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Why does particle displacement A increase sound pressure?

  1. Aug 28, 2010 #1

    Surely, when loudness-intensity-pressure increases, so does the maximum displacement A of the speaker.

    Wrong thread title. My question is: Why increasing the maximum molecule displacement A increases sound pressure.

    They also say that the A of the speaker is equal to the maximum displacement "A" of the molecule. Why that?
    Last edited: Aug 28, 2010
  2. jcsd
  3. Aug 28, 2010 #2
    Re: Why does particle dispacement A increase sound pressure?

    Because the situation is analogous to a person pushing on a box. The displacement of the person is equal to the displacement of the box, isn't it?
  4. Aug 28, 2010 #3
    That's not an answer. I can understand the assumption that it is so for the molecule that is "always in contact" with the speaker (assuming something per average etc as it moves before the speker starts moving). But why should this apply for all other molecules too?
    Last edited: Aug 28, 2010
  5. Aug 29, 2010 #4
    Because each molecule acts as a speaker for the next one?...

    You could say that it is an assumption. The assumption then allows for a model that is sufficiently close to reality, where each molecule has an average position.

    You may also want to read about the model for sound waves in solids which is closely related: http://en.wikipedia.org/wiki/Phonon
  6. Aug 29, 2010 #5
    What about my first question: (assuming that the A of the speaker is equal to the A of each molecule):Why increasing the maximum molecule displacement A increases sound pressure?

    Is it because when the A of the molecule increases then the maximum speed
    umax=(√(k/m))A=ωΑ=2πfA and all other speeds also increase? And if indeed so, then that's the only reason?
  7. Aug 29, 2010 #6
    Ok, the basic model is this. Picture a row of molecules end to end. Each one has an average position that is fixed and it is allowed to oscillate left and right about that position. The displacement is called y (which may be - or +). The MAXIMUM displacement is A (when y = A or y = -A). This (+ a few other conditions) is called the harmonic oscillator model and it is used all throughout physics for all kinds of waves because it allows relatively simple calculations with sines and cosines and gives results that are often very close to reality. The assumption is thus a very classical and fundamental one. You get a lot of physics for relatively little math.

    Now that slide on pressure. V is volume and S is the area of a plane parallel to the speaker. B is the bulk modulus of the medium (air). The math shown is then pretty straightforward. The end result pmax=BkA simply shows that if A increases, so must pmax, linearly. You just can't argue with that math.
  8. Aug 30, 2010 #7
    I am not arguing with the math. I am asking what they are claiming. If you do not know the answer do not answer me that "there is an answer".

    B is constant, so does k, right and the function of sin can only take values between 1 and -1, right? So the Pmax and (?) the Σ|p(t)|dt of a period or half a period, is dependent on A only. And I am asking why that. One answer is that the max speed of each molecule increases as A increases, and its average speed doubles when A doubles because the period T remains the same (see also the simulation http://www.ngsir.netfirms.com/englishhtm/Lwave.htm [Broken])
    So its seems that the reason is the increase of speed of the molecules (note that all molecules in a wavefront are moving towards the same direction as the wavefront). So I am asking whether I concluded correctly and whether the increase of their speed is the only reason. And because I get such answers from physists, now I changed my mind and I AM "arguing with the math". The doubling of the speed just because the A of the speker doubled a little, means that there is a considerable increase at temperature, which is false or at least it seems to contradict that "the speed of sound in air is 343 for room temperature no matter what the speaker does". So what's going on: What have I grasped wrong or what is wrong.
    Last edited by a moderator: May 4, 2017
  9. Aug 30, 2010 #8
    Not saying you are, I just didn't see what you wanted more.


    Pmax depends on BkA, it's the only I can say it, and all three constants have equal ponderation.

    I say it's because of the math. You're looking for something else.

    The RMS speed of the molecules does increase, and that would increase the pressure, but I think that this is beyond the scope, and besides the point, of the talk.

    This was unexpected. If you were offended, it was not my intention.

    Personnally, I would not take the temperature into account, because IMO, for each area of increased pressure, there is an area of decreased pressure as well, so that if you were to calculate T(x), the average temperature along x would be the same whether or not there was a wave.

    I thank you sir for giving me the opportunity to review sound, and I hope I was at least a little helpful.
    Last edited by a moderator: May 4, 2017
  10. Sep 6, 2010 #9
    Excuse me?
  11. Sep 6, 2010 #10
    Last edited by a moderator: May 4, 2017
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