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How do free aerophones differ from wind instruments with enclosed air columns?

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shakuhachi (end-blown flute) — wind instrument The Japanese end-blown flute, or *shakuhachi*.

Under the Sachs-Hornbostel classification system, all aerophones produce sound using air as the primary vibrating medium. The fundamental difference between free aerophones and other wind instruments lies in whether the vibrating air is contained within a tube.1

Wind instruments proper: These instruments feature an enclosed air column within a tube. Sound is produced by setting this specific column of air into motion, such as by directing an airstream against an edge (as in flutes), using a vibrating reed (as in reedpipes), or vibrating the lips against a mouthpiece (as in trumpet-type aerophones). The pitch is primarily determined by the length of the enclosed air column.12
Free aerophones: In these instruments, the vibrating air is not contained by a tube. Examples include the bull-roarer, which disturbs the air directly, and instruments utilizing free reeds, such as the accordion or harmonica. In free reed instruments, the pitch is determined by the physical characteristics—such as length and thickness—of the vibrating reed itself, rather than the dimensions of an enclosed air column.12