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B How a whistle produces a sound?

  1. Jan 5, 2017 #1
    I have 3 question:
    1) How whistle produces a sound? I mean sport whistle with small ball inside.
    2) How we whistle (by mouth)?
    3) If you have a hole in exhaust tube in car. Many people say to me: "It is louder when the car drove long time. (a tube is hotter)" My easy question is, why? And what causes the noise when it is hotter and not cold? (if it is true...)

    I know what is a sound. It is mechanical vibration of air. A change of pressure of air.
    For case 1) I believe that the small ball is very important and I guess that when the ball is moving in the whistle it opens and closes a hole and also flow of air out and it changes the pressure and causes a sound wave. Do you agree?
    In case 2) I can't see any moving mechanism, so in this case I have no clue. Same as in question 3).
    Thank you for your response and discusion ;)
     
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  3. Jan 5, 2017 #2

    jedishrfu

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  4. Jan 5, 2017 #3

    ZapperZ

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  5. Jan 5, 2017 #4

    FactChecker

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    Suppose some air flow is straight through, but other air flow is circling round in the bowl of the whistle. Then there is a collision between the two flows that causes an unstable flow. The circling flow also causes a feedback that determines the frequency of the whistle tone. I don't think that the ball is essential. It just causes a flutter that is more attention grabbing than a steady tone. There is good detail in https://en.wikipedia.org/wiki/Whistle
     
  6. Jan 6, 2017 #5
    Thank you for post. You are right. Next time I will start with searching. But both of them finished where I'm starting. If we talking about 2) whistling by mouth. At the end of the first link is unanswered question.
    "Maybe you should start with simpler arrangements - you can make a tone just blowing air through your lips ro your teeth right? How does that work? "

    I have read both but still don't have answer but ok I have a good base, I believe. So more general question how is possible that air flow coming through tiny enough cavity with the holes at the ends causes a vibration of the surranding air around the hole? Why isn't there a steady flow? Or steady momentum or force?
     
  7. Jan 6, 2017 #6

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    Some of the air stream doesn't fit directly through the exit hole. It swirls around and comes back to collide with the entering flow. Because of that collision, it is not a steady flow.

    Compare just blowing through your lips with whistling. To whistle, you keep your tongue low, out of the way so air flow can circle around backward.
     
  8. Jan 16, 2017 #7
    So if I understand good
    1) whistle works: I blow to the whistle. The air coming in and one part directly come out from the hole. Other part circulates in bowl and time to time (period of one circle) "hit" incoming air and stops outcoming flow. This change in flow through the hole causes periodical change in pressure of ambient air - sound. Is it right?

    2) whistling by mouth: I can transform this situation on to a wall with very small hole. If air hit the wall all the air can't come through and causes backflow which periodically stops outcoming flow coming through the hole. This change in flow through the hole causes periodical change in pressure. Ok?
     
  9. Jan 16, 2017 #8

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    That sounds about right. The only thing I would change is this: I don't think that the flow completely starts and stops, but the collision causes the flow to not be steady and to oscillate at some frequency.
     
  10. Jan 16, 2017 #9
    Ok, but the part about pressure...is my idea good?
     
  11. Jan 16, 2017 #10

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    Yes.
     
  12. Jan 16, 2017 #11
    So now whe
    So and now my 3rd question:
    If you have a hole in exhaust tube in car. Many people say to me: "It is louder when the car drove long time. (a tube is hotter)" My easy question is, why? And what causes the noise when it is hotter and not cold? (if it is true that it is louder, people said).

    Are the ideal conditions for backflow (sound) affected by temperature (higher speed of flows?). What do you think about it?
     
  13. Jan 16, 2017 #12

    Nugatory

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    That is a waaaaaaay more complicated question.... Here are some of the factors to consider:
    1) The components of an exhaust system expand as the system heats up, changing the shape of everything and hence the details of the gas flow through and around obstacles..
    2) The size of the hole often changes with the temperature. It's not unusual to find exhaust leaks that open and close as the temperature changes and things shift around.
    3) Exhaust gas doesn't flow smoothly. It's released as a series of pulses as the valves open and release high-pressure gas into the exhaust system tens of times every second. This leads to alternating pressure waves inside the exhaust system, and very complicated pressure behavior at any hole in the system.
     
  14. Jan 16, 2017 #13

    I see. It seems to be very complex. I let it be as a open question for future generations :) Thank you all, who contributed to this conversation ;)
     
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