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Why can't we whistle in a closed space?

  1. Aug 20, 2013 #1
    Try whistling in a small cup or with your palms around your mouth (with some space inside, but try to cover the holes between your fingers). It seems much harder, if not impossible.

    What could be the reason? If you try talking, it will sound muffled but at least you can produce and hear the sound. But the whistling doesn't even start.

    My guess is that this is related to air pressure, but I'm not really sure.
     
  2. jcsd
  3. Aug 20, 2013 #2

    jfizzix

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    To whistle, you have to create standing sound waves in your mouth, with a constant flow of air from your lungs (or from outside since some people can whistle while breathing in too). The same principle applies to flutes and other tube-like musical instruments. Anything that interrupts the flow of air, prevents you from setting up standing waves so you won't be able to sound a note.

    It would be interesting to see if playing a flute in a similarly closed environment would be similarly difficult. I would expect so, but experiments are where the truth lies.

    Hope this helps:)
     
  4. Aug 20, 2013 #3
    This seems to make sense. Thank you for your answer!

    Just wondering, in this case, why does our voice still work? Also, can't standing waves happen even in closed spaces, provided the nodes get exactly at the boundary of the space?
     
  5. Aug 20, 2013 #4

    jfizzix

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    To answer these questions, you would have to consider where the standing wave is being set up. Your vocal chords are deep in your throat so there is enough space in front of them for the air to be flowing long enough to set up a standing wave.

    when you whistle with your hands almost over your mouth, there's not enough space for the air to be flowing long enough to set up a standing wave.
     
  6. Aug 20, 2013 #5
    When you talk, the sound is created in your throat and merely propagates from your mouth into the cup. For the whistling sound, you need a specific air volume (your mouth) to create a resonance frequency. It's like a flute that resonates at a certain frequency depending on the internal volume of the flute, which you can alter by opening and closing holes. Your open mouth is simply an open boundary (hole) where pressure waves escape your mouth. When you place a cup near your mouth opening, then the volume of the cup is changing the resonance frequency of the mouth volume.
    Come to think of it, you probably can't play a flute or recorder when you stick it inside a big bottle or something.
     
  7. Aug 20, 2013 #6
    Alright, this explains everything. Thanks again for these explanations!
     
  8. Aug 20, 2013 #7
    I can whistle into a cup as long as the cup does not form a tight seal around the mouth.
    To whistle one has to blow air through the lips and as long as air can escape, whistling is possible into a cup. With a cup sealed against the skin, the buildup of pressure prevents the blowing through the lips part.

    One can talk even if a cup is sealed against the skin. To talk air from the lungs has to flow over the vocal cords. Exhaling through the mouth is impossible with the cup sealed against the skin. Instead the air takes a different path throught the nasal sinuses and exits through the nose. The mouth should be open so the resonance of the sinuses and mouth can produce words, Sounds that are produced by the lips such as that from the letters "p" and "b" are just about impossible to make.
     
  9. Aug 22, 2013 #8
    A flute works because the flute imposes some boundary conditions on the pressure waves that can exist in the tube. The open end is a node in the pressure wave and an antinode in the displacement wave. This is because the outside air quickly moves to cancel out the pressure wave at the end, and you can't generate enough airflow to meaningfully change the pressure of the outside air.

    Your mouth is similar. The air outside your mouth is a node in the pressure wave. If you put something outside your mouth, it changes the boundary conditions, and it is no longer a node, since you can change the pressure in the cup.
     
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