sounds and playing a recorder

Hello

If you play a recorder why does the pitch of the note vary when you cover different holes on the tube. How does covering these holes affect the frequency of the sound wave?

thanks
andiejeAsked:
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Dave BaldwinFixer of ProblemsCommented:
This tells more about it than I could : http://en.wikipedia.org/wiki/Recorder
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quizwedgeCommented:
Covering and uncovering holes lengthens and shortens the air column. The shorter the air column (uncovering more holes), the higher the note. See http://www.flute-a-bec.com/acoustiquegb.html
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TommySzalapskiCommented:
Since the medium is the same (air) then the speed of the wave is constant. The length of the wave is the distance from the hole you are blowing in to the first opening (or a factor of it). When you cover a whole, that distance changes. So the wavelength is greater thus forcing the frequency to decrease making the sound lower.
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andiejeAuthor Commented:
I probably should have been more specific in the question. Sorry
Perhaps there isn't a simple one sentence answer to this.

I think i was confused because i was thinking of covering one hole or the top of the recorder and just one hole at the bottom, which isn't how you play it.

So more holes uncovered shortens the air column? Thus if you cover just the top whole the air column is the distance between the mouth piece and the top hole?

And you cover all of the holes to the bottom the air column is the length of the recorder?

And shorter air columns gives a higher pitched note.

IS there an intuitive reason (for kids) why a shorter column gives a higher note. I can see from the formula relating to tubes on the acoustic resonance page here http://en.wikipedia.org/wiki/Acoustic_resonance why a shorter column would give higher frequency, but I can't show that to my 11 year olds
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quizwedgeCommented:
I'll start off by saying that despite the site's name, I'm not a expert in the area of acoustics. :) Here's what I've gathered.

1. If you cover one hole at the top and one hole at the bottom, if the holes were big enough it would be the same as just covering the top. With the number of holes between the top and bottom, it probably sounds the same, but since the holes are small enough you can do something called forked fingering http://www.flute-a-bec.com/colonnedair-fourche-gb.html From that page, "Fork fingerings consist in opening a hole closing one or two holes below it. Because the open hole is too small to "cut off" the vibrating air column, the pitch is lowered by a semitone." Possibly too advanced a topic for 11 year olds, but figured I'd include it for completeness.

2 & 3. You're correct. Less holes covered = a shorter air column and covering all of the holes means the air column is the length of the recorder.

4. Correct, the shorter the column, the higher the pitch.

For the intuitive reason, I'm not sure I've got the math and science completely right, but I think this is right or at least close enough for explaining to an 11 year old. Hopefully if I'm wrong, someone can correct me.

The longer the wave, the lower the pitch. With a longer tube, the wave has to be longer / the frequency is lower. For a real life example of a wave, try getting something pliable that will make a sound when you bend it back and let it go. Show how the pitch is lower when you bend it back a little bit vs bending it back a lot where it will have a higher frequency.

Hope that helps.
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