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Homework Help: Blowing across a coke bottle (sound physics)

  1. Apr 25, 2006 #1
    Here is the question I have:
    When you blow across the opening of a bottle, how will the sound vary with taller and shorter bottles? With fatter and thinner bottles? How will the shape of the bottle opening affect the sound?

    Ok, now I am not totally clueless on this subject matter, let me tell you what I have. The flow of air in the tall bottles has to travel further, producing a larger air column, which causes the air to vibrate slower and thun produce a lower pitch. The opposite happens with a short bottle.
    For fatter and thinner bottles, would it be the same relation (fatter = slower vibrations)?
  2. jcsd
  3. Apr 25, 2006 #2

    Andrew Mason

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    Not bad for a start. Look up: RESONANCE and STANDING WAVE.

  4. Apr 25, 2006 #3


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    This works with Pepsi bottles too.
  5. Apr 25, 2006 #4
    Ok, I remember standing waves being formed when two equal waves travel in opposite directions and combine (superposition). And resonance is when the amplitude reaches a maximum when the frequency of a driving force equals the frequency of the system.
    I cant seem to understand how to apply these principles to the question?
  6. Apr 26, 2006 #5


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    Blowing across the top of the bottle will create a region of low pressure at its entrance - a pressure node. This disturbance in the local air pressure is propagated to the innards of the bottle at the speed of sound. Left to its own, that is if the pressure disturbance is not driven at a specific frequency, a standing wave forms in the botttle. If the bottle widens the rarefication will be weakened as it spreads throughout the volume of the bottle.
    Last edited: Apr 26, 2006
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