Why do open holes in air instruments act as pressure nodes?

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yosimba2000
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Just out of curiousity, why is this?

I'm reading this page right here:
http://newt.phys.unsw.edu.au/jw/fluteacoustics.html#registerhole

A dark hole on the flute means the button is closed, and each white hole means the button is open (open to atmospheric pressure).

You can see from the diagram that for every open hole, a node is formed in the corresponding wave. What about this exposure to atmospheric pressure makes this happen?

Thanks.
 

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I don't interpret it that way. It mentions in the article that opening the holes is effectively making the instrument shorter, so essentially moving the node to this end point.
 
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The "shortening" interpretation is consistent with the "node" interpretation. After all, what's significant about the "end" of the instrument, except that it's the place where the oscillating air column is open to the atmosphere? At that location, the pressure is essentially equal to ambient, or "zero," pressure.