An aerophone is any musical instrument which produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes, and without the vibration of the instrument itself adding considerably to the sound. It is one of the four main classes (class 4) of instruments in the original Hornbostel-Sachs scheme of musical instrument classification. The other three are idiophone, membranophone and chordophone. A fifth class, electrophone, was added in a later revision.
Hornbostel-Sachs divides aerophones by the fact whether vibrating air is contained in the instrument itself or not.
The first class includes instruments where the vibrating air is not contained by the instrument itself, such as the bullroarer. Such instruments are called free aerophones. This class includes 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 includes instruments where the vibrating air is contained by the instrument. This class includes almost all the instruments generally called wind instruments in the west, such as the flute, the oboe and the trumpet.