I frequently use the tuned percussion tubes Boomwhackers in my elementary music teaching. I noticed something odd about them.
When I compare the smaller and larger C tubes, they sound one octave apart. The sound decays too quickly for me to verify the intonation with a tuner but it sounds like an octave to me.
I know that the wavelength of the two resulting sound waves should have a 2:1 ratio, as should their physical length. And it is nearly so... the large is 62.7cm and the smaller 30.3cm.
But why isn't it exactly 2:1? The small ones seem just a little too short. This must be on purpose -- the intonation is reasonably good. Also I have multiple sets to compare and so I know it's not a manufacturing defect.
These are the tubes. I'm using them with both ends open, if that matters.