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Homework Help: Exercise involving acoustics physics engineering

  1. Dec 28, 2016 #1
    1. The problem statement, all variables and given/known data
    A disturbance signal propagates in a ventilation duct with cross-sectional area A=0,04m2. The fundamental frequency of the disturbance signal is 250Hz and its spectrum consists of odd harmonics only.

    Choose the appropriate filter options for rejecting this particular disturbance.
    Justify your choice (advantages)
    Choose the dimension of your filter in order to attenuate by 20 dB

    2. Relevant equations

    3. The attempt at a solution

    I hesitate between two approachs: using a expansion chamber with transmitted power formula
    = 4 / ( 4cos²(kl)+ (A1/A2 +A2/A1)²sin²(kl) ) where A area of the duct and A2 area of the chamber and l lenght of the chamber, and getting a transmitted value in order to attenuate 20dB, in the lectures the expansion chamber was not presented as a filter so maybe it is completely stupid thie approach

    the other idea is using a helmotz band stop resonnator but then it become possible to attenuate completely 250Hz by designing a helmotz resonnator with resonnance frequency equal to 250Hz, so i don't understand why it is asked to attenuate 20dB

    In both case i don't know what means "its spectrum consists of odd harmonics only" and how to use this.

    Happy end of year for everybody (sorry for my english i am not a native english speacker)
  2. jcsd
  3. Dec 29, 2016 #2


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    Gold Member

    Sorry I can't quite make the leap to acoustics just now, but if it were a problem in electrical engineering, involving a transmission line, we might use a stub of transmission line shunted across the main line. By choosing a suitable length, the fundamental and odd harmonics can be reduced greatly.
  4. Jan 5, 2017 #3
    It's okay i solved it by using expansion chamber, when it attenuates a frequency f0 (at maximum attenuation) it is possible to show that it attenuates the same each odd harmonics (2n+1)*f0, so the expansion chamber is the good filter
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