does the sound becomes softer when I open the holw which is not at a node?

    3. The attempt at a solution
    I tried to understand what happens when I open a hole of the tube. I thought I would shorten the length L of the tube and would receive a new fundamental harmonic which would give me a higher frequency and therefore a higher pitch. I am not sure why the amplitude goes up. and I totally do not understand why there is a difference in opening a hole where there is a node located or not.

    Thanks for any hint!
  2. jcsd
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