Aerophones are one of the four main classes of instruments in the original Hornbostel–Sachs system of musical instrument classification. The first class  includes instruments when played, do not contain the vibrating air. For example the bull roarer would fall into this category. Bull-roarers are called “free aerophones” due to the lack of a body containing the air to create the sound.
This class includes [412.13] free reed instruments, such as the bawu, hulusi, sheng, harmonica, concertina, accordions, harmoniums. The second class  includes instruments which contain the vibrating air when being played.
The term aerophone is broad in this definition as it denotes classifying musical instruments that are sounded by a breath of air, including the didgeridoo , brass instruments [e.g., trumpet, french horn, baritone horn, tuba, trombone] and [421 & 422] woodwind instruments [e.g., oboe, flute, saxophone, clarinet].
A free reed aerophone is a musical instrument that produces sound as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows.
Hornbostel–Sachs number: 412.13 [a member of interruptive free aerophones]. Free reed instruments are contrasted with non-free or enclosed reed instruments, where the timbre is fully or partially dependent on the shape of the instrument body, Hornbostel–Sachs number  flute, reed, and brass.
|Aerophones / Hornbostel Sachs No#:|
41 Free aerophones
Instruments in which the vibrating air is not contained within the instrument, for example sirens, or the bull-roarer.
411 Displacement free aerophones
The air-stream meets a sharp edge, or a sharp edge is moved through the air. In either case, according to more recent views, a periodic displacement of air occurs to the alternate flanks of the edge. Examples are the sword-blade or the whip.
412 Interruptive free aerophones
The air-stream is interrupted periodically.
412.1 Idiophonic interruptive aerophones or reeds – The air-stream is directed against a lamella, setting it in periodic vibration to interrupt the stream intermittently. 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 finger-holes.
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.
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.
412.121 Independent percussion reeds.
412.122 Sets of percussion reeds. – Earlier organs
422.3 by the lack of finger-holes.
412.131 Individual free reeds.
412.132 Sets of free reeds – Accordion, harmonica, and reed pipes of the pipe organ.
412.14 Band reed instruments – The air hits the sharp edge of a band under tension. The acoustics of this instrument have so far not been investigated.
412.2 Non-idiophonic interruptive instruments. The interruptive agent is not a reed.
412.21 Rotating aerophones (interruptive agent rotates in its own plane and does not turn on its axis)- Siren disk.
412.22 Whirling aerophones (interruptive agent turns on its axis) – Bull-roarer, whirly tube.
413 Plosive aerophones – The sound is caused by a single compression and release of air. Examples include the botija, the gharha, the ghatam, and the udu.
42 Non-free aerophones (wind instruments proper)
The vibrating air is contained within the instrument. This group includes most of the instruments called wind instruments in the west, such as the flute or French horn, as well as many other kinds of instruments such as conch shells.
421 Edge-blown aerophones or flutes
421.1 The player makes a ribbon-shaped flow of air with his/her lips
421.2 His/her breath is directed through a duct against an edge
421.11 End-blown flutes – The player blows against the sharp rim at the upper open end of a tube.
421.111 Individual end-blown flutes.
421.111.1 Open single end-blown flutes – The lower end of the flute is open.
421.111.11 Without finger-holes.
421.111.12 With finger-holes.
421.111.2 Stopped single end-blown flutes – The lower end of the flute is closed.
421.111.21 Without finger-holes.
421.111.22 With finger-holes.
421.112 Sets of end-blown flutes or panpipes – Several end-blown flutes of different pitch are combined to form a single instrument.
421.112.1 Open panpipes.
421.112.2 Stopped panpipes.
421.112.3 Mixed open and stopped panpipes.
421.112.11 Open (raft) panpipes – The pipes are tied together in the form of a board, or they are made by drilling tubes *in a board.
421.112.12 Open bundle (pan-) pipes – The pipes are tied together in a round bundle.
421.12 Side-blown flutes – The player blows against the sharp rim of a hole in the side of the tube.
421.121 [Single] side-blown flutes.
421.121.1 Open side-blown flutes.
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.
