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How much effect?

  1. May 20, 2008 #1
    Hi all,

    I am new to this message board and respectfully ask a question that has been on my mind for years. I have very little knowledge of physics.

    My question deals with I suppose, reactions between atoms. If one atom moves, over what distance does it affect other atoms?

    For example, if several good microphones were set up in different spots, they would record the sound of someone snapping their fingers. This means that the snapping of the fingers moves atoms in the air in all directions. I can hear sounds of cars on the freeway about half a mile from my home during the night when things are more quiet.

    I'm assuming that if you had very sensitive measuring devices, it would be possible measure the movement of atoms of someone snapping fingers a half mile away? Is this correct or does the movement of atoms somehow get absorbed after a certain distance?

    I am asking this question because I want to know how much of an effect the movement of one atom has over what kinds of distance.

    I appreciate any help,
    thank you,
    B
     
  2. jcsd
  3. May 21, 2008 #2
    I think the movements would get damped by random collisions with other particles..The noise from vehicles is loud enough for you to be able to hear it even after half a mile of damping..but the sound of a finger snap wouldn't be measurable all that long.

    The thing is that air molecules already are moving with some random speed. If the motion of particles because of finger snap becomes so low that the vibration due to that is smaller than the normal movement of air particles, then no matter how sensitive your instrument is, you can not distinguish motion due to finger snap from the normal movement of air particles.
     
  4. May 21, 2008 #3
    Raze2dust,

    Thank you very much for the response.
     
  5. May 21, 2008 #4

    Mapes

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    To expand on Raze2dust's answer a bit, there are some tricks you can use to detect sounds whose signal-to-noise ratio are very poor--sounds that would be undetectable by looking at an output signal vs. time from a single microphone. One is to set up multiple microphones and combine the signals, making sure to time-correct the signals to account for the different distances between microphone and target. (If the distances are unknown, different correction times could be tried to see if a signal emerges.)

    Another trick, useful if the sound source is periodic like well-timed snapping fingers, is to look at the signal in the frequency domain rather than in the time domain. That is, signal strength is plotted vs. frequency rather than time. This kind of chart gives better, cleaner results the longer you feed in a signal. The benefit here is that a periodic signal stands out as a spike at a certain frequency even though any single pulse would be essentially undetectable due to noise.
     
  6. May 21, 2008 #5
    Hello Mapes,

    Also to you thank you for the response. I do get your point about looking for things like a frequency versus a single pulse.

    I do find it amazing that I can hear the sound of cars a half mile away (even though it would seem pretty common) because it means that all of the air molecules must have necessarily moved so that I could hear it with my ears.

    By how you answered, am I correct in thinking that you're saying that although the sound of regularly snapping fingers would not be easy to detect, that it would be possible that air molecules in my ear would move due to the snapping of fingers half a mile away?

    Thanks,
    B
     
  7. May 21, 2008 #6

    Mapes

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    It is certainly predicted by our current knowledge of mechanics.

    The range of sound (compressive wave power) that our hearing can detect is truly amazing, as is the range of photon intensity that our vision can evaluate. It's just too bad that we can't feel, for example, a range of temperatures from 1 K to 10,000 K...
     
  8. May 22, 2008 #7
    That would be truly amazing. Mapes, you are saying that there is no doubt that the finger snapping does move molecules/atoms that are a very far distance away from the snapping fingers right?

    I just want to get it a little more accurate in my mind because it is so amazing to me that "small" things like fingers snapping can move atoms that are a very far distance away. I find it mind boggling. And yes, I wasn't really even thinking of photons and how sensitive that must be!

    Thanks,
    B
     
  9. May 22, 2008 #8

    Mapes

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    Well, let's go into a little more detail. Classical mechanics says that a finger snap creates an initially spherical compression wave transmitted through atomic collisions that extends throughout the existing air and other media. As the compression wave expands, its intensity at any point decreases because its finite area is spread over a larger and larger volume. Eventually the magnitude of induced atomic motion is negligible compared to random thermal atomic motion, which is also transmitted through atomic collisions. At this point the sound is essentially undetectable, but not zero.

    Quantum mechanics is a more complex, but more accurate theory. A fundamental idea in QM is the indeterminacy principle, which says that you cannot know the exact position of an object, even in theory. According to QM, a sound wave that is sufficiently attenuated cannot be detected even in theory, even ignoring noise, because the induced atomic motion is simply too minuscule.

    So I shouldn't have implied earlier that a detectable sound can travel arbitrarily far. Hopefully this makes things clearer and opens up more topics to think about.
     
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