There are no simple one frequency sounds in nature.A Sound is made up of a large number of frequencies in whole number relationships to each other i.e.X, 2X, 3X, 4X and so on (taken that X is the sound I'm talking about)
So if you blow on a tube the first sound you hear is the fundamental X. If you blow harder a phenomenon called overblowing occurs and the sound jumps up an octave which is the first overtone 2X. If you blow still harder it jumps yet again producing 3X and so on.The overtone flute artfully uses this phenomenon to produce a musically useful scale by stringing together their overtones.