421.121.11 Without finger-holes.
421.121.12 With finger-holes – Western concert flute.
421.121.3 Stopped side-blown flutes.
421.121.31 Without finger-holes.
421.121.32 With finger-holes.
421.121.311 With fixed stopped lower end – [apparently non-existent].
421.121.312 With adjustable stopped lower end – piston flutes.
421.122 Sets of side-blown flutes.
421.122.1 Sets of open slide-blown flutes.
421.122.2 Sets of stopped side-blown flutes.
421.2 Flutes with duct or duct flutes – A narrow duct directs the air-stream against the sharp edge of a lateral orifice.
421.13 Vessel flutes (without distinct beak) The body of the pipe is not tubular but vessel-shaped – Xun.
421.21 Flutes with external duct – The duct is outside the wall of the flute; this group includes flutes with the duct chamfered in the wall under a ring-like sleeve and other similar arrangements.
421.211 [Single] flutes with external duct.
421.211.1 Open flutes with external duct.
421.211.11 Without finger-holes.
421.211.12 With finger-holes.
421.211.2 Partly stopped flutes with external duct.
421.211.3 Stopped flutes with external duct.
421.212 Sets of flute with external duct.
421.22 Flutes with internal duct – The duct is inside the tube. 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).
421.221 [Single] flutes with internal duct.
421.221.1 Open flutes with internal duct.
421.221.11 Without finger-holes – Whistle
421.221.12 With finger-holes – Recorder
421.221.2 Partly stopped flute with internal duct.
421.221.3 Stopped flutes with internal duct.
421.221.31 Without finger-holes.
421.221.311 With fixed stopped lower end.
421.221.312 With adjustable stopped lower end.
421.221.4 Vessel flutes with duct.
421.221.41 Without finger-holes.
421.221.42 With finger-holes – Ocarina.
421.222 Sets of flutes with internal duct.
421.222.1 Sets of open flutes with internal duct.
421.222.2 Sets of partly stopped flutes with internal duct.
421.222.3 Sets of stopped flutes with internal duct.
421.222.11 Without finger-holes – Open flue pipes of the pipe organ.
421.222.12 With finger-holes – Double flageolet.
422 Reed aerophones – 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.
422.11 (Single) oboes.
422.111 With cylindrical bore.
422.111.1 Without finger-holes.
422.111.2 With finger-holes.
422.112 With conical bore – Oboe, bassoon.
422.12 Sets of oboes.
422.121 With cylindrical bore.
422.122 With conical bore.
422.2 Single reed instruments – The pipe has a single ‘reed’ consisting of a percussion lamella.
422.21 (Single) clarinets.
422.211 With cylindrical bore.
422.211.1 Without finger-holes.
422.211.2 With finger-holes – Western clarinet.
422.212 With conical bore – Saxophone.
422.22 Sets of clarinets.
422.3 Reed-pipes with free reeds – The reed vibrates through [at] a closely fitted frame. There must be finger-holes, otherwise the instrument belongs to the free reeds 412.13.
422.31 Single pipes with free reed.
422.32 Double pipes with free reeds.
423 Trumpets – 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. Examples include the bugle, the didgeridoo, and the shofar.
423.11 Conches – A conch shell serves as trumpet.
423.111.1 Without mouthpiece.
423.111.2 With mouthpiece.
423.12 Tubular trumpets.
423.121 End-blown trumpets – The mouth-hole faces the axis of the trumpet.
423.121.1 End-blown straight trumpets – The tube is neither curved nor folded.
423.121.11 Without mouthpiece.
423.121.12 With mouthpiece.
423.121.2 End-blown horns – The tube is curved or folded.
423.121.21 Without mouthpiece.
423.121.22 With mouthpiece.
423.122 Side blown trumpets.
423.2 Chromatic trumpets – The pitch of the instrument can be altered mechanically
423.21 Keyed trumpets – Ophicleide.
423.22 Slide trumpets – Trombone.
423.23 Valved trumpets – French horn, euphonium, baritone horn, trumpet, and tuba.
423.231 Conical bore
423.232 Semi-conical bore
423.233 Cylindrical bore
43 Unclassified aerophones