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Homework Help: Highest frequency of sound waves in a thin aluminium rod

  1. Feb 17, 2015 #1
    1. The problem statement, all variables and given/known data
    Estimate the highest possible frequency (in Hertz) and the smallest possible wavelength, of a sound wave in aluminium due to the discrete atomic structure of this material. The mass density, Young's modulus, and atomic weight of aluminium are 2.7x103kg m-3, 6x1010 N m-2, and 27 respectively.

    2. Relevant equations
    Second partial of Ψ(x,t) WRT t = second partial of Ψ(x,t) WRT x multiplied by (Young's modulus / mass density)

    3. The attempt at a solution
    I've obtained the expression v = sqrt(Y/p), but v=frequency x wavelength
    so v = 4714.045 = frequency x wavelength.
    I understand that the minimum wavelength occurs at wavelength = the interatomic spacing between the aluminium atoms, but I am unsure as to how to obtain an expression for it's value.

    The maximum frequency occurs at the minimum wavelength so I can just algebraically sub in the min wavelength to find the max frequency.

    I'm assuming I have to use the atomic weight of aluminium somewhere in the question, but I can't seem to find the next step.

    I have a feeling I may have strayed in the wrong direction ; perhaps I could use some boundary conditions and solve the wave equation? I don't know if I'm oversimplifying the problem.

    Any help is massively appreciated.
  2. jcsd
  3. Feb 18, 2015 #2


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    Staff: Mentor


    You have 2700kg/m3 and every atom has a weight of 27 u. How many aluminium atoms do you expect per m3? What does that tell you about the distances between those atoms?
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