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?