▁is ▁actually ▁the ▁sound ▁of ▁different ▁harm onic ▁components ▁of ▁same ▁sound ▁coming ▁into ▁domin ance ▁at ▁different ▁times . ▁ ▁John ▁W . ▁Col t man , ▁in ▁a ▁detailed ▁analysis ▁of ▁fl ute ▁ac oust ics , ▁describes ▁two ▁types ▁of ▁war bles ▁in ▁Native ▁American ▁fl utes : ▁One ▁" of ▁the ▁order ▁of ▁ 2 0 ▁H z " ▁caused ▁by ▁a ▁" non linear ity ▁in ▁the ▁jet ▁current ", ▁and ▁a ▁second ▁type ▁" in ▁which ▁amplitude ▁mod ulation ▁occurs ▁in ▁all ▁partial s ▁but ▁with ▁different ▁phases ". ▁The ▁first ▁type ▁is ▁analyz ed ▁by ▁Col t man ▁in ▁a ▁controlled ▁setting , ▁but ▁he ▁concluded ▁that ▁analysis ▁of ▁the ▁second ▁type ▁of ▁war ble ▁" is ▁yet ▁to ▁be ▁explained ". ▁ ▁The ▁war ble ▁can ▁be ▁approxim ated ▁by ▁use ▁of ▁v ibr ato ▁techniques . ▁The ▁phase ▁shift ▁that ▁occurs ▁between ▁different ▁harm on ics ▁can ▁be ▁observed ▁on ▁a ▁spect ro graph ▁of ▁the ▁sound ▁of ▁a ▁war bling ▁fl ute . ▁ ▁Writ ten ▁Music ▁▁ ▁Writ ten ▁music ▁for ▁the ▁Native ▁American ▁fl utes ▁is ▁often ▁in ▁the ▁key ▁of ▁F - sharp ▁minor , ▁although ▁some ▁music ▁is ▁scored ▁in ▁other ▁keys . ▁However , ▁the ▁convention ▁for ▁music ▁written ▁in ▁F - sharp ▁minor ▁is ▁to ▁use ▁a ▁key ▁signature ▁of ▁four ▁sh ar ps . ▁This ▁convention ▁is ▁known ▁as ▁" N ak ai ▁tab l ature ". ▁Note ▁that ▁the ▁use ▁of ▁finger ▁diag rams ▁below