Concept
An aerophone (ˈɛəroʊfoʊn) is a musical instrument that produces sound primarily by causing a body of air to vibrate, without the use of strings or membranes (which are respectively chordophones and membranophones), and without the vibration of the instrument itself adding considerably to the sound (or idiophones). According to Sachs, Aerophones or 'air instruments' include what are usually called 'wind instruments,' with the addition of a few instruments with a different acoustical principle called 'free aerophones.' A wind instrument has two essential factors: a tube enclosing a column of air, and a device for setting that air into vibration by interrupting into pulsations the steady breath of the player (or the wind of a bellows). These may be lips, a mechanical reed, or a sharp edge. Also, an aerophone may be excited by percussive acts, such as the slapping of the keys of a flute or of any other woodwind. A free aerophone lacks the enclosed column of air yet, "cause a series of condensations and rarefications by various means." 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). The wind factor is not only provided by the players' lungs function.
