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 of instruments in the original Hornbostel-Sachs scheme of musical instrument classification. Hornbostel-Sachs divides aerophones by whether vibrating air is contained in the instrument itself or not. The first class (41) 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 (42) includes instruments where the vibrating air is contained by the instrument. This class includes almost all the instruments generally called wind instruments - including the didgeridoo, brass instruments in the west, such as the trumpet, the french horn, and the trombone, and woodwind instruments such as the sheng, the oboe, and the clarinet. Additionally, very loud sounds can be made by explosions directed into, or being detonated inside of resonant cavities. Instruments such as 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.
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