Sounds waves in recorders and octaves

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Nathan Massey
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Hello,

I've searched the internet for an answer to this question but cannot find it. Not a homework question, I'm just designing a recorder and don't understand this.

A recorder is a fipple flute, meaning that it generates standing sound waves by finger placement over holes that change the wavelength. However, on the back of a recorder there is a hole that you hold fully closed with your thumb to play the standard octave, (C4), when the player half covers this back hole, the octave increases to C5. In terms of sound waves, how does half covering this hole accomplish this? Is it changing the wavelength, or is it something to do with pressure?

Thank you in advance.

-Nathan
 

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Not sure exactly what you mean by "pressure" but I think it is changing the wavelength by creating a nodal point of the pressure wave. I would guess that the hole would be located pretty close to 2/3 the length of the recorder?

What happens when you fully open this hole? My guess it probably changes the note only a tone or semitone?
 
Nathan Massey said:
A recorder is a fipple flute, meaning that it generates standing sound waves by finger placement over holes that change the wavelength. However, on the back of a recorder there is a hole that you hold fully closed with your thumb to play the standard octave, (C4), when the player half covers this back hole, the octave increases to C5. In terms of sound waves, how does half covering this hole accomplish this? Is it changing the wavelength, or is it something to do with pressure?
I have played the recorder very much, but I do not know exactly. I can give some theory, though.

On all woodwinds, going from the base tone to the octave means "short-circuiting" the base tone. This is done by opening a hole at a specific point relative to the base tone. Having one octave hole for each tone is theoretically perfect, but in practice it is impossible to play. That conundrum is solved in different ways:
  • In a saxophone, there are two octave holes. Each of them is a compromise for half the octave and a complicated mechanical linkage changes between them.
  • In a recorder, you only have one octave hole (since no complex mechanics are available). Since you cannot change between several octave holes, your thumb regulates the octave by altering the size of the octave hole. If you think about it, you automatically increase the thumb pressure when you play the higher notes. This has the effect of making the octave hole smaller.