Aerophones are one of the four main classes of instruments in the original Hornbostel–Sachs system of musical instrument classification, which further classifies aerophones by whether or not the vibrating air is contained within the instrument. The first class (41) includes instruments which, when played, do not contain the vibrating air. The bullroarer is one example. These are called free aerophones. This class includes (412.13) free reed instruments, such as the harmonica, but also many instruments unlikely to be called wind instruments at all by most people, such as sirens and whips. The second class (42) includes instruments which contain the vibrating air when being played. This class includes almost all instruments generally called wind instruments — including the didgeridoo, (423) brass instruments (e.g., trumpet, french horn, baritone horn, tuba, trombone), and (421 & 422) woodwind instruments (e.g., oboe, flute, saxophone, clarinet).
Additionally, very loud sounds can be made by explosions directed into, or being detonated inside of resonant cavities. Detonations inside the calliope (and steam whistle), as well as the pyrophone might thus be considered as class 42 instruments, despite the fact that the "wind" or "air" may be steam or an air-fuel mixture.