This can include a column of air being set in vibration (as in wind instruments) or an air-flow being interrupted by an edge (as in free-reeds).
In either case, according to more recent views, a periodic displacement of air occurs to the alternate flanks of the edge.
In this group also belong reeds with a 'cover,' i.e. a tube in which the air vibrates only in a secondary sense, not producing the sound but simply adding roundness and timbre to the sound made by the reed's vibration; generally recognizable by the absence of fingerholes.
412.11 Concussion reeds - Two lamellae make a gap which closes periodically during their vibration.
412.12 Percussion reeds - A single lamella strikes against a frame.
- Earlier organs 412.13 Free-reed instruments feature a reed which vibrates within a closely fitting slot (there may be an attached pipe, but it should only vibrate in sympathy with the reed, and not have an effect on the pitch - instruments of this class can be distinguished from 422.3 by the lack of finger-holes).
Whirling aerophones (the interruptive agent turns on its axis) The sound is caused by a single compression and release of air.
The player makes a ribbon-shaped flow of air with his lips (421.1), or his breath is directed through a duct against an edge (421.2).
421.1 Flutes without duct - The player himself creates a ribbon-shaped stream of air with his lips.
421.11 End-blown flutes - The player blows against the sharp rim at the upper open end of a tube.
421.12 Side-blown flutes - The player blows against the sharp rim of a hole in the side of the tube.
421.121.2 Partly stopped side-blown flutes - The lower end of the tube is a natural node of the pipe pierced by a small hole.
421.121.311 With fixed stopped lower end - (Apparently non-existent).
This group includes flutes with the duct formed by an internal baffle (natural node, bock of resin) and an exterior tied-on cover (cane, wood, hide).
421.222.11 Without fingerholes - Open flue stops of the organ.
421.222.12 With fingerholes 421.222.2 Sets of partly stopped flutes with internal duct.
The player's breath is directed against a lamella or pair of lamellae which periodically interrupt the airflow and cause the air to be set in motion.
422.1 Double reed instruments - There are two lamellae which beat against one another.
There must be fingerholes, otherwise the instrument belongs to the free reeds 412.13.
The player's vibrating lips set the air in motion.
423.1 Natural trumpets - There are no means of changing the pitch apart from the player's lips.
423.121.1 End-blown straight trumpets - The tube is neither curved nor folded.
423.121.2 End-blown horns - The tube is curved or folded.