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Sound and air movement: do they affect each other?

  1. Feb 6, 2012 #1
    Hello Forum.

    I have two basic sound questions:

    I know sound (large wavelength) diffracts a lot when it exits an aperture. But if someone is speaking and he/she is not facing us we still hear them, even if we are in an empty field.
    Does sound diffract so much that it ends up even behind us?

    2. If we created a flow of compressed air in front of a speaker playing some music, would the receiving microphone simply detect the noise from the compressed air or would the compressed air disrupt the air molecule behavior (sound coming out of the speakers) and change it?
    After all, sound is simply compressions and rarefactions of air density, i.e. molecules that bump into each other...
    If we push some air in front of the microphone we should be able to randomize the air molecules motion and not here the music.....

  2. jcsd
  3. Feb 6, 2012 #2
    Sound can get carried by air movement.

    You can hear this on a windy day if you walk around someone speaking.

    I'm not sure that all the sound you hear behind someone speaking in still air is the result of diffraction alone. The human voice is not a single point source.

    Flowing air has a lower pressure than still air. So if you blow air at or across the speaker it will experience a lower pressure than if you didn't.
  4. Feb 6, 2012 #3
    Well, when someone speaks the aperture is his/her mouth. It would seem that sound would move forward, in a hemispherical region of space....but not be present in region behind the the person speaking.....
    Reflections have an effect, but if we are in the middle of a football field we still hear a person's voice even if they are not facing us...How do air molecules get to vibrate behind the person speaking?
  5. Feb 6, 2012 #4
    If you look at the acoustic radiation pattern of many chamber type sources you will see the characteristic 'doublet pattern' (figure of eight).

    Don't forget that the back of the head/throat/chest is not a very good acoustic barrier.
  6. Feb 6, 2012 #5


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    Sound emitted by a point source (not exactly someone's mouth, but similiar) the sound energy spreads shperically...that is, in all directions.
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