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Nodes in standing waves

  1. Jun 14, 2009 #1
    1. The problem statement, all variables and given/known data
    I don't have an actual problem to solve, it's more of a conceptual problem.

    I'm trying to understand harmonics and standing waves, and how the nodes work out, but it's very confusing.

    (1) If a wave is FIXED at both ends, for the fundamental or 1st harmonic, does it have 0 nodes, 1 node, or 2 nodes? Do you count both "end nodes" where the wave is fixed?

    (2) If a wave is FIXED at one end, and OPEN at the other end, I know that the equation for frequency is f(n) = n*v / 4L, where n=1,3,5,7...
    Does "n" represent the # nodes, or the harmonic level?
    This is super confusing for me, because for f(1), there is 1 node at the fixed end, which makes sense, because this is the first harmonic.
    However, for f(2), there are 2 nodes, but "n" can only equal odd numbers. So there is no second harmonic... Why is that? So to find values of n, we have to go by harmonic number, and not the number of nodes, for all situations (ie. open, closed, open-closed waves)?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 14, 2009 #2

    diazona

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    Homework Helper

    Yes you do.

    Think of it this way: n represents the number of half-wavelengths that fit into the length L. A wave that consists of an odd number of half-wavelengths has a node at one end and an antinode at the other end, which fits perfectly into a pipe that's open at one end and closed at the other. Alternatively, for a wave that has an even number of half-wavelengths, there are two possibilities: either it has a node at both ends, which fits a pipe that's closed at both ends, or it has an antinode at both ends, which fits a pipe that's open at both ends. That's why for pipes that are open at both ends or closed at both ends, n only takes even values.
     
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