Aerophones are one of the four main classes of instruments in the original Hornbostel–Sachs system of musical instrument classification. The first class (41) includes instruments when played, do not contain the vibrating air. For example the bull roarer would fall into this category. Bull-roarers are called “free aerophones” due to the lack of a body containing the air to create the sound.
This class includes (412.13) free reed instruments, such as the bawu, hulusi, sheng, harmonica, concertina, accordions, harmoniums. The second class (42) includes instruments which contain the vibrating air when being played. The term aerophone is broad in this definition as it denotes classifying musical instruments that are sounded by a breath of air, 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).
A free reed aerophone is a musical instrument that produces sound as air flows past a vibrating reed in a frame. Air pressure is typically generated by breath or with a bellows. Hornbostel–Sachs number: 412.13 (a member of interruptive free aerophones). Free reed instruments are contrasted with non-free or enclosed reed instruments, where the timbre is fully or partially dependent on the shape of the instrument body, Hornbostel–Sachs number: 42 (flute, reed, and brass).
411. Displacement Whip
412. Interruptive 412.1. Idiophonic / reed
422.1. Double reed
422.2. Single reed