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I'm making an instrument out of an open-ended tube that produces sound when the top (opening) is struck. To find the lengths that correspond to pitches, I used V=λf and combined it with the fundamental frequency of an open-open system. However, I've found that in testing I am one octave below my desired pitch.

If I were to treat it as an open-closed system where the length equals 1/4 of the wavelength, my calculations would work out.

Why is this? Is it a coincidence?

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Like with Boomwhackers but striking on the end? Yes, that would cause an octave shift. Strike the tube on its side.

Open ends are nodes for the sound pressure of a standing wave. A closed end is an antinode for the sound pressure.

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