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I Mean free path and acoustics

  1. Aug 8, 2017 #1
    Hello everyone,

    I have a question regarding the implication of mean free path and pressure acoustics.

    I have seen several publications on the internet and also calculated the minimum displacement of a wave in air using the formula

    dp=v * rho * *2*pi*freq. * dx

    The values of displacement can be as low as Angstrom or even lower.

    A mosquito 3 meters away could create a 10nm displacement of air which is above the 20uPa hearing threshold for humans. doi: 10.1098/rspb.2000.1021

    The mean free path in air is around 60nm.

    The question is how come the continuity assumptions of classical acoustics are preserved here?
    In other words, I don't understand how we could talk about a wave if we don't have a continuity in the influence of the air molecules on each other.

    Thanks in advance for your help

    Best regards

    Last edited: Aug 8, 2017
  2. jcsd
  3. Aug 8, 2017 #2


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    2017 Award

    Staff: Mentor

    It is an average displacement. The individual displacements will vary by at least these 60 nm, but averaged over something like 1020 atoms a mean displacement of 10 nm is well-defined.
  4. Aug 9, 2017 #3
    thank you very much for your answer,

    do you know of any book/article where the theoretical limit of movement for creating an acoustic wave is calculated/shown (considering the pressure/temperature as you have mentioned)?

    thank you again for your help
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