An aerophone is any musical instrument that 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 distinguishes aerophones based on whether or not vibrating air is contained within the instrument itself.
The first class includes instruments in which the vibrating air is not contained within the instrument, such as the bullroarer. Such instruments are called free aerophones. This class includes free reed instruments, such as the harmonica, and also many instruments unlikely to be considered wind instruments at all, such as sirens and whips.
The second class includes instruments where the vibrating air is contained within the instrument. This class includes almost all the instruments generally considered wind instruments in the West, such as the flute, the oboe, and the trumpet.