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In page Pan flute:

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The pan flute is an end-blown flute. Sound is produced by the vibration of an air-stream blowing across an open hole at the end of a resonating tube. The length of the tube determines the fundamental frequency. An overblown harmonic register is near a 12th above the fundamental in cylindrical tubes, but can approach an octave jump (8th) if a decreasing taper is used.[citation needed]